Mike Berry
Preventing Loss of Control: Maintenance Issues

Preventing Loss of Control: Maintenance Issues

Loss of control is a hot topic among the NTSB, FAA and other aviation organizations that promote aviation safety. 


Last year, the NTSB named the prevention of loss of control in flight in General Aviation as one of its “Most Wanted” transportation safety improvements. The NTSB report issued in 2015 stated that “between 2001 and 2011, over 40 percent of fixed wing GA fatal accidents occurred because pilots lost control of their airplanes.” 

This is unacceptable and mostly preventable. All pilots should be aware of the possibility of loss of control and take the time to review actions to prevent it. 

One way is to study flight manual procedures specific to the aircraft you fly. Another is to be aware of common situations that can contribute to loss of control. As a former air carrier pilot, air taxi pilot and mechanic with inspection authorization, I will focus this article on equipment and mechanical malfunctions that can also contribute to loss of control. 


Preflight actions

As aircraft control systems, flight management systems and electronics have advanced to make the task of flying easier, it is still necessary to have sufficient skills not only to fly the aircraft but to do all of the tasks we learned to do as a student pilot. 

Preflight actions, and proper flight planning—such as checking the weather, notams, aircraft preflight, weight and balance checks and the use of checklists—are all part of the job of flying. Without regard to who actually does all these tasks, as pilot in command, you are responsible for your actions in the operation of your aircraft. Even if it’s just a local flight and you are in hurry, don’t shortcut the process. 

How does this relate to loss of control? Preparing to fly a plane is a process of building blocks, and each item on the preflight list is important to the safe operation of an aircraft. The winter season can bring about additional challenges as cold weather, snow and ice can contribute to accidents because of a rushed or skipped preflight. 

Consider the flight crew that attempted to take off with control locks in place and crashed. Did someone actually do a preflight check and didn’t notice the gust locks were in place? And then, in addition, missed an important item on the pre-takeoff checklist—flight control check? 

Several years ago an aircraft was destroyed when a screwdriver was used in place of the control lock mechanism normally found with Cessna aircraft. While the control lock mechanism is not a required piece of equipment for flight, it is definitely not safe to use a screwdriver as a substitute. You can certainly duplicate the original Cessna-supplied control lock, or make one of your own—just be sure that it is marked is such a way that it is not overlooked during preflight. 

I have written in the past about checking an aircraft after maintenance, and again it is the pilot in command that has the last word as to whether an aircraft is acceptable for flight or not. Occasionally, errors are made and flight controls are improperly rigged, with the worst scenario being a control rigged in reverse. 

This actually happened a few years ago, when an aircraft elevator trim control was rigged in reverse. The aircraft was taken up for a test flight with the pilot not realizing this significant error. The autopilot was engaged, altitude hold selected; and the elevator trim and elevator control opposed each other until the autopilot disconnected in an extreme out-of-trim condition with suspected full elevator input to oppose the mis-rigged trim. The aircraft was not flying high enough to allow sufficient time for the pilot to correct the problem and the aircraft crashed. 

While this would be a difficult error to detect on a preflight without the help of a second person, it is something you must be aware of any time an aircraft flight control has had recent maintenance performed. Do you as a pilot not only move the flight controls on preflight, but actually confirm that the controls work in the proper direction? Which way should the aileron move when the control wheel or stick is moved? Are you sure? 

We will never know the exact details, but it’s entirely possible that these items in the building-block approach to flight were completely bypassed. 

How about the elevator and rudder, and the way that they move in relation to the control movement in the cockpit? Are the controls smooth, or are there unusual noises or resistance to full control movement during a preflight ground check of the flight controls? 

Winter weather can add environmental concerns such as snow or ice, which can build up on flight controls when the aircraft is parked and restrict full control movement or even jam a control in a partially deflected position. 

When preflighting your aircraft in
winter, check flap travel incrementally;
if the flaps are obstructed by snow or ice, or one flap is blocked from movement by ice, the flap actuator mechanism can be damaged and lead to an asymmetrical flap condition preventing full control of the aircraft on takeoff.  


Flight control maintenance

Lack of proper flight control maintenance can lead to unauthorized patches or repairs; broken, cracked or missing parts (such as elevator and rudder tips); and slack control cables. 

Good housekeeping

Errors occurring during maintenance can usually be identified by a thorough preflight. A missing cotter pin on elevator cable attachments, a tool or part left behind after maintenance, and any missing or incorrect fasteners attaching the tailcone to the fuselage will likely be detected during a thorough preflight. 

An extreme example of just plain bad housekeeping that cost two people their lives is attributed to a WD-40 can found lodged in an aircraft tailcone after an accident. There have been other bad housekeeping situations, too, such as plastic water bottles and even a flashlight discovered on the cockpit floor ahead of the rudder pedals. 

Never be casual about a preflight, especially after maintenance. 

Primary flight controls

In the case of unauthorized repairs, at least one popular aircraft manufacturer, as stated in their maintenance manual, does not allow stop-drilling of cracks in any primary flight control and does not allow scab patches, either. The control surface must be re-skinned, primed, painted and then balanced to be considered an authorized and airworthy repair. 

Flight controls must be inspected, repaired and returned to service by following all up-to-date manufacturer’s instructions exactly with no shortcuts. 

It is without question that any damage to a flight control requires maintenance, or, at the very least, an inspection by a licensed mechanic to determine if repairs are in order or the control is okay for return to service. 

While working for an aircraft manufacturer’s service center, I recall a situation where a pilot flew in with a complaint that his right aileron was binding and making a scraping sound when he operated the controls. 

The mechanics took a look at the aileron and discovered damage to the underside of the wing and aileron which was caused by the pilot. He admitted that he had struck a tree on a night approach to landing several weeks earlier. This pilot had been flying the airplane multiple times in this condition and had no idea what the extent of the damage was. 



Cable tension is another maintenance issue that must be considered as important to proper flight control operation. When the weather turns cool, control cable tension is reduced because the aircraft and the cables shrink slightly. Slack in the cables will create lost motion and reduction in control surface deflection. This could result in your inability to obtain a perfect landing. 

In the case of extreme weather conditions, when full control deflection is necessary (such as in a strong crosswind) control cable slack could contribute to an accident. Consider checking and adjusting control cable tension when the aircraft is exposed to extreme weather conditions.



As aircraft become more technologically advanced, autopilots, flight management systems and avionics all enter into the discussion of loss of control. In order to rely on an autopilot, it must have regular maintenance checks and service—and, as part of that service, control cable tension and proper control travel (rigging) must be within limits for the autopilot to perform properly. 

Autopilots and associated avionics certainly enhance safety by reducing the pilot’s workload, but at times, pilots—especially during single pilot IFR operations—can become task-saturated. Mix in bad weather, a long duty day, extensive holding, being low on fuel… and things can go bad quickly. You must know your equipment and be very familiar with all modes of operation. 

In my travels as an air carrier pilot I have overheard other pilots’ statements regarding autopilots including, “Why is it doing this?” and “Look what it’s doing now.” While this may be amusing to some, the pilot in command, not the autopilot, must be in control at all times. Take the time to study, ask questions and really dig into the manuals to find out why it’s doing what it’s doing. 

Many new aircraft are now being delivered with autopilots and flight management systems as standard equipment, and accident reports seem to suggest that some pilots fly into conditions that they are not capable of handling on their own. 

Autopilots have limits, and when weather conditions get bad and those limits are exceeded (such as moderate to extreme turbulence, or icing conditions), expect that the autopilot will disconnect and leave the flying to you. 

Never use an autopilot to fly into conditions that you are not capable or experienced enough to handle when hand-flying the aircraft. Realize your limits, and give yourself an out if conditions get bad so you may live to fly another day. Comply with manufacturer’s maintenance and service recommendations for autopilots to ensure that your autopilot functions correctly when you need it the most. 

Prevention is an ongoing process

Recurrent training, studying the manuals on autopilots, and knowing how to operate your avionics and flight management systems can help reduce stress, especially in high-workload IFR environments. 

Loss of aircraft control can happen as a result of many factors, but it is a given that proper maintenance, preflight actions and always performing a complete flight control check prior to each and every takeoff can go a long way toward preventing loss of aircraft control. 

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 .


Further reading

NTSB 2015 Most Wanted Transportation Safety

Improvements “Prevent Loss of Control in Flight in General Aviation”


2016 Most Wanted list
How to Evaluate a Fabric-Covered Aircraft

How to Evaluate a Fabric-Covered Aircraft

Fabric-covered planes in good condition are available, but you need to know what to look for.

Aircraft have been covered in cloth since the Wright Brothers took flight, and the material had to be as light as possible yet strong enough to withstand the demands of flight. 

The standard material used in the early days was cotton or linen. Vintage aircraft typically had wood wings and steel tubing used in the fuselage. 

This is a good example of rib lacing cord on an antique aircraft.
Wrinkles are an indication of possible damage to underlying structure. So are blisters or any rough areas, which could be signs of rust in the steel tubing.
The materials

The use of cotton or linen cloth is still approved; however, it is rarely used today because synthetic materials and improved processes are available. 

Synthetic materials and associated application processes not only reduce the amount of labor required, but also provide longer life, resistance to rot and fungus, and are safer in the case of fire (during material application, and while in flight). 

Polyester cloth specific to aviation applications is almost exclusively used in the recovering (or initial covering) of an aircraft today. Fiberglass cloth has been used as well, and other synthetic materials have been experimented with and/or are in development. 

The most important difference between newer synthetic materials and the original cotton and linen cloth is the fact that cotton is more difficult to work with. In addition, cotton is subject to attacks by fungus, mildew, chemicals (such as acid rain) and is susceptible to damage from rodents and sunlight. 

While synthetic fabric is deteriorated by sunlight too, it has better resistance to the effects of ultraviolet light. Synthetic fabric is also resistant to fungus attack, and while it can be damaged by chemicals, it is more resistant to damage than cotton. 

Cotton and the compatible nitrocellulose dope used to stiffen the fabric in the recovering process are flammable. Nitrate-based dope is extremely flammable even after it dries, and is seldom used today.

Synthetic fabrics sometimes call for cellulose acetate butyrate dope according to the STC, but oftentimes a material that is less flammable and more suitable to the synthetic fabric process is used. 

A significant factor regarding polyester cloth is that the tautness of the fabric
is controlled by heating the fabric with
a temperature-regulated device similar to a clothes iron. Application of dope or sealant materials will not appreciably shrink polyester, as is the case with
cotton fabric. 

Aviation-specific synthetic fabric can be much stronger than cotton fabric. This is a key issue in the pull testing (strength) of the raw fabric to determine continued airworthiness years after the initial fabric application process has been completed. Fabric is considered airworthy until the strength degrades to less than 70 percent of the original design strength. 

The FAA testing specification has always been in reference to the original material the aircraft was designed and certified with. Aircraft produced under the CAR 3 rules were approved with cotton or linen cloth of different grades depending on wing loading and maximum airspeed limitation. For example, aircraft could be certified with grade A cotton, intermediate cloth or glider cloth, depending on the never exceed speeds and wing loading, and then could be later recovered with a fabric of a higher rating. 

Drain grommets should be installed and clear of any obstructions so that air may circulate and moisture can escape.
The process

Working with cotton or linen requires special techniques and processes for a good-looking and airworthy cover job. When recovering an aircraft, the structure has to be carefully inspected and all defects repaired; then it can be primed and protected prior applying the fabric. 

The fabric has to be cut and sewn to the shape of the wing or fuselage and cemented or tacked into position. After the fabric is installed and secured to the frame, it’s permanently attached to the wing ribs with a special lacing cord using a designated knot. 

The spacing of the rib stitches varies according to the VNE (never exceed) speed of the aircraft and if the area is in the propeller slipstream or not. Some aircraft, like the Cessna 120 and early 140, use screws or fabric clips in place of the rib stitching. 

After the rib stitching, the next procedure is the application of cloth tape to cover the stitching and the installation of inspection rings, grommets and patches in various locations to protect the underlying fabric. 

A plasticized liquid lacquer (i.e., dope) is applied to the fabric in several applications initially by brush and then by spray gun to form an airtight and waterproof bond that also tightens and stiffens the fabric materials. 

The proper fit of cotton or linen fabric prior to doping is important, as extremely taut fabric caused by multiple applications of dope will shrink and distort or damage the underlying structure requiring removal, repairs and reapplication of the fabric. 

Proper health precautions must be followed when applying doping agents, especially when applying urethane in a spray form as it is extremely toxic. 

Multiple applications of various mixtures of dope are applied generally by spray gun. Mixtures may include dope with silver metallic compounds for resistance to light, dope with fungicide for resistance to fungus, and pigmented dope for the final color applications. 

After reviewing an airplane’s logbooks and AD compliance, carefully inspect the condition of the fabric. Check for cracked and missing paint or dope, and note the condition of the underlying structure.
Purchase considerations

An aircraft covered with polyester fabric—if it is applied according to STC, properly maintained and kept in a hangar—can have an almost indefinite life. However, when considering the purchase of a fabric-covered airplane, it is important to seek a mechanic that is familiar with this type of aircraft and knows what to look for. 

With the cost of a complete recover job for a tube-and-fabric airplane in the $30,000 to $40,000 range, you must be certain of the condition of not only the fabric, but what lies underneath. The cost to recover Cessna 120 and early 140 aircraft which have fabric-covered wings and a metal fuselage would be significantly less—$15,000 to $20,000—if no major repairs are required. 

As with most airplane purchases, it is always good to look for an aircraft that is in excellent condition and pay the asking price rather than look for the bargain. That bargain plane could require recovering that would make the final cost exceed the value of the aircraft. 

Prior to contracting with a mechanic to do a pre-purchase inspection, there are areas which you can check yourself just to see if the fabric-covered aircraft is in a condition that you would consider purchasing it. 

Keep in mind that vintage tailwheel aircraft probably have had a few ground loops, with airframe and/or engine damage and major repairs. Damage history is almost a given—but what this means for the purchaser is that the repairs must have been done correctly and that the aircraft flies like it should. 

The first order of business is to check the aircraft records, including any FAA Form 337 documents, to get an idea of the history of the repairs done to the airframe and engine. 

After checking the aircraft records, including compliance with all ADs, it would be wise to make up a written list of items to check on a pre-purchase walkaround. Make notes of anything you have a question about. 

Start with the condition of the fabric, and what the finish looks like. Check for cracked and missing paint or dope that would allow sunlight to directly access the fabric. Look for ringworm in the fabric; this indicates that the paint job is failing and will cause the cloth to deteriorate in a short time if exposed to direct sunlight. 

Check for patches, noting any especially large patch areas—these would require a logbook entry, or possibly a 337 form indicating a major repair. If there is no logbook entry indicating a repair was made where a large patch is located, be suspicious. There could have been major damage to the airframe structure that was repaired improperly, or not at all. 

Wrinkles or sags in the fabric most likely point to structural damage. For example, a dent in the metal leading edge of a wing would cause a sag or wrinkle in the fabric that would be visible from the outside. 

Blisters or rough areas under the fabric along lower longerons are an indication of rust in the steel tubing. Other areas could also have blisters or rough spots, such as the horizontal stabilizer, elevator or rudder; water is often trapped in these areas and eventually causes rust or corrosion. 

A fabric-covered aircraft should have sufficient drain holes or grommets installed—not only to allow moisture to escape, but also allow air to circulate and expel any moisture created by condensation. 

With the owner’s permission, pull a few inspection plates off from under the wings, especially in the area where the lift struts attach to the spar. Use a flashlight to take a good look at the wooden spar around the bolt holes, checking for obvious defects such as cracks or splits in the wood.

Move the strut at the upper end and see if there is any evidence of movement between the spar and the lift strut attachment fitting. Whether the spar is wood or metal, any movement is not good and could cause the spar to crack in this location, which would be an expensive repair or replacement. 

While the inspection plates are off, take a look up through the wing. Sunlight is the number-one enemy of fabric, and any daylight showing through the upper wing surface means a reduction in the useful life of the fabric. A very dull indication of light is okay, but if you can see a shadow of a person’s hand blocking the sunlight, then there probably isn’t enough light-resistant silver or pigmented dope remaining on the fabric. 

While the inspection plates are off, take a look at the rib stitching to see if the lacing cord is intact. Rodents have been known to get into a wing and chew the lacing cords, requiring expensive repairs. Rodents and birds can destroy an aircraft, especially if the structure is compromised by droppings or if drain holes are plugged with debris. 

While on the subject of wing ribs, note that over the years, several aircraft accidents (and at least one fatality) have occurred as a result of missing rib nails that secure the rib to a wooden spar. 

Another problem with wings and ribs is that of dissimilar metal corrosion when steel clips are used to secure fabric to the individual aluminum wing ribs—such as those used in the fabric-covered Cessna 120 and 140 aircraft. 

Since tailwheel equipped aircraft are sometimes involved in ground loops, check the wingtips for damage. Look at the fabric to see that there are no scrapes or tears, and check the wingtip for cracks or damage by looking up and out toward the tip through an inspection hole near the wingtip. 

Take a look at the lower rudder area and tail post for signs of damage such as loose fabric, wrinkles or sags, and possibly bent tubing from a hard landing on the tail. 

Get up on a stepladder and check the center section and inboard wing fabric directly in the propeller slipstream. This area sees a lot of vibration and heavy airstream deflection from the propeller, which induces wear/chafing and weakening of the fabric. 

The use of a suction cup on the fabric—attempting to pull up on the fabric in this area—is a simple test to see if the fabric is weak and/or not secure, requiring repair or replacement.
Final thoughts

When evaluating a fabric-covered aircraft, you really need to take enough time to go over the paperwork and the aircraft completely. Repairs to structure or a complete recover job are considered major repairs; they are expensive, and legally must be done by (or supervised by) an experienced and licensed mechanic with inspector status to complete the FAA Form 337. 

Recovering a Type Certificated aircraft is a job I recommend you leave to the experts. Errors in the fabric replacement process are easily made—and these can be difficult and costly to correct. Mistakes may even require starting the job over. 

Replacement of aircraft fabric is a big job because it never is just a plain recover job—there may be repairs required along with the preparation involved, such as completely disassembling the fuselage frame and sand blasting the fuselage, inspecting for damage and rust, and applying dope proof primer. 

Woodwork requires proper preparation with cleaning, sanding and application of special dope proof sealer. 

Multiple repairs to the structure, to include welding prior to the recover process, are more common than one may anticipate. These repairs can become overwhelming unless the job is properly planned and executed by an experienced person. 

How much a recover job costs depends on the process used, how many repairs are required prior to covering, and if you are able to assist in the process. Published cost and time estimates can be overly optimistic, especially if you don’t have experience or close supervision. 

When considering the purchase of a fabric-covered aircraft, look for a well-maintained aircraft with a quality fabric cover job. A quality job should last 20 years or more, depending on environmental conditions and exposure to sunlight. Any bargain-priced fabric-covered plane will most likely cost more to own. 

Vintage tailwheel aircraft can be a joy to own and fly. Enjoy the experience, buy the best—and leave the recover job to someone else!   

Tubing repairs or manufacture require a skilled welder to do the job correctly.

Michael Berry is a former aircraft repair shop owner. He is also a multi-engine rated ATP (757/727), A&P/IA, airplane owner, turbojet flight engineer and Part 121 air carrier captain. Berry has over 15,000 pilot hours. Send questions or comments to .


Consolidated Aircraft Coatings
While this Cessna T182 Turbo Skylane has a Garmin 1000 glass display, it still has the same basic pitot and static systems as older aircraft.

Aircraft Instrument Systems: A Brief Guide

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. 

This panel of this Skylane is equipped with a low vacuum warning and voltmeter.
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.

EGT instrumentation must use the correct color-coded wiring and may not be spliced, repaired, or in any way modified from the original configuration, including length.
How instruments operate, and why they fail

Traditional (steam gauge) aircraft instruments can be grouped according 
to their operating systems. 

Instruments such as these are often sold on eBay in unknown condition, and may not operate or cannot be repaired. If you are buying an instrument at a flea market or on eBay, you should not only ensure that it can be repaired and certified, but make certain that it is appropriate to your aircraft.
Pressure instruments

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.

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 specific repairs while others may not.
Vacuum instruments

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. 

Inaccurate readings of an airspeed indicator can have a definite impact on performance and overall safety. This example is unairworthy.
Electric instruments

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. 

The compass is a required instrument and must have a correction card. According to 14CFR 65.81, General Privileges and Limitations, repair to or alteration of instruments are reserved for certificated repairmen at an authorized repair station.
Repair options

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. 

Fuel gauges must be operational, and most can be repaired.
Authorized shops

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 airspeed indicators are aircraft-specific; know your requirements before purchase.
Radioactive components

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.

Fuel quantity senders wear out and need to be serviced by an approved facility. 
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. 

Post World War II instruments such as the ones installed in this early model Cessna 172 may have luminous dials that contain radium. Few repair shops are equipped to handle this material and may refuse the shipment.
Buyer beware

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. 

System leaks

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. 

Altitude discrepancies

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. 

Be proactive

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 .


Air Parts of Lock Haven 

Further reading
FAR 91.205
“Powered civil aircraft with standard category U.S. airworthiness certificates: Instrument and equipment requirements”

FAR 91.411
“Altimeter system and altitude reporting equipment tests and inspections”

Appendix E to FAR Part 43 
“Altimeter System Test and Inspection”

14CFR 65.81 
“General Privileges and Limitations”

All of the above documents are available at the FAA website: