FYI for DIYers

If youre doing maintenance yourself, follow accepted maintenance procedures - and ask a mechanic for help if necessary


A friend of mine was an FAA inspector in Alaska for part of his career. He once showed me a collection of pictures he had taken of aircraft repairs, and I could barely believe my eyes. I have never seen so many creative uses of duct tape in my life.

On an experimental aircraft, you have a large degree of freedom to modify the aircraft as you wish. The situation on a certified aircraft is completely the opposite, and for many good reasons. When that aircraft was certified, it underwent a lengthy review of possible failure modes, the effects of those modes and criticality of those failures to make certain that the failure of one component would not lead to a catastrophic failure of other adjoining components or systems.

There are engineering aspects to this topic, and there are legal issues. From the legal standpoint, if you are operating an aircraft that has unauthorized parts or unauthorized repair methods, technically you are operating an un-airworthy aircraft. The aircraft hasnt been maintained in accordance with its original type certificate or the parts dont conform to the original configuration list, both of which make the aircraft legally un-airworthy.

For anyone contemplating a professional pilot career, getting caught operating such an airplane is difficult to explain at an interview. Pilots who arent interested in a career of furloughs, checkrides, bad hotels and aviation-induced divorces often find it more difficult to accept the legal argument in and of itself. Pilots who fly for pleasure often dont think its a big deal to run afoul of the airworthiness issue because they see it as a legal technicality.

The local FSDO will say those are the rules, and theyre there to enforce the rules. Fair enough. But from a practical standpoint, adhering to proper airworthiness criteria makes sense because, no matter how silly a particular rule might seem, it was likely written in the aftermath of an accident. Unapproved modifications on an aircraft can directly jeopardize your safety.

Proper Maintenance
While researching cockpit fires, I found that more than a quarter of the engine fires were caused by improper maintenance. In many of these, the NTSB was unable to ascertain who performed the maintenance – generally because the work was not logged, which implies that either the mechanics were not properly certified or were embarrassed about the work theyd done.

Pilot-performed maintenance can also fall into this category. You may think changing the oil is no different than changing the oil in your car, and perhaps the basics are the same, but remember that the typical general aviation aircraft vibrates more and the engine endures greater chances in pressure.

More than a couple do-it-yourself pilots changed their oil but failed to properly torque the oil filter during the installation. This resulted in the oil filter vibrating off and causing oil starvation.

Part of the certification of parts such as oil filters includes testing them under various torque settings and determining an acceptable range. When you watch an aviation maintenance technician work with an open manual in the workshop, its because the manufacturer has specified the exact steps to be taken for the maintenance.

Authorized Parts
Everyone has heard the argument that the alternator in many aircraft is the exact same alternator you can find at an auto parts store for a third the price. It may be, but according to the FAA, its still a violation of the FARs to use non-certified parts in the aircraft.

Part of the certification process is meant to trace the components to make sure theyre quality parts. However, the quality is often something you cant really see. When a component is certified for use on an aircraft, the certification process will often specify the steps in the manufacturing process. This may seem like overkill, but for every sentence in the rules, there is usually a bloody example behind it.

Lets say you have a simple metal pin that has worn out in the aircraft, and you dont want to pay for a factory-approved replacement, so you ask your nearby metal lathe operator to crank out a look-alike pin. But will the pin actually be identical? Doubtful.

Besides using a specific metallic composition, the manufacturers approval may even specify the speed of the metal lathes operation and the sharpness of the cutting tools used in the process. Using dull cutting tools will create excess friction, which will heat the surface of the component and can heat treat it. Heat treating will change the metallic character of the surface grains, and during the subsequent cooling stage, stresses within the surface can be created that will substantially affect the surfaces resistance to fatigue and cracks.

Once the part has been manufactured, the engineers often specify how it should be stored and transported. If the part has bearings, theyll need a protective lubrication layer in between the bearings during transport. Maybe the part has hydraulic seals that need to avoid drying out. Maybe the part requires a certain temperature range. All of these factors are commonly specified in the certification and manufacturing process of an aircraft.

Proper Materials
A huge percentage of in-flight fires were caused by the use of improper or unauthorized materials in heat-sensitive areas. Especially egregious: using unapproved plastic vinyl hoses close to a turbocharger.

It should go without saying that potentially flammable materials should be kept away from sources of ignition, but the veracity of that statement seems lost on some. Engine cowlings generally fit fairly tightly, allowing heat to build up during ground operations and initial climb. In such tight quarters, its often difficult to keep such things as hydraulic hoses away from exhaust stacks and electrical wires.

But materials problems can crop up almost anywhere on the airframe – and they go far beyond the potential for fire. Consider the operator of a rental airplane who wanted to save some money and replaced aircraft parts with what he thought were comparable parts made for a refrigerator.

When he was done, there were two metals in contact with each other that had very different galvanic potential. The result was what some people refer to as dissimilar metals corrosion. The story doesnt end there. One of the engines failed during flight when the airplane was over open water. Several critical parts had corroded to the point where the engine would not restart, and with Murphys help they picked a bad time to do it.

If you think that specifying the metal allowed in a fastener for a general aviation aircraft is a sign that the federal bureaucracy has mushroomed far beyond itself, you might want to consider whether substituting them might have unintended consequences. When a manufacturer undergoes the tedious process of certification, both the manufacturers engineers and the certification authority will review the materials used in every junction. They are analyzing the potential for corrosion, because undetected corrosion can easily compromise the structural integrity of the parts.

If the possibility of corrosion exists, then proper preventive measures, such as protective surfaces or painting will be designated. The proper inspection method will be specified, as well as the inspection interval.

Lets say you find a great bargain on a panel-mounted GPS at AirVenture and take it to your local avionics shop for an installation quote. It isnt going to be cheap, and you think back to your days of installing car stereos for your friends and decide to install it yourself.

More than a few of the smoke in the cockpit victims tried this very kind of act. There are a number of different failure modes with avionics installations that pose a real hazard to flight if the installer didnt do a good job.

For example, some cowboy mechanics had not accounted for the extra heat output of the new avionics, which caused the adjacent avionics to overheat and begin to smoke. Some of the pilots had to stretch the wires a bit to hook everything up, unknowingly causing the wire to rub up against a metal post. That leads to chafed insulation and an electrical short circuit.

Some of the pilots had not properly secured the cables behind the panel, and vibration took its toll, leading to cracked insulation and short circuits. A couple unfortunate individuals didnt secure the new avionics properly in the stack, and the entire stack of avionics collapsed, causing more smoke. Incorrect selection of protective circuit breakers was another mistake.

Any electrical component necessary for the safe conduct of the flight must be properly protected from lightning. This requires that electrical systems be grounded, shielded and surge-protected.

Most pilots who arent avionics technicians cant begin to guess the proper methods for ensuring lightning protection. If there are any discontinuities or gaps in the conductive nature of the skin, the electricity will not be able to dissipate easily and may cause a failure around the gap or damage to components underneath it.

It seems that Murphys Law applies equally well to mounting avionics. There are plenty of failure modes that arent immediately obvious. Risking the chance of smoke or fire in the cockpit is an awfully steep chance to take. This is best left to the pros who know what they are doing.

High-Pressure Washing
Proud owners appreciate cleanliness, and who doesnt enjoy finding a hangar that is clean enough to sit down and eat? When the underside of an aircraft shines nearly as well as the sunny side, its obvious the airplane is well-loved. Undercarriages certainly pick up the mud, dirt and grime, but these areas can be difficult to clean. Be careful how you do it.

While walking through the large discount warehouse lately, I spotted a pressure washer pumping out more than 1,500 PSI. The package showed a powerful stream of water being directed onto the landing gear of a nice turboprop. Think it would be handy to have a high-pressure device like that to keep the underside clean?

It just so happens that high-pressure water directed on wheel bearings can force the lubricant out. In fact, several manufacturers specifically advise against using high-pressure water for cleaning around ball bearings and axles. The maintenance manuals describe using hot water and a brush to clean excessive amounts of grime and grease from the landing gear area, but point out that a certain amount of excess grease should be left on to protect the surfaces. Little details like these may not be obvious to well-meaning pilots, and they can cause a lot of expensive damage.

Carpets and Cleaning
The FAA is very specific about the types of materials that can be installed in an aircraft, and that even includes the type of carpeting. Its tempting to go out to the local carpet sales outlet to buy carpet at a fraction of the cost youll be quoted by an aircraft interior refurbisher.

However, you cant be assured the carpet meets FAA fire-resistance standards, and technically are making your aircraft non-airworthy by installing non-certified carpets. The unfortunate fact with some aircraft interiors is that, while they do comply with standards for flame resistance, they can produce lethal gases if they are near a constant source of combustion.

By the way, you might want to be careful with cleaning solvents and solutions while cleaning your aircraft interior. Using the wrong chemicals can strip the upholstery or carpet of its flame-resistant coating.

Repainting Control Surfaces
Maybe your aircrafts paint job looks a bit pitted and youd like a new paint job, or maybe one of the control surfaces suffered a little hangar rash and youd like to touch up the paint. Before you take the plunge, remember that even something as simple as repainting control surfaces can cause induce flutter at slower speeds than the airplanes certification standard.

Something so small as a bit of paint on a relatively light control surface can substantially change the mass of the overall control surface, and this change can lead to premature flutter. Some of you are chuckling to yourselves, saying the FAA just wants pilots to roll over and play dead.

If you dont think this is a serious matter, consider the pilots of a Grob 115-D whose rudder was repainted without being rebalanced. Unfortunately youll have to ask them in the afterlife because the last persons to see them alive were hunters who reported seeing pieces of the airplane falling to the ground.

The top portion of the rudder assembly was found almost a half-mile from the main wreckage. Balance checks of the left aileron showed that its residual hinge moment exceeded the manufacturers specifications.

A review of the aircraft maintenance logs revealed the airplane had been repainted, but the flight control surfaces had not been rebalanced. The NTSB determined that the rudder surface fluttered. The elevator attachment delaminated, causing an empennage overload leading to wing overload and separation.

Good Intent, Unintentional Failures
Some of the home grown maintenance attempts were done with good intent to spruce up the aircraft. Its hard to blame a pilot for wanting to apply some TLC to his aircraft. In general, a pilot who does some preventive maintenance will have a much better understanding of the systems of the aircraft and how to care and operate them.

The FAA is quite clear about what kinds of maintenance the aircraft owner is allowed to do. They are included in the FARs and the agency has published a handout for its accident prevention programs outlining just exactly what maintenance an owner can perform. Attempting to do any other refurbishment, repair or modification to an aircraft beyond this list is unauthorized on a certified aircraft unless you hold an A&P certificate.

Maintenance and repair are just like flying – they are best done by the book. Proper maintenance with certified parts may be expensive, but improper maintenance can be many times more expensive. A lifetime spent recovering from skin grafts is a steep price to pay for saving a few bucks on some maintenance.

-by Pat Veillette

Pat Veillette is an aviation safety researcher and instructor pilot for a large carrier.


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