Jacqueline Shipe
Alternators and Electrical Systems

Alternators and Electrical Systems

Many single-engine aircraft rely on alternators to power aircraft systems, avionics and cockpit gadgets. A&P Jacqueline Shipe guides you through how alternators work and what to do when yours isn’t functioning correctly. 

Nowadays, with everything from glass cockpits to auxiliary power outlets for iPads and phone chargers, there are more demands on the average aircraft’s electrical system than ever before. Most General Aviation airplanes rely on an alternator to provide a steady, reliable source of electrical energy to power electrical components and recharge the battery. 

Electrical system components

The main components in an average airplane’s electrical system are the battery, alternator, voltage regulator, bus bar and wiring. 

The battery provides stored power for starting the engine. It also provides a reserve of electrical power in case the alternator malfunctions in flight. 

The electrical bus bar provides a central point of power distribution to almost all electrical components (except the starter). The bus bar receives its power from the battery or alternator. Electrical components are connected to the bus bar through a circuit breaker or fuse.

Electrical system specifications

The electrical system on most airplanes is either a 14- or 28-volt system. 14-volt systems have 12-volt batteries. 28-volt systems utilize 24-volt batteries. 

The system voltage refers to an airplane’s operating voltage, which is always higher than battery voltage. The system voltage has to be higher than battery voltage in order to recharge the battery. 

Most airplane electrical systems are “single-wire,” meaning that the airframe itself is used as a ground, eliminating the need to run two wires for a positive and ground connection to each electrical component. They are also DC systems, meaning that the components operate on direct current rather than alternating current (AC).

How alternators work

Alternators generate electric current based on the principle of magnetic induction. Any time magnetic lines of force have relative motion with a conductor that is in close proximity, a voltage will be induced in the conductor. 

Alternators have an electromagnet (called a rotor) that spins inside multiple windings of a conductor (called a stator). As the rotor spins, the varying north-south lines of magnetic force induce an alternating current in the conductor. 

Copper-colored stator windings are barely visible under alternator case.

The alternating current is converted to direct current through a series of diodes (rectifiers) that allow current to flow in one direction, but not the other. 

Aircraft alternators are usually “three-phase,” meaning that the stator has three separate conductor windings.

Stator windings.

The rotor is an electromagnet whose magnetic strength is controlled by the amount of current it receives from the voltage regulator. This allows the voltage output of the alternator to be regulated. If the rotor were made with a permanent fixed-strength magnet, voltage output would be unregulated and would vary with engine rpm. 

Rotor inside a disassembled alternator case half.
Voltage regulator

Aircraft alternators are externally regulated by a voltage regulator (sometimes called an alternator controller), which is usually mounted either on the firewall in the engine compartment or under the instrument panel.

Voltage regulator installed on the firewall of a Cessna.
This regulator has only three wires—a common configuration. The red wire is the current supply for the voltage regulator from the bus. The black wire is the ground connection. The blue wire is the field wire that connects to the F1 terminal of the alternator.

The voltage regulator controls system voltage by controlling the electrical circuit (called the field circuit) that energizes the electromagnet of the alternator rotor. On most single-engine planes, this is accomplished by varying the flow of electric current to the rotor.  

Many voltage regulators have only three wires, but this aircraft’s regulator has five wires in total. The orange wire and the black wire are used to sense alternator voltage directly from the B+ terminal and ground connection at the alternator. The orange wire connects to B+. The red wire is the current supply from the bus that powers the voltage regulator. The yellow wire is the field wire that connects to the alternator’s F1 terminal. There is another ground wire (not shown) for the voltage regulator itself that attaches under one of the voltage regulator mounting bolts.

Older-style voltage regulators had contact points that would wear over time. Modern voltage regulators are fully electronic and, other than a voltage adjustment screw on some models, are maintenance-free. Whenever one of these units fails, it is simply replaced. 

This Plane-Power voltage regulator is similar to a three-wire regulator. The sense wire and the aux wires are connected to the enable wire. The enable wire runs current from the bus to power the regulator. The field wire is the output to the alternator rotor, and the ground wire goes to ground under one of the mounting fasteners for the voltage regulator. This regulator is set up for a 14-volt system and has a built-in 16-volt overvoltage sensor. 
Aircraft alternator design

The original manufacturers for most of the single-engine alternators used on Cessna aircraft were Ford, Chrysler, Prestolite or Delco Remy. Ford alternators were the most commonly used type on Cessnas. Many of the original alternator designs are still in use. 

This alternator’s tag shows Chrysler as the original manufacturer.
Ford-style alternator on a Cessna.

Most aircraft alternators have only three wires connected to them. A field wire connects to the F1 (field) terminal on the alternator, an output wire comes from from the B+ terminal of the alternator and a ground wire connects the alternator frame to a suitable ground connection. 

Ford-style alternator on a Cessna. This alternator has the most common setup with only three wires attached. The braided ground strap is for the ground connection. The heavy wire in the front that connects to the red B+ terminal is the alternator output wire. The field wire connected to F1 is barely visible behind the B+ terminal.
All of the wires attach to terminals on the rear of this alternator.
This alternator has a ground wire from the F2 terminal to the ground connection for the alternator.
Many voltage regulators have only three wires, but this aircraft’s regulator has five wires in total. The orange wire and the black wire are used to sense alternator voltage directly from the B+ terminal and ground connection at the alternator. The orange wire connects to B+. The red wire is the current supply from the bus that powers the voltage regulator. The yellow wire is the field wire that connects to the alternator’s F1 terminal. There is another ground wire (not shown) for the voltage regulator itself that attaches under one of the voltage regulator mounting bolts. 

On most of these types of alternators, the F2 terminal has no wire attached to it and is grounded to the alternator frame. Some alternators have a ground wire routed from the F2 terminal to the ground wire connection on the alternator frame.

F2 terminal. No wire attaches here. The terminal itself is grounded to the alternator case.

The alternator’s rotor gets its electrical connection through two carbon brushes that ride on separate slip rings. One of the brushes transmits positive current from the voltage regulator to the rotor windings and the other provides the ground connection, either from the F2 terminal or from an internal ground to the alternator frame. 

Rotor slip rings. The carbon brushes ride on the slip rings to transmit electric current and provide a ground connection to the rotor windings.

This article covers the most common three-wire system. A pilot/owner performing troubleshooting or wanting specific details on how his/her system operates should reference the current wiring diagram for their aircraft’s model and serial number. 

Electric current flow through a typical system 

When the aircraft master switch is turned on, the battery contactor closes and battery power energizes the aircraft’s electrical bus bar. Current from the bus flows through the alternator field circuit breaker and aircraft master switch to the voltage regulator. 

Some aircraft are also equipped with an overvoltage sensor that is wired in series between the master switch and the voltage regulator. The sensor must be in the closed, unfaulted position to allow current to flow.

The regulator senses that alternator output is zero and sends full current flow through the field wire, which is connected from the voltage regulator output to the F1 terminal of the alternator. The electromagnet of the rotor becomes energized since it has a complete electrical circuit with power at F1 and a ground at F2. 

As the engine starts and the alternator is rotated mechanically with either a belt or gear, the stator windings have an alternating current induced into them from the magnetic field of the rotor. 

Insulated stator winding with a soldered connection to the diode. If the alternator mounts or bushings become loose, the alternator can vibrate and the windings can crack or break. This failure creates an open circuit.

There are usually three stator windings and a total of six diodes in each alternator. The alternating current from each stator winding is converted to direct current after passing through two diodes.

Recently-replaced diode. The diodes have soldered connections to the stator windings.

The output of all three stator windings is rectified and passes as direct current out the B+ terminal, through a heavy-gauge wire to the alternator circuit breaker (usually 60 amp), then to the aircraft bus bar. 

Alternator output terminal (B+).

The voltage regulator senses the alternator output voltage, though the monitoring location varies. Some voltage regulators sense bus voltage, and some are wired to sense voltage directly at the B+ terminal of the alternator. 

The voltage regulator then adjusts current flow to the rotor field as needed to maintain the set system voltage. Always refer to the aircraft maintenance manual for the correct voltage range when setting or checking system voltages. 

If the alternator output or bus voltage is too low, the regulator increases current flow to the rotor field, which in turn increases the strength of the electromagnet and increases the alternator output voltage. 

If alternator output voltage is too high, the regulator decreases current flow to the rotor field, decreasing the strength of the electromagnet, which in turn decreases alternator output voltage.

Troubleshooting basics

A good electrical multimeter and a current wiring diagram that is pertinent to the installed equipment on an airplane are necessary to accurately troubleshoot the charging system for faults. 

Red electrical multimeter probe inserted in auxiliary power port to check bus voltage with the engine running and the alternator on and charging. The black lead is connected to the seat track for a ground connection. When checking voltage using a meter probe in the auxiliary power port or cigarette lighter, make sure the probe isn’t touching both the center and side of the port at the same time. The tip of the probe should only contact the furthermost interior center portion. If the probe touches the center and side at the same time, a direct short to ground occurs. This will either cause the circuit breaker that powers the port to trip, or if the port is installed with an inline fuse, it will blow the fuse.


Always check to see if the field (sometimes called the controller) or alternator circuit breaker is tripped before troubleshooting the system. 

Assuming a tripped breaker is not the issue, the first step in checking a charging system is to run the airplane and check bus voltage from either an auxiliary power port (e.g., the cigarette lighter receptacle, not a 5-volt USB power port) or from the bus itself. 

When checking bus voltage on a 28-volt system, the voltage will have to be checked on the bus itself since the power ports on these planes usually have stepped-down voltages. 

You may be able to check system voltage using a panel-mount voltmeter, an engine monitor or a GPS, if so equipped.

Checking the bus voltage is best accomplished with two people, one to run the plane and the other operating the voltmeter. 

With the red probe on the bus bar, and the black probe connected to a good ground (the seat track usually works well), the voltmeter will read the bus voltage. 

Bus voltage should be checked before starting the engine, and then checked again with the engine running to compare battery voltage (not running) to alternator output voltage (running). If the two voltages are identical, the alternator is not charging at all.

If the alternator is charging, the voltage should be noted and then all the electrical equipment on the airplane should be placed in the on position. With the lights, radios, pitot heat and other electrical components turned on, the alternator should carry the full electrical load and still maintain a positive charging rate.

Alternator is not charging

One of the first and easiest checks a pilot/owner can do when troubleshooting an alternator that isn’t charging is to turn on the aircraft master switch (both the alternator and battery side) and see if the alternator rotor becomes magnetized. 

Alternators that are pulley-driven are very easy to check. The pulley should be magnetized enough to hold the steel tool in place. If the alternator is gear-driven, the rotor will still be magnetized enough to detect a magnetic pull on the steel tool held near or on the alternator case. 

If there is a magnetic tug on the tool, it confirms that the field circuit is intact, the voltage regulator is sending power to energize the rotor and the rotor circuit is intact. 

Checking for magnetization of the alternator is done with the engine not running, magnetos off and mixture full lean. It’s also good safety practice to not be near the propeller whenever the battery master switch is being turned on. A stuck starter solenoid could allow the propeller to rotate even though the ignition switch is off.

The integrity of the connections on the alternator itself should be checked to be sure they are secure. A loose connection can allow current to flow intermittently. All alternator ground connections should be checked to make sure they are secure and have a low resistance (less than 0.2 ohms) between them. This includes the connection from F2 to the alternator ground terminal, the alternator-to-engine ground and the airframe-to-engine ground.

Electrical multimeter showing bus voltage with alternator charging.

If the rotor is magnetized and appears to be working properly, the next step is to see if there is bus (battery) voltage on the B+ terminal of the alternator. If there is bus voltage there, it means that the circuit between the alternator output and the bus bar is intact. 

Check next for a mechanical defect in the drive mechanism for the alternator. A loose belt that slips or a failed gear coupling on a gear-driven alternator can cause the alternator malfunction. 

If all the above checks pass, then the most likely problem causing the alternator not to charge is within the alternator itself. It should be removed for repair. 

Alternator is not charging; rotor is not magnetized 

If the rotor is not magnetized whenever the master switch is turned on (make sure the field circuit breaker isn’t tripped), the next step in troubleshooting is to see if the rotor is getting both the current and ground it needs to energize the windings. 

Checking the field circuit and rotor with the master switch turned on. The magnetized rotor and pulley will hold a wrench in place.

Check all ground connections for excessive resistance (anything above about 0.2 ohms). Turn the master switch on and check for voltage at the F1 terminal. It should be within a couple of volts of battery voltage.

In the situation where there is voltage at F1 and all the ground connections have good continuity, the alternator rotor itself is probably faulted or the alternator’s brushes have excessive wear. 

Rotor brush.
Disassembled alternator. The two brushes for transmitting positive current and an electrical ground to the rotor are at right angles to each other. The square white plastic brush holder is surrounding the brush itself. The brushes are spring-loaded to keep them pressed firmly against the slip rings. 

When there is no voltage present at the F1 terminal, there are several things that could be faulted upstream, interrupting current flow. The aircraft’s wiring diagram will show all the installed electrical components in the circuit as well as the wire numbers. 

One of the first things to check in this case is the integrity of the field wire, especially right at the F1 terminal. The crimped terminal connection often breaks over time with vibration. Some engine compartments have a very small space for the alternator terminal attachments and the wires are sometimes routed with sharp bends, causing stress on the crimped terminal ends. The field wire often breaks at or near the alternator terminal. 

Repaired field wire. The field wire is much smaller and more fragile than the alternator outwput wire or the ground wire. It often breaks at or near the alternator terminal where it is attached. This wire has had a prior repair.

If the terminal end is in good shape, the field wire between the F1 terminal and the voltage regulator should be checked for continuity and/or shorts to ground. 

Cessna voltage regulator.

The field wire should only be removed from the alternator with the master switch off. If the master switch is turned on with the wire removed, take precautions to prevent the field wire from touching anything that would allow it to short to ground. The voltage regulator can be ruined if that happens.

The voltage regulator powers the field wire and should be checked next. Disconnect the plug for the regulator and see if the regulator is receiving current from the bus. The red-colored wire is usually the wire that provides power to the regulator. Always consult the wiring diagram. 

If the regulator is receiving power from the bus, check next for a good ground connection for the regulator. Some regulators have a ground wire that is connected under one of the mounting fasteners, and some regulators require a good ground connection between the regulator body and the airframe ground. These connections can corrode over time, causing too much resistance in the ground connection. Cleaning the connections with Scotch-Brite or a small piece of sandpaper usually re-establishes a good ground connection. 

If the voltage regulator has a good ground connection and is receiving bus voltage but is not sending current to the field terminal of the alternator, it is most likely faulted. 

Field terminal (F1).

Some regulators use two wires to sense alternator voltage, one going to the B+ terminal of the alternator and one going to the alternator ground wire. Both of these wires should be checked for continuity and proper attachments at the alternator before assuming the voltage regulator is faulted.

Checking for bus/battery voltage at the B+ terminal.

If the voltage regulator is not receiving power from the bus, the next upstream component is usually an overvoltage sensor. Again, always consult the wiring diagram—some airplanes don’t have overvoltage sensors at all, and some overvoltage sensors are incorporated into the voltage regulator itself. 

The overvoltage sensor opens the power circuit to the regulator if it detects a voltage higher than its preset upper limit. Occasionally the sensor fails in the open position and will not conduct bus voltage to the voltage regulator at all.

If the overvoltage sensor has normal bus voltage on the input wire (usually coming from the master switch), but no voltage on the output wire (between the overvoltage sensor and voltage regulator), the sensor itself is faulted and should be replaced. 

If there is no power to the overvoltage sensor, check for power on both sides of the master switch (alternator side), and for power on the output side of the circuit breaker. Although it is rare, sometimes circuit breakers erode or internally fail in the open position and will no longer conduct current. The master switch contacts can also erode over time. 

Aircraft alternators and electrical systems aren’t complicated once you understand how current flows through the system’s various parts. With a solid understanding of how the system works and an aircraft-specific wiring diagram in hand, combined with these tips for troubleshooting and the assistance of a friendly mechanic, a pilot/owner can diagnose many common problems.

Know your FAR/AIM and check with your mechanic before starting any work. Always get instruction from an A&P prior to attempting preventive maintenance tasks.

Jacqueline Shipe grew up in an aviation home; her dad was a flight instructor. She soloed at age 16 and went on to get her CFII and ATP certificate. Shipe also attended Kentucky Tech and obtained an airframe and powerplant license. She has worked as a mechanic for the airlines and on a variety of General Aviation planes. She’s also logged over 5,000 hours of flight instruction time. Send question or comments to .


Alternator overhaul and repair– CFA supporter
Airmark Overhaul
New OEM alternators– CFA supporter
Plane-Power (Hartzell Engine Technologies LLC)
Cessna’s Retractables: The System, Demystified

Cessna’s Retractables: The System, Demystified

A&P Jacqueline Shipe lists dissects the system and offers troubleshooting tips for Cessna’s retractable singles.


In the late 1940s and through the decade of the 1950s, General Aviation began to really take off as airplane sales increased. The postwar economy was favorable for the production and sale of both single- and twin-engine models, and the “big three” in the aviation industry were Beechcraft, Piper and Cessna. 

It was 1947 when the Beechcraft Bonanza appeared on the aviation market. The Bonanza was the first retractable single-engine plane on the market that had a wide appeal to a large number of customers. Sales were good, and in the late 1950s, Piper also entered the single-engine retractable gear market with the debut of the Piper Comanche. 

Cessna enters the RG market

Cessna wasn’t going to be left out, and in 1959, the Cessna 210 made its debut. Early 210s were essentially Cessna 182 frames with a stronger engine and a retractable gear system that was very complicated in its design. 

The main landing gear doors had their own actuators in addition to the gear actuators, along with an accumulator for the main gear doors. The hydraulic pump on the earliest models was engine-driven. The whole system had a vast array of hydraulic tubing and was plagued with glitches. 

As years went by, Cessna kept making improvements and major changes to the hydraulic system until it had a relatively trouble-free gear design by the end of the 1970s. 

Cessna’s 1979 and later models had no main gear doors (eliminating all the main gear door actuators, accumulators and associated tubing) and the engine-driven pump had been long since replaced with an electric pump. The Cessna 172, 177 and 182 RG models all have the improved design for which the 210 was the predecessor. 



RG system overview

The general system (with a few differences in specifics on differing models) operates hydraulically and uses hydraulic pressure and an electrically-driven pump to provide the means of extending and retracting the gear. 

The major components in the system consist of a power pack assembly, two main gear actuators, the nosegear actuator, main gear down-lock actuators, a hydraulic gear selector lever, an emergency hand pump and all the associated plumbing and electrical circuits for the system.
Power pack

The power pack assembly houses several components in one location. It contains a hydraulic fluid reservoir, an electric motor and hydraulic pump assembly, two pressure relief valves (one for controlling thermal overpressure, and one for controlling pump overpressure), and a pressure switch which turns the pump on and off based on system pressure. 

The pressure switch shuts off the pump when system pressure reaches 1,500 psi and kicks on the pump when pressure drops below 1,000 psi. There is also a dipstick, so the fluid level in the reservoir can be regularly checked. The power pack is located behind the central console. 



Main gear 

The main gear actuators are installed in the floor just aft of the doorframes. They are mounted at roughly a 45-degree angle to the longitudinal axis of the airframe. These actuators have large teeth on the end of a very strong steel shaft, which extends and retracts according to the flow of hydraulic pressure. 

The main gear legs are bolted into a Y-shaped steel housing called a pivot assembly. The pivot assembly is mounted on a bearing, allowing it to rotate. 

The pivot assembly has a splined shaft with a “sector” gear installed on the shaft. The sector gear has large teeth that are meshed with the actuator teeth. As the actuator shaft is extended or retracted, the sector gear, pivot assembly and main gear leg rotate. 

The angles at which the pivot assembly, actuators and gear legs are connected and mounted in the frame, in addition to the shape of the main gear leg and pivot assembly, cause the gear leg to swing downward, inboard, and then aft as it retracts. 

The plane has to be jacked high enough to raise the main wheels about 18 inches off the ground to have enough clearance to swing the gear when performing gear maintenance or checks. 



The nosegear actuator is attached to a rod end that is connected to the nosegear trunnion. It has a steel shaft with a piston made onto the shaft. The shaft/piston combo moves back and forth depending on which side of the piston has hydraulic pressure on it. 

Fluid from the gear-up line extends the shaft and pushes it forward, which in turn pushes the nosegear forward causing it to pivot upward into the up position. Pressure from the gear-down line presses on the opposite side of the piston causing the shaft to retract into the actuator, pulling the nosegear into the down position. 

Built-in safety

The main gear legs have down-lock actuators and hooking mechanisms that prevent the main gear pivot assemblies from rotating with the gear in the down position. The actuators are located under the floorboard forward and outboard of the main gear actuators. The end of the shaft on the actuator is connected to an arm that disengages the down-lock hook; on some models, the shaft is connected directly to the down-lock hook. 

The landing gear selector lever on all but the earliest 210 models is fully hydraulic. The pilot is opening and closing off ports when the lever is moved, allowing fluid to flow in the opposite direction. 


Warning systems

The system also has a series of switches that make contact or not depending on gear position, and/or the down-lock hook position. A green light is illuminated when all three gears are down and locked. 

An amber “gear unsafe” light is illuminated on some models anytime the gear is in the up and locked position, staying on continually in flight. On later models, the amber light was replaced with a red light that is illuminated anytime the pump is operating. 

The gear warning system consists of throttle micro-switches and a switch on the flaps. Lowering more than 25 degrees of flaps or retarding the throttle below approximately 12 inches of manifold pressure sets off a horn if the gear is not in the down and locked position. 


Retracting the gear

A general flow of what happens as the gear is retracted is as follows: after takeoff, the squat switch located on the nose strut closes as the nose is lifted off and the strut moves to the fully extended position. 

As the pilot selects gear up after takeoff, pressurized fluid from the power pack is routed through the gear selector valve to the nosegear actuator and main gear down-lock actuators, causing them to release the down-lock hooks. 

There is not a down-lock actuator on the nosegear. It is locked down by a mechanically linked hook that goes over center when the nosegear is fully down, and pivots out of the way as the gear actuator pulls the nosegear up. The nosegear doors are mechanically connected through linkages and are pulled closed after the nosegear comes up. 

Pressure is then routed to the main gear actuators, and they begin the process of rotating the pivot assemblies and swinging the gear legs aft and up. 

Once all three gears are fully retracted, system pressure builds to 1,500 psi and the pressure switch on the power pack shuts off the pump. 

Electrically, the three up-lock switches close and provide continuous power to an amber light that stays on whenever the gear is retracted, or on 1983 and later Cessnas, the red in-transit light stays on until the up-lock switches close and system pressure builds enough to shut the pump off. 

Lowering the gear

The gear lowering cycle begins with the selector valve allowing pressurized fluid to flow to all three gear actuators and to the main gear down-lock actuators. 

On some of the later models, the down-lock actuators are internally spring-loaded to the latched position and are configured so that the fluid is returned through them from the main actuators on the gear lowering cycle. With no fluid pressure to overcome the spring pressure, the locks pop into place as the main landing gear pivot assembly is rotated to the down position. 

Once all three gears are down and locked, the system pressure builds and the pressure switch on the power pack opens, shutting off the pump. Electrically, the down and locked switches on each gear leg close and send power to illuminate the green light. 

Maintenance concerns

Maintenance of the system includes routine inspections for leaks or chafing of hydraulic lines, regularly greasing moving parts and checking for proper rigging. 

One of the most common malfunctions involves the brake line that is installed in the hollow main gear leg through a swivel fitting in the pivot assembly. These leak occasionally or the brake line itself rubs on the gear leg and develops a leak. 

The switches that indicate gear position wear with use, and will stop making contact when they should. These switches have to be adjusted or replaced periodically. The gear-up bumper blocks also get worn down over time, and if they are not replaced, they can allow the gear to cause damage to the up-lock switches.

The actuators all have O-rings and seals that wear out and develop leaks, and these have to be rebuilt occasionally.  

Troubleshooting the system 

When troubleshooting a problem, a thorough understanding of how a particular system works is important. The following are general suggestions, but the exact schematic for each plane should be consulted before making a diagnosis.* 

If the gear won’t retract and the pump doesn’t run, and there is a proper amount of hydraulic fluid in the system, a faulty squat switch (open when it should be closed); pressure switch (failed in the open position); or power pack solenoid could be to blame. These are the three main electrical items that must function correctly for the pump motor to receive power. 

If these items are functioning correctly, the pump motor could be defective. The first thing that should be checked is battery voltage to ensure the battery is putting out enough power. The pump motor draws a heavy current load on the electrical system and requires a lot of juice.  

If the gear will retract but the pump runs all the time, assuming there are no leaks in the system, the pressure switch or solenoid on the power pack could be sticking in the closed position. 

If the power pack pump kicks on often during flight, the pilot might want to consider lowering the gear and keeping it extended for the duration of the flight, as this could be a sign the system is losing hydraulic fluid and/or pressure somewhere. If too much fluid is lost, or if the system malfunctions and is bypassing fluid internally not allowing pressure to build up, the hand pump will not lower the gear.  

A note about tires

Tire replacement on the retractable models is the same as on other models, but re-capped tires shouldn’t be used because they usually have a larger outside diameter than other tires; this could cause them to jam on the sides of the wheel well. 

Also, if the tire is wearing excessively on one side, the toe-in and toe-out adjustments are fairly easy to make on these models by adding shims where the axle is attached to the gear leg.  

When sitting level on the ground, the tire should not lean inboard or outboard, but should be perpendicular to the ground.


Further information 

A thorough understanding of the gear system on the plane a person flies makes it easier to handle a malfunction and helps with troubleshooting a problem. 

One of the best resources a pilot can use for learning about any system is the maintenance manual for his or her specific make and model. It gives a system description as well as a troubleshooting table for common faults. 

There are free Cessna manual downloads available through Red Sky Ventures (RSV), and paper copies of manuals can be purchased from aviation suppliers including Aircraft Spruce & Specialty. 

The POH is also a good resource to review for a general system description. Learning as much as possible about your plane makes you a safer pilot.

*There are differences based on each airplane model and serial number, so the schematic and manual for a particular plane should be consulted for exact details. Jacqueline Shipe grew up in an aviation home; her dad was a flight instructor. She soloed at age 16 and went on to get her CFII and ATP certificate. Shipe also attended Kentucky Tech and obtained an airframe and powerplant license. She has worked as a mechanic for the airlines and on a variety of General Aviation planes. She’s also logged over 5,000 hours of flight instruction time. Send question or comments to .


Cessna manuals

Aircraft Spruce & Specialty
– CFA supporter
Red Sky Ventures
DIY Wheel Bearing Service

DIY Wheel Bearing Service

A&P Jacqueline Shipe describes how to service wheel bearings in this article, the second in a DIY series for pilots who wish to take on preventive maintenance of their aircraft.

FAR 43 Appendix A lists the preventive maintenance items owners may legally perform on their planes. This list is fairly long—and some of the items are a little involved for a person to perform the first time by themselves, while other tasks on the list are pretty straightforward.  

There are several preventive maintenance tasks pertaining to the landing gear, including tire changes, strut servicing and servicing the wheel bearings. (Last month, Shipe discussed the steps involved in changing an aircraft tire. See the June 2016 issue for more information. —Ed.) 

A tapered roller bearing with pits on the rollers caused by corrosion due to water. 
Bearings: small but mighty

While cleaning and greasing wheel bearings doesn’t seem like too difficult a task, there are some guidelines that need to be followed. The failure of a wheel bearing can cause major damage to the wheel and can even allow the wheel assembly to slide off the axle.

Wheel bearings are relatively small, but are incredibly strong. They have to support the weight of the plane while allowing the wheel to spin freely in all types of temperatures and conditions. In addition, wheel bearings and races on airplane wheel assemblies also have to be capable of withstanding hard landings and both vertical and horizontal loads without failing.  

The race with lots of pitting; the race is in the process of being removed from the wheel half. 
Types of bearings

The bearings on most airplane wheel assemblies are the tapered roller-type. The outer part of the bearing is larger than the inner part, and the rollers are installed at an angle. 

The bearing itself rides in a metal cup called a race. The race has a “pressed in” fit in the wheel half, and is tapered on the inside to match the bearing. The biggest advantage of tapered bearings is the high load capacity that they can withstand. 

Automotive wheel bearings, on the other hand, usually use spherical rollers (i.e., balls). Ball bearings can withstand prolonged high speeds without building up too much heat, but cannot take high impact loads. 

Tapered bearings will bear up under the not-so-good landings that occur from time to time with an aircraft. In addition, proper servicing of these bearings will keep the wheels spinning freely and will last for a long time. 

The wheel half with the race removed. 
Removing the clips

Once a wheel assembly is removed from the axle, the wheel bearings are easily removed by taking out the metal retaining clips that secure the bearings and grease felts. 

There is an indention in the outer part of one end of the clip to allow a screwdriver to be used to pry it out. The clips don’t have a lot of tension on them and can be easily removed. 

Once the clip is off, the bearing, metal rings and grease felts can all be lifted out together. 

Be sure to keep all the rings and clips organized so they can be reinstalled into the same wheel half and in the same place. The metal rings that retain the bearing are sometimes slightly smaller on the outer half than the double rings used on the inner half, and can be easily mixed up. 

The race being installed. It has to be started evenly all the way around otherwise it will damage the wheel half as it is driven in.
Cleaning the parts 

A small bucket with 100LL Avgas works well to clean the bearings. Swishing the bearing around and spinning it by hand while it is submerged will clean all of the old grease and gunk out. 

The metal rings and clips should also be cleaned, but the felt material needs to be set aside; it should not be submersed in anything. There is really no way to clean the felt, anyway—as long as it is still in one piece, it’s good to go. Any grease felt that is torn or missing a section needs to be replaced. 

Once all the parts are cleaned, they should be blown out with compressed air (if available) or laid out on paper towels to dry. The parts need to be thoroughly clean and dry before fresh grease is applied.
Inspecting the parts

After the bearings, metal rings and clips are clean and dry, the bearing and race should be inspected for pitting or damage. If the race is smooth and has no corrosion, the bearing is generally corrosion-free as well. 

Races that have light surface corrosion can sometimes be smoothed out with a piece of light grit sandpaper (800 to start and 1200 to finish). Deep pits in a race mean replacement is needed. 

Discoloration on the bearing or race, such as a rainbow or gold color, can be a sign that these parts have generated excessive amounts of heat, in which case they should be replaced. 

The tiny section of aluminum at the bottom of the recess for the race is easily cracked if the race is driven in too far.  
Preventing corrosion

Wheel bearings typically fail for two reasons: corrosion or overheating. 

The greatest threat to airplane wheel bearings is usually corrosion. Almost all bearings and races will eventually require replacement due to water getting past the grease seals and accumulating in the bearing cavity, causing rust and pitting. 

When cleaning a plane, strong degreasers should not be used on wheel assemblies and wheels should never be sprayed with a water hose. The pressurized water will get past the grease seals and ruin the bearings. 

Folks that want their wheels clean can wipe them out with a rag that is lightly moistened with a little Gojo original white cream hand cleaner (the non-pumice kind). Then the wheels can be wiped clean with a dry rag. 

The back side of the wheel assembly should be closely inspected for any signs of cracking after a new race is installed.
Replacing the races

Wheel bearing replacement is easy, but replacement of the races is a little tough to do without the proper tools. 

Because the race has a pressed-in fit in the wheel half, it has to be driven out. This can be accomplished by using either a hammer and punch or a bearing driver tool. 

Occasionally a person encounters a wheel assembly with a race that has broken loose and is spinning in the wheel half itself. In this case, the wheel assembly has to be replaced; there is no permanent way to hold the race in place if the wheel assembly has lost enough metal that the race is no longer fitting tightly. 

The wheel is made of cast aluminum. When reinstalling the steel race, it is very important that it be driven in straight. If it gets cocked—even a little—the much softer aluminum will be gouged and damaged. 

The best tool for the job is a bearing driver, as it allows each blow of the hammer to be applied equally around the circumference of the race. 

Once the race is almost near the bottom of its recess, very light blows should be used to seat it in the wheel half. Many mechanics have driven the race in too far and cracked the fairly thin aluminum ring that retains the race. 

The wheel should always be thoroughly inspected for any sign of cracking on the front and back sides, whether or not a race is replaced.

A freshly greased bearing alongside its grease felt and retainer clip.
Packing the bearings and reinstalling 

Once all of the races are installed and the wheel halves are inspected, the bearings are ready to be packed and installed. A high-quality wheel bearing grease that has good water resistance should be used. 

The grease has to be pushed up through the bearing until it comes out the top between each roller. If it doesn’t squeeze through each opening, the inside of the bearing will have gaps and inadequate lubrication. 

It takes a little while to pack a bearing by hand. There are bearing packers sold in almost any automotive store that make the job a little faster and a little less messy. 

Once the bearing is packed, apply a layer of grease to the entire surface of the race to ensure it is covered as well. 

The bearing can then be reinstalled along with the correct order of retaining rings and grease felts. 

Lastly, reinstall the clip. It is a good idea to make sure the clip is pressed into place all the way around by pushing it outward with a screwdriver. 

After all the clips are in, the wheel bearing service is complete.

The greased bearing assembly installed in the wheel half. The wheel is ready for installation.
Jacqueline Shipe grew up in an aviation home; her dad was a flight instructor. She soloed at age 16 and went on to get her CFII and ATP certificate. Shipe also attended Kentucky Tech and obtained an airframe and powerplant license. She has worked as a mechanic for the airlines and on a variety of General Aviation planes. She’s also logged over 5,000 hours of flight instruction time. Send question or comments to . 
This Cessna nose strut is overinflated. Overinflation of a nose strut can make steering difficult because there is a centering cam that is designed to keep the nose wheel straight once the strut is extended. Too much air pressure can cause the cam to engage on the ground and make steering difficult or impossible.

Strut Servicing: The Ins & Outs


In the third article in a DIY series for pilots, A&P Jacqueline Shipe goes through the steps an owner can take in order to properly service the struts on their aircraft. 

Among the preventive maintenance items listed in FAR 43 Appendix A that pilots may legally perform on an airplane that they own is strut servicing.

The struts on any airplane serve a critical purpose. They provide the shock absorption necessary to prevent the airframe structure from enduring too much stress from the impact loads incurred on landings.

Even taxi operations impose stress on an airframe every time the gear hits a bump or uneven surface.

The strut absorbs the bulk of these loads and prevents them from being transmitted to the airframe.

Overinflation or underinflation can affect the operation of the squat switch on this Cessna 210.
Types of struts

There are several different kinds of struts used for shock absorption. Over the years aircraft manufacturers have used different materials to limit the stress from the impact of landing. Some have used rubber biscuits, bungee cords and spring steel.

The most common type found on most planes (and the only type used on fairly heavy planes from light twins all the way up to airliners) is the hydraulic air/oil cylinder, also referred to in some manuals as oleo struts. The oleo strut is very reliable, can withstand tremendous loads and is fairly simple in its design.

The oleo strut uses air pressure and hydraulic fluid to create a spring effect. The strut consists of an outer housing called a cylinder and an inner piston that is connected to the nose fork or to the main wheel axle. The piston portion of the strut is the part that actuates up and down.

There are different styles and configurations, but all struts house hydraulic fluid in the lower section of the strut and compressed air (or nitrogen) in the upper section. As the piston is driven into the cylinder upon landing, the fluid is forced through an opening called an orifice that slows the rate of the flow.

Some manufacturers make use of a metering pin connected to the piston. The pin is mounted so that it is forced upward through the orifice along with the fluid. It protrudes up through the orifice, is slim in the middle and wider on both ends.

Its shape is tapered so that as the piston reaches the top portion of its travel, less and less fluid can fit throughthe opening. This gradually slows the fluid flow and decelerates the piston. Meanwhile the pressure of the compressed air is being steadily increased as the piston travels upward and reduces the volume of space in the upper chamber.

Eventually the increased pressure of the compressed air overcomes the decreasing fluid pressure and forces the piston to extend. As the fluid flows in the other direction, its flow is impeded at a steady rate by the opposite end of the metering pin, gradually slowing the fluid flow in the opposite direction. This results in dampening out any oscillations and returning the airplane to its normal static height above the ground.

Some models don’t use metering pins but have metering tubes with various sized holes in them that slow the rate of flow as the piston reaches either end of its travel. Some manufacturers don’t use either metering pins or tubes, but instead use restrictor plates with orifices in them to produce the same effect.

Clear tubing is connected to a Cessna nose strut in preparation for strut servicing.
Fluid and air: both are vital

On any model, the strut has to have the correct amount of fluid and air to work properly. The fluid used for strut servicing is MIL-H-5606 (red) mineral-based hydraulic fluid.

5606 is sold by the gallon and in quarts. It is nice to keep a supply on hand not only for struts, but also to refill brake and gear reservoirs. Typically it takes around a gallon of hydraulic fluid to service three struts.

Nitrogen is better than compressed air for strut servicing because it is drier and doesn’t vary in pressure as much as air; it is also less corrosive to the inside of the strut housing.

However, nitrogen is not always readily available. A person needs a regulator and high-pressure hose in addition to a nitrogen bottle, and the cost for all the items can exceed $500.

If nitrogen is not available, air pressure from a standard air compressor is usually sufficient to air up a nose strut. Nose struts don’t require as much pressure as main struts.

All the single engine Cessna series only have an inflatable strut on the nose; the mains are solid or tubular steel.

If the pressure required is beyond the capability of a standard air compressor, a booster can increase compressor air to a high enough level to inflate the struts. These are available for around $200.

Though Piper aircraft can require 200 psi, single engine Cessna nose struts don’t require that much pressure. As long as the tail can be lowered a little, a regular air compressor that produces at least 130 psi works fine for Cessna singles.

Twin Cessna main and nose struts, however, do require a lot of pressure. It would be best on a twin Cessna (or any twin, period) to always use nitrogen, due to the increased weight of a twin engine plane and the more extreme temperature changes.

A discarded can or container and a section of clear hose will be needed in order to catch old hydraulic fluid as it is pushed out during strut servicing.
Servicing a strut

The tools a person needs to service a strut include about three feet of clear flexible tubing with a ¼ inch (inside diameter) opening to fit over the Schrader valve; a valve stem tool; and an empty gallon size container to catch the old fluid.

The airplane needs to be jacked, or at least have the nose raised if only the nose is being serviced.

With the airplane jacked, remove the valve stem slowly from the filler valve in the top of the strut. It is best to loosen it enough to release the air pressure, and then remove it the rest of the way after the pressure has bled off. A small spray of hydraulic fluid comes out with the air pressure, so it’s a good idea to have a rag handy.

Once the valve stem has been removed, push the tubing over the open Schrader valve and insert the other end into the empty container. Next, push the strut up to its fully collapsed position. Any old fluid will be shoved out.

Then remove the container with the old fluid and insert the hose into a can with at least a half-gallon of clean new hydraulic fluid. Next, pull the strut down to its maximum extended position. The suction will pull in the fluid; it will continue to siphon for a few seconds after the strut is fully extended.

Next, slowly push the strut up to its fully collapsed position. As some of the fluid is pushed back out, air bubbles will come out too.

Extend the strut again, and repeat the process until all of the fluid comes out as a solid stream on the compression stroke.

Once all of the air bubbles are removed, the strut will be considerably more difficult to push up to its collapsed position. Once the strut is fully collapsed, the hose should then be removed from the valve and the valve core reinstalled.

This process is called bleeding the strut, and it’s the only way to get the proper amount of hydraulic fluid into the inner chambers of the cylinder.

There is no way to simply pump a little fluid in to the strut; the strut must be filled using this bleeding process. If the process isn’t followed, large air pockets in the lower chamber can cause the strut to collapse under a load.

Once the strut is filled with fluid, it can then be aired up with either nitrogen or compressed air through a strut booster.

After the airplane is lowered off the jacks, the final adjustments can be made by releasing a little of the pressure by depressing the valve core for a split second at a time.

Generally main struts should be inflated so that around five inches of the piston is exposed, and nose struts to around four inches.

The exact range for each model can be found in the service manual. The struts should be inflated so that they are within the proper range even when the airplane is fully loaded.

The last stroke of the bleeding process used to fill a Cessna nose strut with hydraulic fluid shows the fluid in the tubing is coming out in a solid stream with no air bubbles.

Troubleshooting struts

Properly serviced struts should have a certain amount of buoyancy about them.

Struts that are filled with air pressure but are low on hydraulic fluid tend to stick in place. Struts that stay extended for a period of time after a plane has landed and then suddenly collapse are also typically low on fluid.

Any sort of a knocking noise from the nose strut during taxi operations or upon landing is an indication that it is bottoming out due to it being low on fluid, air, or both.

If a strut is low on fluid, it is usually because the rubber seals have gotten old and hardened. There is generally a rubber wiper ring and a large rubber O-ring with one or more backup rings in the strut housing. These rings harden and become brittle over time, especially in cold weather.

Granville’s Aircraft Hydraulic and Strut Sealant is an FAA approved product that can be mixed with hydraulic fluid and added to the struts during the servicing process. It doesn’t cause the seals to swell, but it does cause them to soften and become more flexible, much as they were in their original state.

This additive works well, once enough of it gets into contact with the seals. After it is first added, the strut may still go flat a time or two and need to be re-aired before it finally holds.

Properly serviced struts help to soften landings and prevent damage to the airframe, and keeping the struts in good shape will pay off big in the long run.

A quantity of MIL-H-5606A hyraulic fluid is needed to service struts. Granville’s sealant added to the hydraulic fluid can help revitalize old seals and is FAA approved. 

Jacqueline Shipe grew up in an aviation home; her dad was a flight instructor. She soloed at age 16 and went on to get her CFII and ATP certificate. Shipe also attended Kentucky Tech and obtained an airframe and powerplant license. She has worked as a mechanic for the airlines and on a variety of General Aviation planes. She’s also logged over 5,000 hours of flight instruction time. Send question or comments to .


Appendix A to Part 43,
“Major Alterations, Major Repairs and Preventive Maintenance”
Shake, Rattle, & Roll: Fixing a Cessna Nosewheel Shimmy

Shake, Rattle, & Roll: Fixing a Cessna Nosewheel Shimmy

Various things can cause nosegear shimmy. Here’s what to do.

There’s nothing worse than completing a near-perfect landing and rollout only to have a sudden shimmy in the nosegear cause the whole front end of the airplane to vibrate. The shaking can be alarming to pilots who have never experienced it before, and can be worrisome for passengers. The vibration is also very hard on the airplane itself. 

A nosewheel shimmy is a rapid back-and-forth oscillation of the steerable part of the nosegear and wheel. It can be caused by a variety of problems, and it sometimes takes more than one trip to the shop to get the issue resolved. 

The nosegear has several points at which it pivots and rotates. These pivot points naturally wear over time, and excessive play in any one of them can cause the nose to shimmy. 

The photo is a close look at torque link bearings and bushings. Regular servicing these with clean grease helps prevent wear.
The torque links (shown disassembled here) are made of aluminum castings.
The torque links attach to a steerable nose collar on the top
Cessna nosewheel design

The nosewheel is turned left and right by means of a steering collar that is connected to the lower piston part of the strut through the torque links (i.e., scissors). The collar is connected to the rudder pedals via spring-loaded steering rods. 

As the pedals are pushed fore and aft, the collar is swiveled left and right. The bottom of the collar has a lowered keyed section in the front that travels in a slot in the lower section of the strut housing, providing a stop to limit how far the nose can be turned left and right. 

A shimmy damper is mounted with its housing attached to the steering collar and its inner rod attached to the upper immovable section of the strut housing. The rod has a fixed piston with an orifice in the center of it. 

The shimmy damper is filled with hydraulic fluid on both sides of the piston. As the collar and the attached damper turn left and right, fluid is forced through the orifice to the opposite side of the piston. This provides a slight resistance to turning, and helps eliminate most of the minor oscillations.

The torque links section of the nosegear is made of aluminum castings that attach to the steerable nose collar on the top, and to a fitting on the lower section of the strut. They have pressed-in bushings and inserted bearings that are subject to wear since they have to flex every time the strut extends or compresses.

The rod ends (on left and right) of the steering collar are where the steering rods are attached.
The steering collar roller bearing rides on the strut housing as it is steered back and forth.
The lower section of the strut housing. The tabs on the left and right provide the stops for the nose steering travel; the left tab also is the attachment point for the shimmy damper rod. The section above the tabs is where the nose steering collar pivots; above that is the snap ring recess.
Potential causes of a nosewheel shimmy

On a plane that develops a nose shimmy, one of the first things to check is the balance and condition of the nose tire itself. An imbalance in the nose tire and wheel assembly can cause a vibration in the nosegear. Uneven tire wear can also cause nose vibrations. 

An overly inflated nose tire can also contribute to a nosewheel shimmy. If a shimmy develops soon after a tire was inflated, check the tire pressure and try letting a little air out of the tire. 

Overly inflated tires have less tire surface in contact with the pavement. This greatly reduces the friction and the resistance to turning that would naturally be present if the tire pressure was less, making it easier to swing back and forth if the slightest imbalance or wear in the gear is present.

A closeup of the steering collar’s grease fitting. The tab on the collar contacts the stops to limit the steering turn radius.
The shimmy damper itself requires servicing from time to time. A damper that is low on fluid or is excessively worn internally will most assuredly result in a nose shimmy.

2. An issue in the torque links or steering rods

As the bushings and bearings of the torque links section wear, an excessive side clearance between the link and corresponding attachment on the nose strut casting can develop, allowing a small amount of free play between the upper and lower sections of the strut. This can cause the nose to shimmy. 

There are washers or shims of varying thickness that can be installed between the torque links and their corresponding fittings to remove any side-to-side free play. 

In addition, misadjusted or excessively worn steering rods can cause a shimmy (as well as pull the nose to the left or right, making taxi operations more difficult).

3. A need for servicing of the nosegear

The shimmy damper itself requires servicing from time to time. A damper that is low on fluid, or is excessively worn internally will most assuredly result in a nose shimmy.

Regularly greasing the collar and scissors with a high-quality grease will help to prevent wear and corrosion. Fresh grease also helps remove a small amount of free play from the steering collar by filling the gaps between the rollers.  

There is a grease fitting on most collars and scissor joints to accommodate a grease gun. Any excess grease should be wiped off with a towel to keep it from collecting dirt. Also, the grease fittings should be wiped clean before the grease gun is attached to avoid pumping dirt into the bearings. 

Whether you use AeroShell 7, 22 or 33, Mobil 28 or another grease, be sure you continue to use the same kind every time you service the nose gear, because they aren’t compatible with each other. Whatever you start with you want to stick with.  

The roll pin secures the strut assembly to the engine mount on most models. It can be tough to remove if it’s been a long time since it was last disassembled.
The strut bearing surface is greased, and the shims are installed in preparation for installation of the steering collar.
Assorted thicknesses of nosegear steering collar shims.

4. A need for additional shim

An airplane that still has a nose shimmy after all the preceding items have been eliminated as culprits may require an additional shim between the steerable nose collar and the strut housing. 

Thin shims installed between the bottom of the strut housing and the collar can remove excessive vertical free play between the collar and strut housing. These shims come in varying degrees of thickness to fill varying sizes of the gap between the collar and strut housing. 

Adding a shim is normally one of the last things that folks resort to when trying to fix a shimmy because the entire strut has to be removed from the airplane, but a newly-shimmed collar can often provide the fix for a shimmy problem. 

Strut removal is not a particularly complex task, it just requires a little more time than simply balancing a nosewheel, or servicing a shimmy damper.

Once the steering collar is in place, the snap ring can be reinstalled.
After the steering collar snap ring is installed, it should be gently tapped to be sure it is properly seated. If the snap ring is difficult to seat in its recess, one or more of the shims may have to be removed.
Adding shims, step by step

The first step in the process is to take the weight off the nosewheel so that the strut can be removed.

Next, the nosewheel and pant are removed, and the strut is completely deflated by removing the valve core in the top. 

The third step is to remove the large roll pin at the top of the strut assembly. 

The roll pin secures the strut assembly to the engine mount on most models, and it can be tough to remove if it’s been a long time since it was last disassembled. 

A rivet gun with a flat set along with a block of wood works well for jarring loose stuck pins or seized bolts, and it usually works well on the roll pin. 

The wood is placed against the roll pin and the rivet set is seated on the opposite side of the wood. As the trigger is pulled the jarring from the rivet gun generally breaks the pin free, allowing it to be tapped out. The air pressure on the rivet gun should be adjusted to a lower setting and gradually increased as needed. 

Once the roll pin is removed, the steering rods are then disconnected from the collar. 

There is one last bolt securing the strut at the bottom of the engine mount. Once this is removed, the entire strut assembly (housing and all) can be removed from the plane.

After the strut assembly is removed, the collar must be disassembled from the housing. The collar is secured to the strut housing by a large snap ring. The snap ring is an outer type that is spread apart to release it. Once the snap ring is removed, the collar and any existing shims can be lifted off the strut housing. 

Next, any needed shims are placed between the collar and the bottom of the strut housing. The greater the total thickness of the shims, the greater the tension and the resistance to turning is on the collar. 

When installing a shim, there is no need to have to remove the lower inner section of the strut from its housing unless the seals are getting a little old or were leaking. However, it is a very convenient time to reseal the strut if there is any doubt about the seals leaking later on. 

On the inside of the collar is a continuous roller bearing that rides on the strut housing as it is steered back and forth. It is a good idea to thoroughly clean the bearing with Varsol or mineral spirits using a soft-bristle parts brush. Be sure the bearing is completely dry, and then give it a good coating of fresh grease before reinstallation.

The shims must be installed first, then the collar, and lastly, the snap ring.  

The collar should move freely but have virtually no up-and-down movement. The snap ring should be gently tapped all around using a punch to be sure it is properly seated.

If the snap ring is difficult to seat in its recess, one or more of the shims may have to be removed. Also, if there is too much resistance to turning once the snap ring is in place, a shim may have to be removed. 

After the snap ring and collar are properly attached to the strut assembly, reinstall the strut on the airframe. It is a good idea to service the strut assembly with fresh hydraulic fluid before reinstalling the valve core and inflating the strut. 

A Cessna nosegear has to pivot and rotate, but it shouldn’t shimmy. The cause of the shimmy can sometimes be hard to find, but it’s not impossible to fix. A shimmy-free, well-greased nosegear helps make taxi and landing operations go much more smoothly.

Know your FAR/AIM and check with your mechanic before starting any work. Always get instruction from an A&P prior to attempting any maintenance tasks.

At final reinstallation of the nose strut, it is a good idea to service the strut assembly with fresh hydraulic fluid (5606) before reinstalling the valve core and inflating the strut.

Jacqueline Shipe grew up in an aviation home; her dad was a flight instructor. She soloed at age 16 and went on to get her CFII and ATP certificate. Shipe also attended Kentucky Tech and obtained an airframe and powerplant license. She has worked as a mechanic for the airlines and on a variety of General Aviation planes. She’s also logged over 5,000 hours of flight instruction time. Send question or comments to .

Keep Up with Upkeep: Aircraft Battery Maintenance

Keep Up with Upkeep: Aircraft Battery Maintenance

Jacqueline Shipe, A&P, explains the technology and preventive maintenance for aviation batteries in her sixth DIY article targeted to owner-pilots.


The bulk of the items listed in FAR 43 Appendix A, paragraph (c) that an owner may legally perform on his or her owned aircraft are primarily maintenance tasks that have to be performed on a fairly regular basis. 

This is definitely true concerning aircraft battery maintenance, and “servicing or replacement of aircraft batteries” is included on the list of 31 preventive maintenance items. 

All batteries begin to degrade in performance from the moment they are placed in service. The constant chemical reactions that take place cause an ever-increasing lack of efficiency within the battery. This is especially true of batteries that are allowed to run down and remain in a low or depleted state.

Lead-acid batteries are the type used in almost all General Aviation planes and are becoming more common for turbines employed in low-cyclic applications like medevac. (Turbine powered planes in high-cyclic applications (i.e., airliners) often have nickel cadmium or “NiCad” batteries installed. These batteries are costly, and the servicing requirements are much more complex than for the lead-acid batteries. NiCad batteries should only be serviced by a professional.) 

Anatomy of a battery

A lead-acid “flooded” battery consists of multiple cells enclosed in a plastic case. Each cell consists of alternating sets of lead plates. 

Half the plates contain lead oxide, and the other half of the plates contain soft spongy lead. The plates are set in an alternating arrangement; each lead oxide plate is next to, but not touching, each spongy lead plate. 

The plates are immersed in an electrolyte solution of sulfuric acid and water. Removable caps allow an owner to inspect and adjust the electrolyte level of the battery. 

Each battery cell produces roughly two volts of electric power. A 12-volt battery has six cells (and six caps) and a 24-volt battery has 12 cells (and 12 caps). 

A flooded-style 12 volt aircraft battery with removable vent caps.
The chemical reaction

Sulfuric acid produces a chemical reaction between the opposing plates, causing the lead oxide plates to become positively charged and the spongy lead plates to become negatively charged. 

As a battery discharges, the sulfuric acid in the electrolyte solution is converted into lead sulfate on both the positive and negative plates. Lead sulfate is not conductive. As it grows on the plates, covering more and more of the surface area, it reduces the efficiency and output of the battery. 

The discharge process also makes the electrolyte far more watery as the sulfuric acid is depleted. Batteries not only discharge under an electrical load, but they also self-discharge when not being maintained in a fully charged state.

If a battery is left for a prolonged length of time in an uncharged state, it will eventually completely discharge once the plates become so coated in lead sulfate that no more exchanges of electrons or ions can take place. 

A voltmeter allows you to see what the charging voltage output of a battery charger actually is. Set the meter reading on DC voltage and place one lead on the positive post and one lead on the negative post as the battery is being charged to check the charging voltage. The battery’s voltage is checked the same way, but without the charger connected.
The charging process

During the charging process, the chemical process is reversed: the lead sulfate on the plates is converted back into sulfuric acid; lead oxide is redeposited back on the positive plates; and pure lead is deposited back on the negative plates. 

A battery which remains in a depleted state of charge for a prolonged period of time forms lead sulfate that eventually hardens and crystallizes on the plates to the point that it can’t be converted back into its original components of lead oxide, pure lead and sulfuric acid—no matter how long the battery is left on a charger. 

Maintenance-free or “sealed” batteries have non-removable covers and the electrolyte level cannot be adjusted.
Maintenance-free batteries

Maintenance-free or “sealed” batteries have non-removable covers and the electrolyte level cannot be adjusted. These sealed batteries go by a variety of names: RG, or recombinant gas; AGM, or absorbed glass mat; and VRLA, or valve regulated lead acid. 

These batteries use a fireproof glass mat separator between the positive and negative plates. The glass mat is saturated with electrolyte and the mat’s microporosity allows the hydrogen and oxygen to recombine. 

VRLA batteries are designed to recombine the gases generated during the charge-discharge process and to maintain electrolyte throughout the lifespan of the battery, which makes them maintenance-free for the aircraft owner. 

A battery box needs regular cleaning and neutralizing of any acid residue to help prevent corrosion.
Extending battery life

The best thing any owner can do to extend the life of his or her battery is to keep it fully charged. The alternator or generator on a plane that is regularly flown helps to keep the battery in a good state of charge. 

A plane that sits for extended periods, however, needs an external charging source to keep the battery maintained in good shape and prevent permanent sulfating of the plates. The Achilles’ heel on any battery is to allow it to completely discharge, especially if the discharge occurs slowly over a long period of time.

The battery box should be flushed occasionally with clean water to be sure it is not leaking and that the drain (shown here) is clear of clogs.
Handling a vented aviation battery

Battery acid is harmful to the skin and eyes, so rubber gloves and safety glasses should be worn any time you are charging or servicing the battery in your aircraft. 

To prevent electric shock, ensure that any metal tool that is in contact with the positive battery terminal is not allowed to touch any metal structure on the battery box or airframe.

Anytime the battery is charged or serviced, the best thing to do is to completely remove it from its compartment. 

This can be difficult to do depending on the location of the battery, and all batteries are heavy and can be tough to lift out of the box. The 24-volt batteries are particularly cumbersome. 

The straps that are occasionally installed on the tops of the batteries are only there to aid in the removal from and installation into the battery box. 

Once it is out of the aircraft, the battery should be supported from underneath; very often the plastic or rope-like straps weaken over time and can easily break. 

Electrolyte should only be added to a flooded-style battery after the battery is fully charged, and then only up to the fill indicator in the cell. Inspect the rubber O-ring seal on the vent cap to be sure it is not damaged or missing. 
Taking care of the battery box

The complete removal of a vented battery from the airplane not only makes it easier to service, but also allows the battery box to be cleaned and inspected. 

A solution of baking soda and water will neutralize any acid residue in the box. 

The drain line should be inspected to be sure it is still attached properly and is clear of any clogs. 

Any corrosion should be thoroughly cleaned off, and the box should be painted with either a zinc chromate primer topped by a good quality epoxy paint or with a bituminous or acid proof paint that is specially made for battery boxes. (Battery box modifications for Cessna aircraft are available by STC from
Bogert Aviation. —Ed.)

This warning label on top of a sealed battery advises that the installer not over-torque the terminal bolts. They only require 70 inch pounds, and overtightening the bolts can cause electrolyte leakage around the battery posts.
Adjusting electrolyte levels

In addition to charging the battery, the electrolyte level should be inspected on flooded batteries. The electrolyte will be low if the battery is in a discharged state and will increase as the battery is being charged; therefore, the final adjustments of the electrolyte level should take place once the charging process is complete. 

Most service manuals recommend adding only distilled water to cells that are low on electrolyte after the battery is fully charged. 

During initial servicing of a new battery, however, only aviation electrolyte should be used and the cells should not be diluted with water. The specific gravity of the electrolyte on a charged battery is 1.285 while electrolyte for an automotive battery has a specific gravity of 1.265. 

When adding fluid, a syringe or a bulb-type battery filler works well so that fluid can be removed if too much is added. 

Any spills can be cleaned and neutralized with a little baking soda and water, but only do so after the battery caps are reinstalled and tightened. Care should be taken to make sure none of the baking soda enters the battery.

Upon reinstallation, be sure not to overtighten the battery terminals. The terminals on a sealed battery require a relatively low torque, and overtightening can cause them to leak.

A battery contactor on the battery box should not be used as a place to hook the leads from an external charger. There isn’t much room to connect the leads, and it can easily lead to a direct short to ground. It is best to remove the battery to charge it. 
External charging of a battery

When using an external charger to charge a battery, it is best to use an
aviation-specific charger. Always
charge the battery to the manufacturer’s specifications. 

Aircraft batteries have thinner plates than automotive batteries and are more susceptible to damage from overcharge. They also require lower charging voltages than automotive batteries. This is also true of float chargers that are typically left plugged in any time a battery is not in use.

Teledyne Battery Products, the com-pany that makes Gill batteries, lists four chargers for its various battery products
on its website; these are available through Gill distributors.

The charger recommended by Concorde for use on its batteries is the Battery MINDer brand. This company has aviation-specific float chargers for aircraft batteries that are temperature compensated voltage regulated. These chargers provide a higher charge rate in colder temperatures and a greatly reduced rate of charge as temperature increases, preventing an overcharge. 

Once the battery reaches a fully charged state, the charger shuts itself off. Battery MINDer also has some solar powered versions for planes that are parked out on the ramp.

Float chargers are nice and lots of folks permanently install them on the battery. If you do, be sure to use FAA approved components like those available from Audio Authority Corp that are designed for aviation use. 


Not designed to last forever

Even with the best care, batteries by design have a fairly short lifespan of usefulness. Periodic replacement is a given—around five years if unmaintained and up to 10 years if properly maintained. When choosing a new battery, pick a high quality product. 

Some folks like flooded-style batteries best, some prefer VRLA. Flooded batteries are typically messier than sealed batteries and cause corrosion, but they are slightly more forgiving of being overcharged since electrolyte levels can be adjusted. Flooded batteries are also less expensive.

Sealed batteries are less corrosive, and they self-discharge at a slower rate than flooded batteries. Sealed batteries typically cost more than flooded batteries.

With either style, the best thing an owner can do to extend the life of his or her battery is to keep it fully charged. With the improved chargers on the market today, that is becoming easier to do. 

Know your FAR/AIM and check with your mechanic before starting any work. Always get instruction from an A&P prior to attempting preventive maintenance tasks.

Jacqueline Shipe grew up in an aviation home; her dad was a flight instructor. She soloed at age 16 and went on to get her CFII and ATP certificate. Shipe also attended Kentucky Tech and obtained an airframe and powerplant license. She has worked as a mechanic for the airlines and on a variety of General Aviation planes. She’s also logged over 5,000 hours of flight instruction time. Send question or comments to .


Aviation batteries
– CFA supporters

Concorde Battery Corp. 
Teledyne Battery Products
(Gill batteries)


Replacement battery boxes

Bogert Aviation
– CFA supporter


Temperature compensated voltage regulated chargers
Battery MINDer


Airframe interface kits and accessories
Audio Authority Corp.
Getting Current: Troubleshooting a Landing Light Circuit

Getting Current: Troubleshooting a Landing Light Circuit

Finding and repairing a broken circuit is the subject of this fourth installment of A&P Jacqueline Shipe’s DIY series.  

Among the many preventive maintenance items listed in FAR 43 Appendix A that a pilot may legally perform on his or her plane is “troubleshooting and repairing a broken landing light circuit.” This specific entry is the only reference to electrical circuit troubleshooting on the list. 

Most electrical circuits for lights or pitot heat, etc. are fairly straightforward, while a wiring harness for a unit like a panel-mounted GPS can be very complex. This article will focus on the tools and expertise required to successfully troubleshoot a landing light.

Fig. 9-35: In this landing lights diagram, power comes from the bus bar to a 20-amp circuit breaker, which is a shared breaker for two separate landing light circuits. The number 14 indicates that the wire size for that section is 14 gauge. L1A and L2A indicate the wire numbers.
Study the diagram

On any electrical circuit, the best troubleshooting tool is always the current wiring diagram pertinent to the model and serial number of the airplane. Learning how to read a wiring or system schematic can help a pilot not only in performing repairs, but also in understanding how a unit or system actually works. 

Everything electrical has to have a power source and a ground to operate. Some circuits contain numerous switches and circuitry that work in conjunction with each other to provide the needed power or ground. 

When a fault occurs, knowing how to dissect that circuit into sections—and understanding when and where voltage or a ground is supposed to be present—is essential. The wiring diagram provides all the needed information. 

There are standard symbols used on these diagrams to indicate different components in a circuit. There is always a symbology chart somewhere in the maintenance manual wiring section that lists the symbols and the components they represent. 

Some of these symbols are drawn to look somewhat like the component they represent, such as a circuit breaker. Switches, contactors and relays are generally shown on diagrams in the open (or “relaxed”) condition unless otherwise noted. 

To get familiar with a specific circuit, follow the flow of a circuit on a diagram and consult the chart when you see an unknown symbol. It doesn’t take long before the symbols all become familiar.

An electrical symbology chart similar to this can be found in the electrical section of any maintenance manual. 
The parts of a circuit

Exterior light circuits are some of the most straightforward circuits on any plane, and the diagrams provide good practice for folks first learning to read a wiring schematic. Generally, it is best to start at the power source in the diagram and read down from there. 

In figure 9-35 on page 28, power comes from the bus bar to a 20-amp circuit breaker, which is a shared breaker for two separate landing light circuits. The number 14 indicates that the wire size for that section is 14 gauge. 

L1A and L2A indicate the wire numbers; “L” representing a lighting circuit. (Each original wire on a plane is stamped multiple times along its entire length with the appropriate wire number, which helps tremendously.) 

Both switches are shown in the open or “off” position. Coming out of each switch is the “B” section of each wire, still 14 gauge, up to a knife connector. Past the knife connector is the “C” section of each wire up to the terminal on the bulb itself. 

The “D” section starts at the opposite terminal on the bulb and goes to ground. This section of wire is a little heavier duty as indicated by the fact that it is 12-gauge wire. 

The wire gauge refers to how big its cross-section is; the smaller the number, the fatter and heavier duty the wire is. Starter and battery wires, for example, are large and heavy duty, generally six gauge or lower. Larger diameter wires can carry much more current than smaller diameter wires.

The airframe itself is generally used to provide a ground on most planes. The exact point where a wire for a circuit is connected to the airframe for a ground is usually not too far from the electrical component itself. 

The airframe should be clean, and the wire terminal should be free of corrosion to ensure there is a resistance-free path for an electrical flow to ground. 

This navigation light bulb has a “post” style base. The brass colored bulk of the base contacts the socket base on the airplane to provide an electrical ground; the black raised portion is an insulating material; and the small raised silver end is the positive contact point.
A previously repaired section of a landing light wire tends to get brittle on lights that are mounted in the cowling. Previously repaired sections should be suspected if the wire does not have good continuity through it.
Check the bulb first

First, be sure that any wires being checked are not touching any other wires or the airframe. Don’t allow any metal tools to touch both a live wire and the airframe at the same time. 

In the landing light circuit, the quickest and easiest place to go to troubleshoot an inoperative bulb is to check the bulb itself. 

After removing individual bulbs from the plane, measure the resistance across the terminals with a multimeter set to ohms. This is a very simple task for landing or taxi light bulbs. 

Navigation lights and several of the interior light bulbs utilize a base that inserts into a socket. The small raised area at the bottom of the base is the positive contact point, and the base of the bulb is the ground contact point on these bulbs. A good bulb will show continuity; the resistance varies a little depending on the type of filament it has. 

Strobe bulbs are different; they can only be tested by an operational check. To check a strobe bulb, put the suspect bulb in a known good circuit, turn it on and see if the bulb works. If one wing strobe bulb works but the other doesn’t, switch the bulbs and you’ll immediately know if it’s the bulb or something else in the circuit that’s causing the trouble.

A multimeter should be set to the lowest ohm reading when checking bulbs, and the meter leads should be placed as illustrated here to check the resistance through the bulb filament.
Meter leads should be placed like this to check for amperage. The wire that feeds from the bus bar that has power on it (i.e, the hot wire) is disconnected and the meter is placed in series with the bulb. The circuit has to be energized and complete for amperage to flow.
Check these things next

The two main checks that are required when troubleshooting a circuit are for voltage and a ground. Voltage is a measurement of the potential amount of electric power coming in to a certain point. It is not an indication of how much power is actually flowing. 

Amperage measurements give an indication of the actual amount of power (current) that is flowing. Most meters only measure voltage, but some do have amperage settings. 

Voltage is easier to measure because it can be checked anywhere in the circuit by placing the meter in parallel with the circuit. Amperage is harder to read because the meter has to be placed in series within the circuit, and the circuit has to be complete so that current is actually flowing. 

To check the bulb filament, the wires should be disconnected from the bulb to ensure the resistance reading is across the filament and not back through the circuit.

Voltage and resistance can be easily measured with an electrical multimeter. There are many different manufacturers of multimeters; even the most inexpensive ones are generally good enough to troubleshoot most electrical issues. 

Before taking any measurements, it is a good idea to set the meter to the lowest ohms setting and touch the test leads together. This tests the connection of the leads to the meter and also the continuity of the test lead wires as well as the internal resistance of the meter. The reading should be zero ideally, but in any case it shouldn’t be much over one ohm. 

When the aircraft battery is on and the landing light switches are closed (i.e., turned on), there should be 12 volts at the terminal of the L1C and L2C wires. There should also be a very low resistance path to ground, which is measured on the L1D and L2D terminals. 

In the landing light circuit, voltage can easily be measured by placing the positive probe of the multimeter on the light terminal for the L1C or L2C wire (depending on which bulb is being checked) and the negative probe on a clean, bare metal area of the airframe for a ground. (Some owner-pilots use a small alligator clip to connect the black lead off the multimeter to a spot on the airframe. —Ed.)

With the appropriate switch flipped on, the voltage reading should be approximately the same as battery voltage if the circuit is working properly. 

In this photo, the wire is being reattached to the light. The receptacles are made of thin metal and can easily be broken if the screws are overtightened.

Most of the time, a voltage measurement is all that is needed to be assured that power is being received, but it is good to know how to check amperage. 

An electric motor that is operating a little slower than normal or is on the verge of shorting out typically begins to draw an excessive current load. A high amperage reading will also be present if there is too much resistance to motion in the mechanical apparatus the motor is trying to move. 

Flaps that are binding, or a landing gear retraction or extension mechanism that is not properly adjusted will cause a motor to draw a high current load and get hot. Checking for a higher than normal amperage reading can allow you to detect a malfunction and fix it before it causes a total failure.

Amperage measurements are also useful to confirm that a component is receiving the full amount of electrical energy it needs to operate. Most of the time, voltage readings are sufficient for this, but there are some circumstances where a voltage indication can be misleading. 

A wire that is barely connected or a switch that has badly burned contacts can still make enough of a connection to show full battery voltage at points in the circuit beyond, but will not be able to actually carry enough amperage to operate different components downstream. This can cause a lot of confusion, but is a fairly rare circumstance.

If all other checks pass with proper voltage and a good ground being indicated, and a known unit that is operable still won’t function, it would be prudent to see how much amperage the unit is getting; there could very well be a poor connection in the circuit somewhere upstream, even though the voltage readings are correct. 

Check the electrical multimeter leads for continuity by setting the indicator to the lowest ohms reading and touching the test leads together. The resistance through the meter and leads shouldn’t be much over 1 or 2 ohms.
Ground connection

Ground connections are measured in ohms of resistance. Generally a reading of two ohms or less is indicative of a good connection to ground; readings that are five ohms or higher are cause for some concern. 

The airframe itself is used to ground most electrical circuits. The airframe often develops corrosion, which can cause excessive resistance in ground connections. Usually disconnecting a ground wire and cleaning the terminal and contacting airframe with 220 grit sandpaper or an abrasive pad (i.e., Scotch-Brite) clears it right up. 

With a little practice and persistence, pilots will be able to interpret wiring diagrams, a multimeter will become easier to use, and electrical problems will seem less complex. 

Most electrical issues can generally be traced to a problem that is fairly easy to fix. Knowing how to troubleshoot a circuit and read a schematic will save a pilot/owner both time and money in the long run.

Know your FAR/AIM and check with your mechanic before starting any work.

Jacqueline Shipe grew up in an aviation home; her dad was a flight instructor. She soloed at age 16 and went on to get her CFII and ATP certificate. Shipe also attended Kentucky Tech and obtained an airframe and powerplant license. She has worked as a mechanic for the airlines and on a variety of General Aviation planes. She’s also logged over 5,000 hours of flight instruction time. Send question or comments to .

Voltmeter and multimeter tools
– CFA supporters

Aircraft Spruce & Specialty Co.
Chief Aircraft
Dissecting a Dry Air Pump

Dissecting a Dry Air Pump

A look inside your aircraft’s vacuum system. 


An elliptically-shaped aluminum housing with the intake and exhaust ports in it. The rotating carbon vane assembly is housed inside.
The air inlet for the vacuum pump. The outer vacuum pump housing has an air inlet port on the front of the pump, and an exhaust port on the rear.
A rotor with carbon vanes extended, as they would be in operation. The vanes are free to slide inward and outward as the rotor spins.
A carbon vane slid out to its maximum travel in its slot. The vanes are allowed to expand outward to their maximum extension at the widest points of the ellipse they travel in.

When the earliest airplane gyroscopic instruments were introduced, the only available source for air pressure to spin them was an outside-mounted air venturi. The venturi accelerated the ram air pressure produced by forward flight through a narrowed opening. The instrument hoses were connected to the venturi at the point of lowest pressure, creating a vacuum that pulled a steady stream of air through the instruments. 

Although some VFR-only planes still use this arrangement, the trouble with this setup is that the amount of vacuum is low until certain airspeeds are reached, and the venturi can become ineffective due to ice buildup during inflight icing conditions. 

In the late 1930s, air pumps were developed that were engine-driven, creating air suction (or pressure) as soon as the engine was started. These early air pumps were lubricated with engine oil and would later be called “wet style” pumps. 

In the 1960s wet pumps were largely replaced with the “dry” pumps. Dry vacuum pumps are self-lubricating and have an oil-free exhaust flow that reduces belly deposits and provides a much cleaner source of air pressure on aircraft models that use the vacuum pump exhaust for inflating de-ice boots. The dry air pumps are also a little less expensive and weigh about half as much as the older wet style pumps.

The air intake through the vacuum pump housing. (Note: the three extended shafts are part of the rotor assembly.)
           The air slots for the intake air into the vacuum pump housing. The ports are open at the bottom and sides of the pump housing to allow air to flow in as the carbon vanes are beginning to                      expand.
How the pump works

The standard dry air vacuum pump consists of a rotating carbon vane assembly housed in an elliptically-shaped aluminum housing. The carbon vane assembly is powered by the engine accessory drive. The outer pump housing has an air inlet port on the front of the pump and an exhaust port on the rear. 

The rotor portion of the carbon vane assembly has slots that house the carbon vanes themselves. The vanes are free to slide inward and outward as the rotor spins. Centrifugal force keeps the vanes in contact with the inner wall of the pump housing. 

As the rotor spins, the vanes in the rotor slide in at the narrow section of the housing and slide outward to their maximum extension at the widest points of the elliptical housing they travel in. 

The intake air from the instrument system is routed through the pump fitting to ports in the forward section of the pump housing. The ports are open at the bottom and sides of the pump housing to allow air to flow in as the carbon vanes are beginning to move outward in the rotor slots. 

The air is then compressed as its compartment is compacted while the vanes rotate toward the narrow part of the housing. It is then accelerated out of exhaust ports located in the narrowest part of the ellipse. This all occurs through the first 180 degrees of rotation. 

As soon as the vanes move past the exhaust openings, they scoop in intake air from a second set of intake air openings—and the entire process is completed again in the second 180 degrees of rotation. 

The exhaust port openings in the outer vacuum pump housing.
          The exhaust openings in the vacuum pump with the rotor assembly installed.
How the vacuum system works

The airflow through a common single-engine aircraft vacuum system begins under the instrument panel. Air enters the system through a central pleated paper filter. The filter is located under the instrument panel. Ambient air is drawn into and through it solely due to the suction of the attached hoses going to the vacuum pump. It then flows through the attitude and heading indicators before reaching the system regulator. 

The system regulator combines additional air as needed to the intake of the pump so that the system suction stays within the parameters the regulator is adjusted to maintain. (The regulator has a slipover “sock” style filter to protect the pump from any particles that might be drawn in with the ambient air.) Airflow continues through the pump and then is exhausted into the engine compartment on most models. Aircraft with de-ice boots utilize the pump’s exhaust air to inflate the boots. 

The artificial horizon and directional gyro flight instruments are usually connected to the vacuum system in parallel with each having its own connections to the intake and vacuum air so that even if one instrument were to fail or become clogged, the other one still functions because it has its own connection to the air source. 

A suction gauge is connected in the system so that it measures the air pressure difference between the supply line from the central paper intake filter and the outlet of one of the instruments prior to reaching the system regulator. The pressure drop from the intake air (which is close to atmospheric pressure on nonpressurized planes) and the air being drawn into the regulator is measured in inches of suction. 

Most vacuum systems are designed to operate with around five inches of suction with the engine rpm at or near a cruise setting. If the system suction is too high, it can cause excessive wear in the gyros and the vacuum pump. If the vacuum system suction is too low, the instruments will not give reliable indications. 

Twin-engine aircraft with two vacuum pumps also utilize various check valves so that a failure of either pump doesn’t cause the system to lose vacuum. 

The exhaust opening on the rear of the vacuum pump.
A vacuum system regulator. The system regulator is usually mounted under the panel on the firewall, and is sometimes difficult to access, which causes some mechanics to neglect it.
The elliptically-shaped aluminum housing on a removed vacuum pump. The indentions and “chatter marks” around the inside surface likely led to the pump failing.
          A failed rotor and broken carbon vanes on a removed vacuum pump. Carbon vanes, by design, will wear down over time as the pump operates. Eventually, the vanes can become so short that               they will either hang up, or come completely out of their slots.
Vacuum pump failure

Vacuum pumps are built to run for several hundred hours—but one of the biggest downfalls of dry air vacuum pumps is that when they do fail, it is usually suddenly and without warning. 

The carbon vanes, by design, will wear down over time as the pump operates. Eventually, the vanes can become so short that they will either hang up, or come completely out of their slots as they rotate through the wide part of the ellipse and cause the sudden stoppage of the rotor assembly in the pump. 

Also, the inner wall of the aluminum housing is prone to developing indentions and slight deformities as the vanes slide in and out against it. These indentions can cause one or more of the vanes to hang up and break apart. 

Some vacuum pump manufacturers have incorporated a wear indicator port on the side (or on some models, the rear) of the pump. The ports allow access to check the length of the vanes, which can help catch an impending failure. (The pumps are designed with a nylon drive coupling that shears in two if the pump does lock up, so that the gears in the engine accessory case are not damaged when a pump fails.) 

Contamination within the vacuum pump can also cause a sudden failure. The hoses used in the vacuum system can become dried and brittle over time. If internal pieces of hose begin to flake off, or if any contaminants get into the vacuum system downstream of the central paper filter, they go straight through the pump. The small sock filter on the regulator only filters the ambient air that is added to the flow as the pressure is regulated—not the already-filtered air from the instruments. 

Some mechanics use Teflon tape or some type of sealing compound on the pipe-threaded instrument and pump fittings in the vacuum system. Teflon tape and other sealants are not recommended for use at all in the instrument system, because pieces of the tape or sealant can make their way into the system as the fittings are threaded into place and these may eventually get sucked into the pump. 

The filters themselves can become sources of contamination over time if they aren’t regularly replaced. The sock filter in particular can become so dried-out and brittle that pieces of it may be ingested into the airflow. Most vacuum pump manufacturers require replacement of all filters at the time of installation in order for the pump warranty to be valid. 

Solvents used to wash down the engine during maintenance are very damaging to vacuum pumps. If any of the solvent material gets into the pump, it causes the graphite powder—which is always present from normal wear—to turn into a paste that gums up the inside of the pump. Pump manufacturers recommend completely covering the pump with a resealable plastic bag and tie wraps before washing down the engine.

Oil contamination is also a big culprit in premature vacuum pump failures. One of the biggest sources of oil contamination typically comes from a leaking oil seal on the engine accessory case adapter drive. The adapter drive gear in the accessory case is made with a splined hollow shaft that spins the vacuum pump drive coupling. 

There is an oil seal that the vacuum pump drive gear is inserted through. It naturally wears out over time because the shaft is spinning inside of it. Once the oil seal begins to degrade, it allows oil—under pressure—to head straight for the pump drive coupling and into the pump itself. This excess oil causes a gummy paste to form that eventually binds the pump. 

Kinked fittings or hoses can cause excess wear on a pump by forcing it to work harder than it should to maintain vacuum suction. (If suction levels begin to degrade, lots of mechanics simply increase the suction by adjusting the regulator—instead of determining the exact cause of the suction loss. If a system starts to become sluggish, the root cause should be determined before simply cranking up the regulator.) 

A typical vacuum pump drive shaft.
Some vacuum pump manufacturers have incorporated a wear indicator port to help catch an impending failure.
          A vacuum pump with fittings that are incorrectly installed with Teflon tape. Pieces of tape or sealant can make their way into the system as the fittings are threaded into place and these may                   eventually get sucked into the pump.
A vacuum pump drive as installed on the engine accessory case. One of the biggest sources of oil contamination typically comes from a leaking oil seal on the engine accessory case adapter drive.
The rear side of the vacuum drive adapter removed from the engine. The adapter drive gear in the accessory case is made with a splined hollow shaft that spins the vacuum pump drive coupling.
Vacuum pump replacement

Vacuum pumps are typically straightforward to replace. 

The hoses and fittings on the old pump should be removed before the mounting nuts are removed, so the pump is held tightly in place as the hoses are pulled off the fittings. 

Most vacuum pumps are mounted on four studs and secured with plain nuts and lock washers. Typically one or more of the nuts are difficult to access with a normal type of wrench, and vacuum pump manufacturers make a specially-curved wrench that helps gain a little access. 

The old mechanic’s trick for breaking loose the nuts that are in a tight place involves using a long flat blade screwdriver placed on the loosening side of the nut. The screwdriver is then gently tapped with a small hammer to break the nut loose. 

A new oil seal should be installed anytime a new vacuum pump is installed—whether the old oil seal is leaking or not. The seals wear out over time and require periodic replacement. They are also reasonably priced (around three dollars each), so cost is not a consideration. 

If an owner is having a shop replace the pump, it is best to specifically request that the oil seal be replaced in addition to the pump, because some mechanics don’t replace them unless they are leaking. 

There are two gaskets that require replacement: the one between the vacuum pump and its drive housing, and the one between the drive housing and the engine accessory case. 

Be sure to check the aircraft maintenance manual to be sure the new vacuum pump being installed is the correct model number. Pumps rotate either counter-clockwise or clockwise as viewed from the rear of the pump and case. (The rotation is specified as “CC” or “CW” in the part number.) Putting the wrong pump on will cause it to spin opposite the direction the rotor slots and vanes are designed for, and the pump will fail in short order—if not immediately. 

The vacuum system hoses and system regulator that are just upstream from the vacuum pump must be checked for contamination whenever a failed pump is being replaced. Pieces of the old pump vanes often get sucked backward into the suction hose—against the normal direction of airflow—as the pump fails because the system still retains a lower pressure for a few seconds even though the pump has stopped. 

If the hose isn’t cleaned or replaced after a sudden pump failure, carbon and vane parts will be sucked into the new pump upon startup.

If compressed air is used to blow out the lines, be sure all the instruments are disconnected so they don’t get blasted with excessive pressure or contaminated by unfiltered particles. Also, as the manufacturers specify, all of the vacuum system filters should be replaced at each pump replacement.

Owners that fly a lot of hard IFR might consider periodically replacing a vacuum pump based on time in use alone, even if that pump is operating properly. 

Many new aircraft are shifting toward a glass panel configuration, but the benefit of the vacuum pump system is that it will still power the pneumatic instruments even in the event of a total electrical failure. A little preventive maintenance and upkeep on the vacuum system can help owners be assured that the indications on the gauges can be trusted in the clouds.

Know your FAR/AIM and check with your mechanic before starting any work. Always get instruction from an A&P prior to attempting preventive maintenance tasks.

The front side of the vacuum pump drive.
The vacuum pump drive gear removed from the housing.
A new oil seal should be installed anytime a new vacuum pump is installed. The oil seal naturally wears out over time because the shaft is spinning inside of it.
An assembled vacuum pump installed on the engine. 

Jacqueline Shipe grew up in an aviation home; her dad was a flight instructor. She soloed at age 16 and went on to get her CFII and ATP certificate. Shipe also attended Kentucky Tech and obtained an airframe and powerplant license. She has worked as a mechanic for the airlines and on a variety of General Aviation planes. She’s also logged over 5,000 hours of flight instruction time. Send question or comments to .

Aircraft Carburetors: Still Delivering for Aviation

Aircraft Carburetors: Still Delivering for Aviation

Many Cessna aircraft depend on a carburetor. Cessna Flyer contributing editor and A&P Jacqueline Shipe explains the operation of this fairly simple— and very reliable—invention.

One of the most recognized carburetor manufacturers for the GA fleet is Marvel-Schebler. The company has been around a long time, having its beginnings in the early 1900s when George Schebler and his friend Burt Pierce worked together to design the first carburetor using a tin can with a flap to regulate airflow. 

They both went on to patent their designs, with Pierce calling his carburetor the “Marvel.” Both the Marvel and the Schebler designs were successful and used on a variety of engine types. 

In the early days of General Motors, the two merged and became known as Marvel-Schebler Carburetor Co. (Author’s note: Burt Pierce also designed the still-popular Marvel Mystery Oil through Marvel Oil Co., which he founded in 1923.) In the beginning, the Marvel-Schebler Carburetor Co. made carburetors for cars, boats, tractors and airplanes. 

The company has since changed hands several times, being purchased and resold by Facet Aerospace Products, Zenith Fuel Systems, Precision Airmotive and the Tempest Group (who called it Volare Carburetors until it acquired the Marvel-Schebler trademark in 2010). Today, Marvel-Schebler Aircraft Carburetors LLC produces a complete line of aviation carburetors and parts.

Although Marvel-Schebler is the most recognized brand for aviation carburetors, there are other FAA approved manufacturers, including AVStar Fuel Systems in Florida. 

AVStar was formed in 2007 and has gone on to become the supplier for Lycoming Engines as well as numerous individual customers. AVStar manufactures a line of carburetors as well as kits and parts for use in almost all carburetor models in the General Aviation fleet.

A throttle body (top half of carburetor) is shown here, disassembled from the bowl and turned upside-down. In the lower left corner of the image is the throttle arm; in the bottom center of the image is the mixture control arm. The mixture control valve is the long slender mechanism on the far right of this image (the end has a half-cut in it). The longer section of the mixture control valve covers or uncovers the fuel inlet opening in the sleeve (which is mounted in the bowl) as it is rotated to control the amount of fuel allowed into the fuel discharge nozzle.
An AVStar Fuel Systems’ original equipment brass float. AVStar manufactures a line of carburetors as well as kits and parts for use in almost all carburetor models in the General Aviation fleet.
The float and attached float valve, shown here lifted up out of its seat. The small slotted tab above the float valve is adjustable by bending the tab to make it closer to, or further from, the top of the valve.
How a carburetor functions

Aircraft engines rely on a steady source of fuel to provide the energy needed to support combustion. Liquid fuel must be vaporized and mixed with the proper amount of air in order to burn properly in the cylinders. 

Many General Aviation planes depend on a carburetor to provide a continuous, reliable source of properly mixed fuel and air to each cylinder. The aircraft carburetor has a relatively simple design and is typically very reliable.

Most aircraft carburetors are fairly straightforward in construction. A top part, called a throttle body, houses the throttle valve, mixture control and venturi; a lower bowl section, called a reservoir, holds a consistent volume of fuel. 

Almost all aviation carburetors are float-style carburetors. This means that a float mechanism regulates the fuel level in the reservoir (i.e., bowl). 

A closeup of a float valve. Here the valve has been removed and is lying next to its seat. This pencil tip-shaped valve is attached to the top rear of the float.
Float level clearance is measured between pontoon and throttle body gasket. It should be 7/32 inch on most of the smaller carburetors; a 7/32-inch drill bit (shown on left side of image) can be used as a gauge.
A carburetor fuel bowl; the large brass nozzle in the center with holes in it is the main fuel discharge nozzle. The smaller tube just to the left is the accelerator pump discharge tube. The fuel drain plug is shown with safety wire attached at the bottom of the plug.
The float mechanism

The float is hinged on the rear, allowing it to pivot up and down. A pencil tip-shaped float valve is attached to the top rear of the float. 

Fuel enters the carburetor through the inlet screen, flows down through the float valve and its seat, and into the carburetor bowl. As the fuel level rises, the float and the attached float valve also rise until the float valve is implanted in the seat, shutting off the fuel flow. 

As the fuel level in the bowl drops, the float and float valve also descend, allowing fuel to once again flow into the bowl.

The float travel from full-up to full-down is relatively short; it is stopped on the descent by a tab on the rear hinge. The level to which it rises up is stopped by the attached float valve and seat. 

The mixture metering valve. When the mixture control is pulled back to leaner settings, the opening becomes more and more narrow until it is completely closed at cutoff.
A venturi in the carburetor throat narrows the airflow opening, increasing the speed of the air, thereby lowering its pressure.
Throttle valve closed; idle air and economizer air openings visible in the bottom of the housing next to the throttle body edge.

Adjusting fuel level

It is important to maintain a correct fuel level in the bowl. If the fuel level is too low, the engine will run too lean; if it is too high, the engine will run rich and fuel may leak continuously from the discharge nozzle. 

The fuel level is adjustable by adding or removing washers under the float valve seat to extend or lower it, or by bending a tab on the float itself at the point of contact with the float valve to extend or lower the valve.

Airflow through the carburetor throat begins at the aircraft air filter and proceeds through the airbox into the throat of the carburetor. 

A venturi in the carburetor throat narrows the airflow opening, increasing the speed of the air, thereby lowering its pressure. (This is based on Bernoulli’s principle of airspeed and pressure being inversely proportionate; the same principle explains how an airfoil generates lift.) The outlet for the fuel discharge nozzle from the bowl is placed in the center of this low-pressure area. 

The air chamber on top of the fuel in the carburetor bowl is vented to atmospheric pressure. The pressure difference from the atmospheric pressure on top of the fuel in the bowl versus the low pressure on the fuel discharge nozzle causes fuel to flow out the fuel discharge nozzle. 

A throttle valve (i.e., a butterfly valve) located just downstream of the venturi controls mass airflow through the carburetor throat. As airflow increases, the suction effect on the fuel discharge nozzle also increases proportionately, allowing more fuel to flow. 

Here the throttle valve is open. Four small holes in the bottom of the throttle body are for the idle fuel delivery and economizer air openings. The large port on the right side, closer to the bottom, is part of the fuel bowl vent.
The mixture metering valve. When the mixture control is pulled back to leaner settings, the opening becomes more and more narrow until it is completely closed at cutoff.
The mixture control arm. This arm rotates the mixture valve, which on most carburetors is made as a flexible shaft attached to the mixture control arm.
Fuel flow

Before fuel flows from the bowl out the fuel discharge nozzle, it is routed through the mixture control valve. The mixture control valve is attached to the mixture control arm. 

The mixture control valve on most models contains a shaft (also called a stem). The bottom of this shaft is shaped like a half-cylinder. It rotates in a cylindrically-shaped sleeve with an opening on the side. 

When the mixture is set at full rich, the open part of the shaft/stem is aligned with the opening in the sleeve, allowing full fuel flow through the valve and out of the nozzle. As the mixture control is pulled back to leaner settings, the opening becomes more and more narrow until it is completely closed at cutoff.

When the mixture control valve is open, fuel flows from the mixture sleeve through the main metering jet (this is a fixed orifice that controls the maximum amount of fuel allowed to exit the main discharge nozzle once the mixture control is set to full rich) and into the discharge nozzle well, where it begins to be mixed with air from bleed holes in the nozzle. From there, it flows up and out the main discharge nozzle and into the intake pipes for the cylinders. 

At low throttle settings with the throttle valve nearly closed, there is not enough suction on the main discharge nozzle to cause fuel to flow out of it, but there is a slight amount of airflow between the edge of the throttle valve and the wall of the throttle body. 

This small area of airflow around the edges of the throttle valve acts as a venturi, forcing airflow to speed up as it passes between the edges of the throttle valve and the carburetor throat and lowering the air pressure. 

In order to provide adequate fuel for idling, small openings are made in the throttle body in this area of low pressure. Ports connect the openings with the inner section of the main fuel nozzle and draw fuel from the nozzle at low throttle settings. This arrangement provides an adequate fuel supply for idle speeds. 

A closeup view of the mixture control valve. Auto fuel deposits can form on the inner surfaces of the fuel system and may seize the mixture control valve in place.
The main discharge nozzle. As the mixture control is leaned, the suction effect and fuel flow out of the discharge nozzle is reduced.
The idle mixture adjusting screw is located on the top rear of the carburetor. Turn the screw counter-clockwise to richen, and clockwise to lean idle mixture. If a plane that was running properly suddenly develops a condition where it will not idle, one of the first things to check is that this screw is in place and hasn’t vibrated loose or fallen out.
Idle adjustment

The idle speed and mixture are adjustable, and are the only two adjustments that can be made on most carburetors. Most planes should idle at speeds of 600 to 650 rpm. The idle speed adjustment is simply a stop screw that limits the rear travel of the throttle arm. (It screws in to increase idle speed; moving the screw counterclockwise decreases idle speed.)

The idle mixture adjustment is a large screw on the top rear of the carburetor that screws a needle closer to or further from its seat, which allows more or less fuel to flow through the idle passageways. 

The idle mixture is made leaner as the screw is turned in and richer as it is backed out. It should be adjusted so that there is a 25 to 50 rpm rise in engine speed when the mixture control is pulled all the way back to shut down the engine. 

If there is no rise when the mixture is pulled back to cutoff, the idle mixture is too lean. If there is a rise of more than 50 rpm, it is too rich. 

There have been instances where the idle mixture screw has vibrated loose and fallen out. If this happens, the engine won’t idle at all, but will try to shut down when the throttle is reduced to idle settings.

A view of the bottom of fuel bowl, looking up through the carburetor throat. The acclerator pump discharge outlet and main fuel nozzle are in the center.
Basic maintenance and troubleshooting

Aircraft carburetors are generally reliable and seldom require much attention. The internal parts of a carburetor rarely need maintenance if the airplane is flown regularly and clean gas is used. 

An inlet screen that the fuel supply line attaches to can be removed for cleaning. Generally it stays pretty clean, because most debris gets caught in the aircraft fuel strainer before it has a chance to enter the carburetor. 

Over time, the throttle shaft bushings wear, especially on training aircraft that endure several power changes and throttle movements every hour. Worn bushings can allow a slight intake leak and cause an overly lean mixture. 

Most carburetors have an accelerator pump that squirts a stream of extra fuel into the intake air as the throttle is advanced so the sudden burst of extra intake air doesn’t create a lean condition and cause the engine to stumble, especially if the throttle is opened suddenly. The accelerator pump has a plunger that gets worn with use and periodically requires replacement.

Any leaks coming from a carburetor are cause for concern. A carburetor that leaks when sitting with the engine off most likely just has a tiny bit of debris trapped between the float valve and seat. Draining the fuel from the carburetor bowl and then flushing it by allowing it to refill and draining it again will most likely clear it up. 

Most carburetors have an accelerator pump. Shown here is the acelerator pump plunger with an outer edge made of leather.
Long-term storage of an aircraft

A carburetor on a plane that has sat with the aircraft fuel shut off may not allow fuel to enter the bowl when the fuel is turned on due to a stuck float valve. Gently tapping the side of the bowl with a small rubber mallet sometimes jars it loose and allows fuel to re-enter the bowl. 

If a stuck valve is suspected, momentarily crack open the supply line with the fuel turned on to be sure gas is getting to the carburetor, then re-tighten. Next, slowly remove the drain plug to see if there is fuel in the bowl. An empty bowl indicates a stuck valve or an obstruction in the inlet.

For folks that have an auto gas STC, it is best to never leave a plane with auto fuel sitting in the tanks, lines or carburetor for extended periods. Auto fuel causes deposits of varnish to form on the inner surfaces of the fuel system and often seizes the mixture control valve in place. 

If a plane is left sitting for a season, it will be far better for it to sit with Avgas in it. (Better yet, you may wish to “pickle” the aircraft. For more information, take a look at Steve Ells’ 2015 article “Flying, Interrupted: Modern Engine Preservation” in the archives at PiperFlyer.org.) 

Aviation carburetors are some of the most reliable inventions ever made. Their simple design and quality construction offer years of trouble-free service as long as they are flown regularly and proper steps are taken to ensure a clean fuel supply.

The accelerator pump outlet. This pump squirts a stream of extra fuel into the intake air as the throttle is advanced, so the sudden burst of extra intake air doesn’t create a lean condition and cause the engine to stumble.
The opening for the mixture valve. This is the fuel entrance for the main metering jet and discharge nozzle.

Know your FAR/AIM and check with your mechanic before starting any work. Always get instruction from an A&P prior to attempting any aircraft maintenance tasks.

Jacqueline Shipe grew up in an aviation home; her dad was a flight instructor. She soloed at age 16 and went on to get her CFII and ATP certificate. Shipe also attended Kentucky Tech and obtained an airframe and powerplant license. She has worked as a mechanic for the airlines and on a variety of General Aviation planes. She’s also logged over 5,000 hours of flight instruction time. Send question or comments to .


Avstar Fuel Systems Inc.
Aircraft Carburetors, LLC
Understanding Your Lycoming Fuel Injection System

Understanding Your Lycoming Fuel Injection System

Direct injection of fuel into cylinders offers better fuel distribution and easy cold starting, without the threat of carburetor icing. Jacqueline Shipe (A&P/IA) walks you through a typical Lycoming fuel injection system and the most common trouble spots to check if your engine starts running rough. 

Fuel-injected engines have been common in automobiles for years, and are gaining popularity in General Aviation aircraft. 

Fuel injection systems have several advantages over carbureted systems. With fuel injection, each cylinder gets an almost identical amount of fuel. This helps each cylinder put out an equal amount of power. This in turn makes the engine run smoother and more efficiently. 

In contrast, carbureted systems are prone to have cylinders which run slightly rich or lean in comparison to the rest because of the differing lengths of the intake pipes.

Fuel-injected engines are much easier to start when the engine is cold because each cylinder gets primed with an identical amount of fuel. 

Fuel injection systems also are free from the threat of carburetor icing. 

Fuel injection systems do have a couple of disadvantages when compared to carbureted systems. Fuel-injected engines can be difficult to start when hot. After shutdown in the hot summer months, they typically require a “flooded” start with the mixture full lean, and throttle full forward as the engine is cranked. This process can be frustrating for folks unfamiliar with the quirks of fuel-injected engines. 

The fuel injection system is also very intolerant of even the slightest piece of dirt or debris in the lines or injectors. 

Carbureted systems are generally easy to start when the engine is hot. They also, by design, tolerate impurities a little better than fuel injection systems do. 

Aircraft owners who fly behind fuel-injected engines will likely enjoy many years of reliable and efficient operation. Wise owners should still want to know what’s under the cowl, in order to make troubleshooting problems with their injection system quick and easy. 

Bendix fuel servo as removed from a Lycoming IO-540.
Idle mixture adjustment wheel on a Bendix fuel servo. For easy adjustment, the wheel can easily be turned by hand with no tools required
Idle circuit linkage.
Fuel inlet.
Fuel screen inlet. The arm to the lower left connects to the mixture cable for manual mixture control.
Major parts of a fuel injection system

The main parts of a typical fuel injection system are an engine-driven fuel pump, a fuel/air control unit (fuel servo), a fuel distributor (flow divider) with its associated fuel lines and the fuel nozzles themselves. Most airplanes also have an electric fuel boost pump, which provides fuel pressure for starting and as an emergency backup. 

The engine-driven fuel pump is designed to provide constant fuel pressure to the inlet of the fuel servo.

Throttle body butterfly valve, in the throttle closed position.
Opening for the duct for impact air pressure on a fuel servo with an automatic mixture control. 
Fuel servo

The fuel servo is a fuel injection system’s fuel- and air-metering unit.

The airflow to the intake pipes of the engine cylinders is controlled through the throttle body and butterfly valve in the servo. The pilot’s throttle movements directly control the amount of air entering the engine. This butterfly valve is similar to the butterfly valve in a carburetor. The throttle body is made with a venturi inside; again similar to those in a carburetor. 

However, the venturi in a fuel servo is only there to provide air pressure settings to an inside chamber in the fuel control section of the servo, not to provide nozzle suction for fuel discharge as it does in a carburetor. 

Fuel flow is controlled by the fuel servo’s ball valve, located in the fuel regulator portion of the servo. The ball valve is regulated by a series of diaphragms and springs. The diaphragms are used to allow the opposing pressures of incoming (impact) versus venturi air and metered versus unmetered fuel pressure to constantly regulate the amount of fuel sent out to the nozzles. 

As shown in photo H (right), the front housing of the fuel servo’s automatic mixture control (AMC) provides the opening for impact air pressure. The shape of the housing creates the venturi for the throttle body. 

Impact air pressure is ducted through impact tubes from an opening in the front of the throttle body (ahead of the venturi) to an enclosed chamber on one side of a diaphragm. Air from the low-pressure venturi section of the throttle body is ducted to a chamber on the opposite side of the diaphragm. 

As airflow through the throttle body is increased or decreased by the pilot’s throttle control, the air pressure in the venturi itself increases or decreases inversely. As airflow increases, venturi pressure drops. As airflow decreases, venturi pressure rises. The pressure difference between the impact air (which stays constant except for atmospheric changes) and the venturi air causes the diaphragm between the two chambers to move slightly whenever there is a change in air pressure on one side or the other. This difference in pressure between impact air pressure and venturi pressure in a fuel servo is known as “air metering force.”

The fuel servo ball valve in the fuel regulator is attached to the diaphragm in such a way that it moves toward a more open or closed position as the diaphragm moves in response to air metering force. Note that the venturi air pressure is the main controlling factor for the amount the servo valve is open at any given time. 

A fuel servo as installed on a Lycoming IO-360. The lower left cable is the throttle cable attached to the throttle arm. The center linkage with the scalloped wheel in the center is the idle mixture adjustment. The screw with the spring under the head is for the idle speed adjustment. The fuel inlet screen is on the top left.
At center: the small, threaded hole for the fuel nozzle.
A fuel flow divider on a four-cylinder engine.
Flow of fuel

Fuel flows from the engine-driven fuel pump through a metering jet in the fuel servo. The metering jet opening is controlled by the pilot’s manual mixture control. This fuel is considered “metered” fuel pressure. It is piped to a chamber in the fuel regulator inside the fuel servo. A separate line of unmetered fuel pressure is piped off before the fuel reaches the metering jet, and sent to another chamber in the fuel regulator. This unmetered fuel pressure chamber is separated from the metered fuel pressure chamber by a diaphragm. 

As changing venturi pressure causes movement in the servo valve, it also causes movement between the metered and unmetered fuel chambers. because the servo valve works in conjunction with both diaphragms. 

A reduction in venturi pressure (increased throttle and butterfly valve opening) causes a slight movement of the servo valve toward a more open position until the metered fuel pressure is increased to the point that the servo valve stops continuing to open and stays set at its new, more open position. Increased venturi pressure (decreased throttle and butterfly valve opening) results in a movement of the servo valve toward a more closed position until the decreased metered fuel pressure causes the valve to stop moving and it stays set at a slightly more closed position. 

This process governs the amount of fuel that is sent to the nozzles throughout all throttle settings.

Fuel nozzle for a turbocharged engine.
Fuel nozzle installed on a turbocharged engine.
Automatic mixture control

The AMC helps keep the fuel-air mixture ratio constant by adjusting the pressure differential between impact air pressure and venturi air pressure. It provides a variable orifice between impact air pressure and venturi air pressure—thus modifying the same “air metering force” referenced above. The AMC doesn’t replace the pilot’s manual mixture control; it works in conjunction with it.

A typical fuel nozzle installed on a normally-aspirated (non-turbocharged) engine. The air bleed screen opening is visible at the bottom of the metal shield.
Flow divider

From the fuel regulator section of the fuel servo, fuel is routed to the flow divider. The flow divider, which some mechanics call a “spider” because of its shape, is mounted on top of the engine. It provides a central point for fuel distribution to each fuel line and nozzle. The flow divider has a spring-loaded diaphragm which opens with fuel pressure from the fuel servo and closes when fuel flow ceases. This setup provides a positive cutoff of all cylinders simultaneously at shutdown. (See photos 01 and 02, page 26.)

Fuel flow test setup. Nozzles have been reattached to the fuel lines.
Each cup has been labeled with the corresponding cylinder number.
Fuel cup after the fuel flow test, ready to compare against other cylinders.
Fuel lines and nozzles

The fuel lines connecting the flow divider to the nozzles are hard lines made of stainless steel.

The last unit in the flow of fuel to each cylinder is the fuel nozzle itself. The fuel nozzles are made of brass and are very simple in their construction. The nozzle is essentially a hollow small tube with a calibrated opening on the outlet and a couple of restrictions that reduce the diameter of the tube internally. Each nozzle is calibrated to provide maximum fuel flow necessary at full throttle settings on the discharge end. The nozzles have a receptacle for the fuel line on the opposite end. There are no internal moving parts in the nozzles themselves.

Some nozzles are the two-piece type, and have a removable center section. These pieces should be kept together as a set any time the nozzles are removed. 

The nozzle is also where the fuel is mixed with air to atomize the fuel to make it combustible. Normally-aspirated engines have air bleed screens on the outside of the nozzle, while turbocharged planes have a sealed connection that vents the nozzle air chamber to the turbocharged “top deck pressure” (turbocharger compressor outlet pressure). (See photos 03 and 04 on page 26.)

On both normally-aspirated and turbocharged configurations, the intake manifold pressure is slightly lower than the pressure in the air bleed chamber of the nozzle, so air is continually drawn through the air bleed into the manifold. (See photo 05, page 26.)

A fuel nozzle with some slight stains around the air bleed screen. This could indicate a need for cleaning the screen.
Fuel injection system maintenance and troubleshooting

Most of the time, fuel injection systems operate trouble-free. When a problem occurs in the fuel injection system, it is often intermittent and sometimes can be difficult to pinpoint at first. 

Rough-running engines are usually fairly straightforward to diagnose. Usually a defect in the ignition system, such as a fouled spark plug or incorrect magneto timing is to blame, but occasionally trouble in the fuel system is the culprit. If the ignition system has been ruled out, it’s time to examine how the engine is getting fuel.

Most mechanics start at the nozzles and work their way backward until the source of the trouble is found.
Clogged fuel nozzles

When a problem occurs in a fuel injection system, it usually is caused by small pieces of dirt or debris that partially clog a line or injector. If one or more of the nozzles becomes restricted, fuel pressure will increase because the servo keeps sending out the same amount of fuel.

The fuel flow meter in the cockpit displays fuel flow in gallons per hour; but this number is derived from a fuel pressure reading at the flow divider. An increase in fuel flow may be seen on the gage if one or more nozzles are clogged, even though throttle settings remain unchanged. Higher pressure at the divider caused by a clogged nozzle shows up as higher flow rates on the fuel flow meter. An increased fuel flow indication along with a rough-running engine is an indication that one or more nozzles may be partially or fully plugged. 

The reason for the roughness is simple; the cylinder with a clogged injector is only getting enough fuel to run intermittently. 

This can be verified if the aircraft has EGT probes on each cylinder. On the cylinder(s) with partially clogged nozzles, the exhaust gases will be hotter than other cylinders; evidence that the cylinder is running too lean.

 A simple way to check for restrictions (flow test) each nozzle and line is to remove all the nozzles from the cylinders. The fuel lines should be unclamped as needed to give enough slack so that they aren’t bent or damaged in the process. After removing the nozzles, reconnect each of them to the correct fuel supply line. 

Place each nozzle in a small clear cup or jar that is labeled for the corresponding cylinder. Have someone in the cockpit turn on the master switch and fuel boost pump, with the mixture rich. Slowly advance the throttle from idle to full and back again while someone else observes the output of the nozzles. Each one should have roughly the same flow. 

Next, remove the jars without spilling any of the fuel. Compare the fuel level of the cups. A partially clogged line or nozzle should have a cup with a lower fuel level than the others. (See photos 06, 07 and 08 on page 28.)

Lycoming Service Instruction 1275C gives instructions on nozzle cleaning. The nozzle should be cleaned with acetone or MEK and blown out with compressed air. No picks or sharp tools can be used in the discharge hole or it will be deformed. 

If a particular nozzle or line has a chronic clogging issue and becomes clogged quickly even after cleaning, it may be best to replace both the line and nozzle. Even though a line or nozzle has been cleaned, microscopic particles or debris often remain and become dislodged with subsequent use, clogging the nozzle once again. 

Caution should be used when removing or installing fuel nozzles. The nozzle is screwed into the intake plenum of each cylinder. The plenum is located outside of the cylinder combustion chamber, in the intake manifold preceding the intake valve. 

The end of the nozzle that threads into the cylinder has fine-tapered pipe threads. The intake plenum is aluminum and the receiving threads in it are also aluminum. It is very easy to accidentally cross thread or overtighten a nozzle. The aluminum threads in the cylinder are easily damaged if this happens. (See photo 09, page 28.)

Generally, nozzles should be threaded in finger tight, then torqued to 40 to 60 inch-pounds maximum. If the threads do get badly damaged in the cylinder head it can be an expensive repair; the cylinder may have to be removed. Also, overtightening the union nut on the incoming fuel line can easily strip the relatively soft brass threads on the nozzle, or damage the nozzle inlet. 

The bottom center line is the supply line coming from the fuel servo.
Dirty nozzle air bleed screen

A dirty air bleed screen on a nozzle causes a higher than normal fuel flow out of the affected nozzle. The manifold suction that is always constant at the discharge end of the nozzle doesn’t have an air bleed to reduce it slightly. The fuel servo sends out the same amount of fuel, but with one nozzle pulling through more than its fair share, the rest of the nozzles run too lean. 

This can cause a rough idle, lower than normal fuel flow indication and a higher than normal rpm rise as the mixture is cut off. For reference, the normal rpm rise at cut off is usually 25 to 50 rpm. (See photo 10 on page 28.)

Opening in the fuel servo with the inlet screen removed.
Fuel lines and clamps

Fuel lines are prone to cracking if exposed to too much vibration, so they are typically clamped at several points along their length to minimize any shaking or flexing. 

The clamps catch a lot of heat and the rubber cushion in them dries out and shrinks over time, allowing the fuel lines to shake a little inside the loose clamps. Lycoming has an AD that requires repetitive inspections of the clamps and fuel lines for tightness and security, and replacement of defective clamps. (See photo 11, page 28.)

The lines have union nuts with threads that are easily stripped if the nut is over-torqued. They should be finger tight plus approximately 1/6th to 1/12th turn (one-half to one flat) more when using a wrench for tightening. New replacement fuel lines come as straight units that must be bent and formed to match the old line being replaced. 

Fuel servo center seal

A leaky center seal on the main fuel servo causes the whole system to run overly rich; so much so that the engine is hard to cut off with the mixture control. 

To check for a blown center seal that is allowing fuel to get over into the air chambers of the servo, disconnect the fuel hose between the fuel servo and flow divider. It is easiest to reach at the flow divider. Install a plug tightly in the line to seal it. Remove enough of the intake ducting so that the impact tubes can be observed and turn on the boost pump with full rich mixture and full throttle settings. If fuel comes out the impact tubes, the center seal is leaking and the servo will need to be sent out for repair. Blue fuel stains around the impact tubes also indicate a leaking center seal. 

Fuel inlet screen

If blue stains are observed on and around the servo, the cause is a leaking seal and there is no need go further (and pull the fuel inlet screen) because the whole servo will need to be removed for repair.

However, if a fuel servo is operating erratically, but no obvious leakage is observed, the fuel inlet screen is the next place to check. A clogged screen will cause the system to run too lean.

This screen should also be removed and cleaned periodically as part of routine maintenance. The screen should be cleaned with a solvent such as acetone and blown out with compressed air. (See photos 12 and 13 on page 31.)

If the screen is removed to troubleshoot erratic fuel servo operation, it should be tapped open side down on a clean towel before cleaning so any contaminants can be inspected.

Lower intake system manifold drain valve

Finally, if the previous steps have not helped locate the source of trouble, it is worth examining the lower intake system manifold drain. The drain is made of brass and has a one-way check valve to allow excess fuel and oil drain out of the intake manifold without allowing any air to come into the intake manifold. If the check valve malfunctions, it can cause the engine to run erratically. 

Pilots and owners who operate a fuel-injected engine may already know the advantages of this type of system, but still need to be able to identify the pieces, what they do and how they fit together. This article should give you a good working understanding of the many parts of a Lycoming fuel injection system.

Know your FAR/AIM and check with your mechanic before starting any work. Always get instruction from an A&P prior to attempting preventive maintenance tasks.

Jacqueline Shipe grew up in an aviation home; her dad was a flight instructor. She soloed at age 16 and went on to get her CFII and ATP certificate. Shipe also attended Kentucky Tech and obtained an airframe and powerplant license. She has worked as a mechanic for the airlines and on a variety of General Aviation planes. She’s also logged over 5,000 hours of flight instruction time.
Send question or comments to .


Lycoming Service
Instruction No. 1275C 


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