Aircraft maintenance is a complex, regulated task. The Federal Aviation Administration requires all persons returning an aircraft to airworthiness after maintenance to be properly certificated. For most maintenance actions, this entails either the common Airframe & Powerplant Mechanic certificates or the less common Repairman certificates, as examples. These certification requirements exist to promote the safety and well-being of all involved in aviation.
But what about pilots? Airplane pilots often find themselves also being aircraft owners. As any owner or operator of a complex mechanical device will attest, times exist when minor maintenance becomes inevitable and no properly certificated mechanic is available. Airplane pilots do, however, possess the ability to return aircraft to airworthiness after certain maintenance actions. The Federal Aviation Regulations, in part 43, refer to these kind of actions as preventive maintenance. In particular, §43.3(g) claims:
“[T]he holder of a pilot certificate issued under part 61 may perform preventive maintenance on any aircraft owned or operated by that pilot which is not used under part 121, 129, or 135 of this chapter.”
This one sentence grants airplane pilots a surprisingly large number of privileges with regards to aircraft preventive maintenance. Part 43, appendix A, explicitly spells out the particular items that fall under the category of aircraft preventive maintenance. The items range from simply servicing oil to performing small fabric patches, repacking wheel bearings, repainting aircraft, updating GPS databases, and replacing fuel lines. Pilots would do well to read this particular section to discover just where their privileges extend.
The most obvious advantage of the broad privilege granted by the FAA is to be miserly. Every small aircraft must, once every year, undergo an annual inspection. This entails the complete inspection of aircraft for security, operation of systems for correctness, and servicing of all aircraft components. Typically, the time investment for an annual inspection ranges from six to eight hours for a small, two-seat Cessna 152 or Piper Cub to twenty, thirty, or forty hours for complex, electronic twins such as the Beechcraft King Air or Cessna 340. Knocking a couple hours of labor off these projects could save an owner a lot of money.
The scope of an annual inspection is immense. Part 43, Appendix D, of the FARs gives another laundry list of items that must be included on an annual inspection for legality. Some of these items must be performed by a properly certificated mechanic, but not all of them. Certificated pilots can accomplish nearly all of the simpler preparatory tasks, leaving big items like spar inspections and control cable tests to a properly certificated mechanic.
To illustrate, consider the simplest task of any annual inspection: opening and closing access panels. Every panel should be opened on an annual inspection to reveal the structure and cable runs hidden behind wing and fuselage surfaces. Removing and installing these panels does not require any skills specific to an A&P mechanic, only the basic ability to remove and install fasteners, and keep track of the removed hardware. Aircraft owners and pilots can accomplish these tasks and save themselves bundles of money, even on the smallest of annual inspections.
Of course, the aircraft involved is a major factor in this calculation. A Piper Cub owner would not save anywhere near this amount, due to the lack of panel removal on these aircraft. A Cessna 310 owner, however, would save much more money, due to the complex nature of the cabin floor structure and the extensive panel requirement for the complex landing gear system. Researching the specific nature and scope of annual inspection pertinent to the aircraft owned will play a big part into just how miserly an owner wishes to be about the aircraft.
This example only represents a single method of shaving billable time off of an annual inspection. A wise aircraft owner would discuss any further options with the mechanic, including things like fluid servicing, repainting, placarding, and other small but airworthiness important items. Additionally, many airworthiness directives become due around an annual inspection, and may have provisions for inspections to be performed by the aircraft owner, another possibility for streamlining an annual inspection.
Pilots have a much more compelling reason to perform aircraft preventive maintenance, however. It pushes the pilot to have a deeper understanding of the aircraft. Pilots typically have a very strong notion of how an airplane works from a cockpit, but a weaker understanding of how an airplane works, mechanically. A pilot would know, for example, that control stick input deflects a control surface, and that it gets harder to push with more deflection, but the pilot may not understand exactly how the stick and surface interface. Even more complex would be the interaction between two control surfaces, such as the left and right aileron, or ruddervators on a V-tail aircraft.
A teenager learning to drive encounters a patch of ice in an intersection on a cross-lane turn. When the car begins to slide, the driver inputs brake control, since the typical response on a dry road to loss of control is arresting the vehicle. Instead of arresting, however, brake input here aggravates the slide, causing a partial loss of control to devolve into a complete loss of control. The issue here is that the teenage driver has a misunderstanding of how the brake-wheel-tire-ground system works to actually slow a vehicle down.
This elementary example highlights a disconnect between the pilot and machine that turns a safe situation into a dangerous one. Misunderstanding of physical principles behind the arresting method of this vehicle directly led to a loss of control. These kinds of situations are much rarer in aviation than they are in normal driving, but they do occur. The infamous “doctor killer” notion behind early Beechcraft Bonanza airframes is a direct offshoot of this phenomena.
Early model Bonanza aircraft had a V-tail system instead of a typical T-tail or cross-tail. Instead of one rudder and two elevators, it had two ruddervators that acted as both yaw and pitch control surfaces. Opening up the tailcone access panels and observing the system of control cables connecting the control surfaces to the cabin revealed two important facts. The first was that this system was remarkably more complex than a typical rudder and elevator setup. The second was that the tail structure did not have any more reinforcement than a typical rudder and elevator setup.
These two factors combined gave V-tail Bonanzas in their neoteric years an accident rate three times higher than their traditional tail counterparts, the conventional tail Debonair. Experienced complex aircraft pilots would encounter a situation in a Bonanza that they encountered before in a comparable, traditional aircraft, and would respond in like. These inputs, spread out on only two surfaces instead of three, would result in over-stressing the empennage, causing immediate, catastrophic structural failure.
This failure is a result of the aircraft pilots misunderstanding the mechanics behind their aircraft’s control surfaces. Ruddervators, in comparison to rudders and elevators separately, are delicate. Thus, they require a lighter control touch than other similarly powerful aircraft. The FAA responded to these failures with multiple ADs, strengthening the tailcones and putting new never exceed speeds on these airframes. Since then, Bonanzas and Debonairs have similar accident rates, and the notion of the “doctor killer” aircraft has faded.
These kinds of accidents could have been avoided if the owners of these aircraft had participated with their mechanic in the annual inspection process. A typical tailcone control quadrant, such as in the contemporary Piper Comanche, has thick metal cables running directly from the cabin, through pulleys, to bell cranks on the control surfaces. Older Bonanza tailcones, however, have thin wires bungeed together with metal springs. A cursory examination and comparison of these two would reveal that the Bonanza tailcone was engineered with much closer tolerances and less structural stability.
While not detracting from the safety of the airframe by itself, these design choices gave pilots that exceeded the airframe limits less of a margin for error, resulting in both more accidents and the notorious reputation. With the knowledge and foresight gained by examining the control surfaces inside the tailcone, the pilots could have had a greater appreciation for the fragility of the system.
There are some FAA rules regarding how airframe sensors must work. Pitot systems must have a heating method for IFR-certified aircraft, engines must have a certain set of oil and temperature sensors, and airframes must have fuel level sensors. These fuel level sensors, however, are only required to read accurately when the system is empty.
Newer aircraft have capacitive sensors, which detect the weight of fuel inside tanks, giving a nearly 100% accurate indication of fuel quantity. Older aircraft, however, have resistive sensors, requiring a floating cork on a stick to give a fuel level reading. These systems are notoriously inaccurate and untrustworthy, yet completely legal. That is, they read accurately when the fuel tank is empty, and read from full to empty, therefore, they are legal. But are they safe?
Pilots understand that fuel measuring is a portion of any preflight. This is due to the fact that older aircraft have notoriously unreliable fuel quantity sensors. In fact, those old Bonanzas mentioned earlier have only two marks on their fuel gauges: Empty and Full. Anything in between could be anything at all. Occasionally, a fuel gauge will read full for a surprisingly large portion of fuel usage, and then suddenly drop very fast to empty. Other times, it may drop normally and read perfectly fine.
Pilots must understand this, but it is not something typically taught in ground school. Examination of accident records, however, reveals that many pilots fail to understand that fuel gauges are not true indicators of anything except an empty tank and a full tank. A cursory glance at fuel systems could indicate nearly a half-tank of fuel, but tapping the gauge or waggling the wings could make it drop significantly. The nature of resistive fuel sending units means that, rarely, they will get arrested in their travel, reading a falsely high level of fuel.
A Fairchild M-62A-3 suffered an incident in flight, lost all power, and was forcibly landed in a field next to an airport in Waukegan, Illinois, in 2012. The pilot noted that he observed the right gauge to indicate 3/4 full but did not perform a physical fuel tank quantity inspection on his pre-flight. Post-crash examination revealed that the right tank fuel gauge read 3/4 full regardless of whether the fuel tank was empty or full. An understanding of the nature of vintage aircraft fuel tank gauges would have prevented this accident.
These two examples, of the fuel level sensors and control systems, are just two examples of how airframe knowledge that exists beyond typical pilot knowledge can give enormous benefits to any aircraft pilot wanting to be a safer pilot. Understanding the principles behind the operation of the aircraft can give a pilot unique insights into potentially dangerous situations and allow proper reactions. By engaging in aircraft preventive maintenance, a pilot allows a stronger notion and understanding of airplane operating principles to cultivate, which can only lead to safer operation.