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.
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.
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.
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.
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.
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”
Do you know what instruments you can rely on to provide accurate information when the unexpected happens? A&P Mike Berry discloses what you absolutely need to know about your aircraft instruments.
Aircraft instruments have been a part of aviation since the first flight of the Wright Flyer, which was equipped with a stopwatch, an anemometer (to measure wind speed) and a tachometer.
With the increase of flight activity in the early years of aviation, aircraft instruments were invented to provide necessary information to pilots for precise control and navigation of their aircraft.
As a pilot and aircraft owner, it is important to understand not only how aircraft instruments work, but also to be knowledgeable of the systems that they interface with.
The maintenance and care of an aircraft, including its systems and required inspections, are tasks that the aircraft owner is responsible for—and they are not easy.
In this article I will give some insight into instrument repair and replacement options as well as the maintenance and repair of systems that drive these systems.
The basics, and some important questions
All modern aircraft, whether the aircraft has digital or analog instruments, share the same basic pitot and static systems. These systems deliver a very slight pressure to the instruments that they serve, and instrument accuracy is impacted by even slight variations. Leaks, disturbed air or even partial blockage in the lines serving instruments such as the altimeter, airspeed, and vertical speed indicators will certainly affect accuracy.
There are other systems that are electrical or mechanical in nature and for the most part are self-energized such as the tachometer, oil pressure and oil temperature gauges. While the latest models of aircraft have electrically powered instrumentation, the majority of General Aviation aircraft still retain the self-powered instruments as a matter of reliability and economics.
It is important as an aircraft owner and pilot to know the basics. In case of a total electrical failure, what instruments can you rely on to continue to provide you with accurate information? For example, fuel quantity gauges on most aircraft require electrical power and will not be reliable with the electrical system shut down.
Consider the vacuum system that powers most General Aviation gyroscopic instruments such as an artificial horizon (AH) and gyroscopic heading indicator (DG). When a vacuum pump fails, what instruments can you rely on?
Will your autopilot work? Will a failure of one vacuum instrument cause the other vacuum instruments to fail shortly thereafter? How about the old turn-and-bank or more modern turn coordinator instrument; how are they powered?
Turn coordinators are electrically powered; a turn-and-bank is powered by vacuum from the engine-driven vacuum pump.
The most important aspect of any gyroscopic instrument is that a failure may not be immediately noticeable unless the aircraft is equipped with a warning system.
In the case of a failed pump supplying vacuum pressure to gyroscopic instruments, the instruments will decelerate and become inaccurate over a minute or two, not in mere seconds. This inaccuracy over time can cause a pilot to lose control of the aircraft by following a slowly dying gyro into the ground. Several fatal accidents have occurred over the years for just this reason, and a low vacuum warning can be a lifesaver.
The rules concerning aircraft instruments
FAR 91.205 specifies required instruments for VFR flight for the most basic aircraft. These consist of an airspeed indicator, altimeter, compass, fuel quantity, oil temperature and pressure, and tachometer. These instruments must be operational for an aircraft to be considered airworthy.
There may be additional required instruments associated with the specific operations of the aircraft (such as instrument flight rules) and even some instrument requirements specified by ADs, Type Certificate Data Sheets, flight manuals or supplements and STCs.
It is up to the pilot in command to determine that the required instruments are operational before flight, and that the instruments are certified for the operation intended. While some instruments may legally be inoperative, consideration must be given as to how an inoperative instrument will affect the operation of the aircraft.
Additional rules concerning aircraft instruments according to 14CFR 65.81, General Privileges and Limitations, are that “… a certificated mechanic… is not permitted to… accomplish any repair to or alteration of instruments. These activities are reserved for certificated repairmen at an authorized repair station.”
This means that anything other than an external adjustment of an instrument—including installing a compass repair kit—is not authorized.
Static systems test and inspection for IFR flight is required by FAR 91.411 and must be accomplished every 24 months or “Except for the use of system drain and alternate static pressure valves, following any opening and closing of the static pressure system, that system has been tested and inspected and found to comply with paragraph (a), appendix E, of part 43 of this chapter; and (3) Following installation or maintenance on the automatic pressure altitude reporting system of the ATC transponder where data correspondence error could be introduced, the integrated system has been tested, inspected, and found to comply with paragraph (c), appendix E, of part 43 of this chapter.”
This means a certificated mechanic with the proper test equipment can certify only the static system (checking for leaks) and not the altimeter or transponder portion which is referenced in FAR 43 appendix E.
How instruments operate, and why they fail
Traditional (steam gauge) aircraft instruments can be grouped according to their operating systems.
Pressure flight instruments operate off of the static and pitot system, are self-powered and extremely sensitive diaphragm-type instruments relying only on variations in pressure to operate. These pressure variations are transmitted mechanically by gears and a jeweled movement as a result of the extension and retractions of the diaphragm.
As with anything mechanical, age takes its toll on the accuracy of pressure instruments such as the airspeed, altimeter and vertical speed indicator (VSI). These instruments are affected by moisture as well as dust and dirt, and should be kept clean.
Cloudy or dusty-looking instruments may mean that the system is contaminated and the static system must be purged of moisture or dust and the instruments promptly repaired or replaced. Leakage sometimes occurs between the instrument glass and outer case as well as inside system fittings. Sealants become inflexible over time and lose their ability to keep the system closed. Leakage must not be tolerated, as the accuracy of all the instruments in that system is compromised.
Aircraft instruments are delicate and require special equipment and training to be successfully repaired.
Vacuum operated (gyroscopic) instruments have been very reliable over the years, with very few actual failures of the instruments themselves; however, these instruments are subject to malfunction when an aircraft vacuum system fails.
Vacuum system failures can be prevented with proper care and maintenance (or replacement of components) as specified by the aircraft manufacturer.
One often-overlooked procedure is to check the vacuum gauge reading in your aircraft against a calibrated gauge. This ensures that the actual vacuum/pressure is set correctly, as over-pressure or under-pressure compromises accuracy, increases wear and creates an opportunity for failure of instruments or the entire system. Another often-overlooked but recommended procedure is to replace both pressure and vacuum filters on an annual basis.
When replacing a vacuum pump due to a failure, ensure that all hoses, filters and fittings are checked for contamination from foreign material as not only is the newly-installed pump at risk of failure, the instruments may also fail due to foreign material contamination.
Vacuum instruments are mechanical devices that operate with a gyro spinning at high speed powered by jets of vacuum or pressure impacting on small cups machined into the gyro rotor. The precision-balanced rotor is suspended by a shaft and supported by tiny bearings which are lubricated when the instrument is assembled. There is no provision for lubrication other than when the unit is disassembled during maintenance or repair.
Gyros rarely fail without some type of warning which may be indicated by excessive drift or precession, noisy or erratic operation. Inactivity really takes its toll on these instruments as the lubrication that is on the tiny bearings tends to drip or wick away from the actual bearing surfaces when the instrument is at rest for long periods of time.
Electrically powered instruments can be of several different configurations, from a simple fuel quantity sender or flap position sender (variable resistor) and indicator, to an electrical tachometer powered by a small generator (though a flexible mechanical cable between the engine and the gauge in the instrument panel is more common).
EGT and CHT gauges are usually self-powered relying on dissimilar metals in the sender or sensor to generate an electrical signal directly to the gauge on the instrument panel. The color coding of the wires is important as senders with different color coding than the instrument will not be compatible.
Sending units and wiring for CHT/EGT gauges must not be repaired, spliced, or in any way modified from the original configuration—including length. If it’s broken, replace it.
Anything electrical is subject to the effects of vibration, corrosion and broken (open) connections; remember this in your troubleshooting routine.
Also significant in any electrical instrument installation is that individual components of a system are in most cases not interchangeable. For example, a Rochester brand gauge must be connected to a specific type of sender unit intended for use with the Rochester gauge; a Stewart Warner brand sender may not work properly with a Rochester gauge.
Mistakes can be costly; check the schematic diagrams for the proper wiring, refer to the parts manual for the compatible component, and physically check that the item is what is actually installed in the aircraft you are working on.
Electrical components do wear out and/or deteriorate over time and malfunction, even if the item is rarely used. Good preventive maintenance practices—such as keeping moisture off of connections, proper routing and attachment of wiring, and reducing airframe vibration—can go a long way in avoiding premature instrument and electrical failures.
Finding a shop that will work on older instruments is becoming difficult if not impossible, and often owner-pilots are left with no option but to replace an instrument.
The rules of requiring approved technical data covering repairs and overhauls, approved parts sourcing and proper repair and test equipment are alive and well in the aircraft instrument arena. For this reason, many instruments that were original equipment on General Aviation aircraft 30 to 50 years ago are no longer supported and are not repairable.
Instrument repair shops operate as FAA approved repair stations and while all instrument shops adhere to the same FAA rules, some shops may be authorized to do repairs while others may not.
Do some checking around to see if you can find a shop that does repair older instruments. There are some, such as Air Parts of Lock Haven, that specialize in older aircraft instruments and in fact have repair station authority to do extensive repairs.
Air Parts of Lock Haven also has access to repair parts sources that other shops may not have. Air Parts of Lock Haven repairs older instruments and can also duplicate original instrument dials and faces.
Many instruments that were supplied as original equipment in the 1940s and 1950s and even into the 1960s came with luminous dials and markings which happen to be radioactive and are now considered hazardous material.
If you have one of these instruments, it must be shipped as hazardous material with all the markings, shipping labels and details that pertain to hazardous material. Few shops are equipped to handle this material and will refuse the shipment.
At last check, Air Parts of Lock Haven can receive these instruments and has authority to handle the material, but the instrument will not be returned with the radioactive dials.
General shipping information
Any instrument that requires shipment to a repair shop must be packaged properly—as if you were shipping eggs—and the package should be marked as fragile and insured.
It would be prudent to call the instrument shop you are shipping to and ask for a carton to ship an instrument in and wait a few days for the container to arrive rather than risk damage to the instrument in shipping.
Unfortunately, shipping companies can and do damage aviation material—and an insurance adjuster’s value of the instrument may be much less than what a functional instrument may actually cost.
A word to the wise: if you are buying an instrument at a flea market or on eBay, not only ensure that it can be repaired and certified, but make certain that it is appropriate to your aircraft.
Markings on a replacement airspeed indicator, for example, must be specific to the make and model of aircraft, and the details may be found in an official flight manual, TCDS, STC-related flight manual supplement, or even AD notes or Service Bulletins.
If you send in an airspeed indicator with a specific aircraft manufacturers’ part number, what you will get back is a repaired or a replacement airspeed indicator that will have the markings appropriate to that particular part number—which may or may not have the correct markings for your aircraft. There is no choice here as to changing the markings, and adding or deleting marks is not permitted by the FAA.
The importance of accuracy for performance
The performance listed for your aircraft was obtained when the aircraft, engine and propeller were new, and the aircraft was rigged properly, loaded to the most favorable center of gravity location and flown by a test pilot under optimum atmospheric conditions with accurate instrumentation. While it is possible to duplicate the published performance numbers with an older aircraft, everything must be nearly perfect to do so, and accurate instrumentation plays a big role.
Although digital instrumentation is replacing analog instruments and equipment, much of the instrumentation still relies on precise pitot or static system pressure which is then delivered to the computer or other device to indicate airspeed, altitude or vertical speed.
So, unless you have precise pressure, the 78 knots indicated you are using to achieve best rate of climb may not be exactly 78 knots. In addition, mechanical tachometers, whether due to age or inactivity, have a history of being inaccurate.
Inaccurate readings from just these two instruments—airspeed and tachometer—can have a very definite impact on performance and overall safety, as the aircraft will not achieve published performance numbers.
Most, if not all, aircraft maintenance shops have tachometer checking equipment and the calibrated tachometer checker should be used to compare required static rpm listed on the TCDS to the aircraft’s actual full-throttle revolutions per minute. An aircraft tachometer can easily differ from the published requirements by 100 rpm or more and some aircraft are rejected during annual inspection because of this. Practical application
Any aircraft owner knows that aircraft are expensive to maintain and there is no indication that costs will come down. Aircraft instruments are no exception; however, there are some economical ways to determine if you do have instruments that are in need of repair or replacement.
Static system leaks, for example, can often be discovered by some simple tests. Does the VSI, airspeed or altimeter needle move when a door or window is closed or opened while on the ground with the engine not running? When you open the cabin heat valve or a window in flight, do any of the three instruments just mentioned move abruptly?
Unusual temporary indications may indicate a leaking system component such as an alternate static port, leaking instrument glass or a broken or cracked moisture trap.
Also consider the effect of modifications to your aircraft, as these may impact the static system and overall instrument accuracy. An example of this was an aircraft that was modified with a cargo pod and several electronic sensors for aerial survey operations.
When the modifications were completed, the aircraft was test flown and at higher altitudes (in the teens). An instrument accuracy check revealed a 900-foot error in the actual altitude versus indicated altitude.
Errors such as this are rare, but can happen, so be especially vigilant when multiple modifications are made to an aircraft. The possible combined effect these may have on actual versus indicated altitude is worth examining.
An unofficial altitude comparison can be made between a GPS unit’s derived altitude and the indicated altitude while in flight. Large errors—such as a difference of a few hundred feet or more—should be cause for further investigation into pressure instrument (altimeter) and static system accuracy.
Electrical fuel quantity
Fuel gauges are another set of instruments that are known to be inaccurate, yet pilots rely on them. A typical electrical fuel quantity system on General Aviation aircraft consists of three parts: the sending unit (using a variable resistor attached to a mechanical arm/float), electrical wiring, and an indicator in the cockpit.
The sending unit attached to the fuel tank can fail mechanically or electrically, or provide inaccurate readings as both parts can wear or age. The float can absorb fuel and partially sink, providing an erroneous indication. Electrical wiring can become corroded or disconnected, and if a complete circuit is not maintained, may indicate full all the time (or empty all the time).
Some basic troubleshooting by a technician with a voltmeter and schematic can determine the offending component fairly quickly—especially when the plane is opened up for annual inspection.
A fuel gauge indicating the quantity of fuel in each tank is one of the required instruments according to FAR 91.205, and most (if not all) components—even on the most ancient aircraft—can be repaired or replaced to make the system work properly.
As a pilot or aircraft owner/operator it is very important that you properly maintain aircraft instruments and associated systems as well as seek repairs or replacement at the first sign of any deficiency. Operating an aircraft with a faulty or inoperative instrument can have serious consequences.
Maintenance personnel conducting an annual or 100-hour inspection should not return an aircraft to service, and pilots should not conduct flights with inoperative instrumentation or equipment required by FAR 91.205.
Michael Berry, a former aircraft repair shop owner, is a multi-engine rated ATP (757/727). In addition, he’s a turbo jet flight engineer, an A&P/IA mechanic, airplane owner and 121 air carrier captain. Berry has 15,000-plus pilot hours. Send questions or comments to .