Preventive Maintenance

Doing things before problems occur in an effort to prevent inopportune failures? What a concept! The term “preventive maintenance” does have an official definition as well as implications assigned by the United States Federal Aviation Administration.

So just what does it mean? According to the Administrator: “Preventive maintenance means simple or minor preservation operations and the replacement of small standard parts not involving complex assembly operations.” This definition has been tweaked a bit to enable pilots to perform certain functions on their aircraft and the guidelines are presented in Federal Aviation Regulation (FAR) 43.3.

Privileges for pilots include tasks such as troubleshooting and repairing landing light wiring circuits but do not include position and panel lights or similar systems that may be more complex. For those not well versed in the art of aircraft repair, words of caution: lack of knowledge may end up costing more in the long run.

Replacing and servicing batteries

Replacing and servicing batteries is another area where frequent attention can prevent missed trips and is suitable for pilots to accomplish. When replacing any aircraft’s battery, use only one approved for that make and model; plus preventive maintenance does include adding water and top charging. If corrosion is noted on the terminals or in the battery box area, a baking soda solution works well in aircraft equipped with lead acid batteries. This should be followed by a complete flush with fresh water so no trace of the baking soda remains.

Emergency locator transmitter battery replacement is also permitted provided manufacturer’s recommendations are followed. The new expiration date for replacement (or recharging) must be legibly marked on the outside of the transmitter and entered in the aircraft maintenance record.

Navigation units frequently contain internal batteries requiring replacement. In certain cases such as with a portable global position sensor (GPS) change out is most often not a complicated process. Many of these devices are manufactured with substances considered hazardous which will impact acceptable means of disposal. In other systems such as onboard flight management system (FMS) computers, internal batteries are used to assist in the shutdown process in the event aircraft electrical power is turned off prior to complete system termination.

Unfortunately devices such as this are not well advertised and their replacement only becomes realized when a malfunction occurs generally in the start-up process just prior to an important trip. It is worth conversing with the FMS manufacturer and determine if an internal battery is used as well as recommended replacement intervals. In some cases these batteries are not field replaceable or may require an alternate power source be applied during the change. A thorough inquiry should be made prior to attempting replacement.

Navigation systems

Pilots are also authorized to remove and replace self-contained, front instrument panel-mounted navigation and communication devices, provided they employ tray-mounted connectors secured in the rack. This does exclude automatic flight control system components, transponders, and distance measuring equipment (DME). The approved unit must be designed to be readily removed and replaced and pertinent instructions must be provided. Prior to the unit’s intended use, an operational check must be performed in accordance with maintenance instructions and where pertinent, the applicable sections of the FARs. Systems such as the auto pilot often involve complex testing to validate airworthiness and if full functionality is not achieved, operation of the aircraft in certain airspace may be impacted. This is particularly true of air data and auto pilot systems when operating in reduced vertical separation minimums (RVSM) regions.

Transponder and altimeter systems are also off limits to anyone not covered by FAR 43.3 and addressed by either Part 91.411 or 91.413. Some gray areas do exist in who can perform a test of a transponder system after a known disconnect of the digital data link. This is clearly covered in Advisory Circular 43-6b and gives return to service authority to those listed in FAR 43.3.

Updating navigational software data bases is another area where pilots can use their sign off authority. This is provided no disassembly of the unit is required and pertinent instructions and training are provided. This privilege can be based on the type of aircraft operation and the sign off in maintenance logs may be an essential step. Prior to the unit’s intended use, an operational check must also be performed.

Heading off failures

Preventive maintenance is by no means limited to pilots. Much of what goes into support initiatives of aircraft today is with the intent of heading off failures. Oftentimes an enhanced situational awareness utilizing many of the body’s sensory organs can have a significant impact in heading off potential problems. Routinely comparing flight displays for uniform brightness and clarity will often provide subtle clues that an instrument may be approaching failure. Touching a unit (carefully) can also provide evidence specifically regarding temperature and vibration. Even audible ques can provide guidance as to the operating state of various components.

As avionic components do depend on electrical power and a byproduct of operation is heat, one of the most important proactive checks is the assurance of proper airflow surrounding the component. Caution must be used when evaluating the location of a device in close proximity to fuselage skin. This is particularly true in aircraft operating at high altitudes for prolonged periods of time. Some documented cases revealed that the heat sink impact of metal component surfaces joined to un-insulated aircraft structure will cause the temperature of the unit to go below the design threshold.

By analyzing the previous repair history of particular units we can identify the parts most susceptible to failure and make arrangement for replacement to extend the component life and overall performance of your avionics. It has been noted that this type of attention will improve system reliability by as much as 50 percent.

Coaxial cables are an area that have for the most part, been considered “on condition.” After all, what could possibly go wrong with a chunk of wire? Antenna cables are generally tuned for the type of system in which they interact and maybe best considered transmission lines. A coax is made up of at least two conductors separated by an insulating material. Breakdown of a coax will result in initial diminished range and clarity of the transmissions and reception and possibly soon followed by transmitter failure. Breakdown in transmission lines can occur from the results of heat, chemical exposure, or even over tightening of cable clamps.

Of course “preventive maintenance” may not always work for the good. Using certain types of automotive wax on aircraft windows or improper erosion tape on antenna leading edges may reduce the ability of these components to dissipate electrostatic charge.

Many in this business are still firm believers in the old adage ”If it isn’t broke, don’t mess with it.” As an ex-airframe manufacturer’s technical representative, my first question to someone calling with a problem “had you done anything to the system prior to experiencing the malfunction?” Most of the time the answer was slow in coming and followed by “I’ll call you back.”

The good news is my phone doesn’t ring that much anymore and I attribute that to being proactive rather than reactive when it comes to the maintenance of aircraft in my care. 


Jim Sparks has been in aviation for 30 years and is a licensed A&P. He is the manager of aviation maintenance for a private company with a fleet including light single engine aircraft, helicopters, and several types of business jets. He can be reached at