Avionics Technology: The Test Flight

As part of the maintenance program

Test flights have been a fact of life in our industry since that fateful day at Kitty Hawk and are undertaken for a variety of reasons. In some cases they are mandated in the event of engine overhaul or required as part of a manufacturer’s ongoing maintenance program. The Federal Aviation Administration (FAA) provides little guidance on the topic with many of the crucial decisions left up to those involved in the event.

Federal Aviation Regulations (FAR) contained in Part 91 include statements implying all equipment on board an aircraft must be functional for flight or otherwise listed in a minimum equipment list (MEL). It is also stated that it is the responsibility of the pilot in command to ascertain the airworthiness of the aircraft and whether it is in a condition that is safe for flight.

The concept of flying an aircraft with a known anomaly, be it intermittent or otherwise, may be a bit of a stretch for those of us with an awareness of the regulations. A damaged aircraft is defined as one that has sustained physical damage, has inoperative/malfunctioning equipment, does not meet applicable airworthiness requirements, and as such will require a “Special Flight Permit” from the local airworthiness authority. This consideration is often overlooked when maintenance test flights are needed.

One of my realizations over the years is that most aircraft are inhabited by gremlins. Many reside in the landing gear wheel wells. When the aircraft is on the ground with the gear extended, these critters are comfortably at rest. Once a flight is initiated and the landing rear is retracted into the wells these insatiable little rascals are forced from their homes and in a fit of rage will frequently wreak havoc with many of the aircraft’s systems.

Troubleshooting techniques employed today include the shotgun approach, involving replacing virtually all the components that in and of themselves or combined with others, may produce the symptoms at hand. There is also the analytical approach which involves technical knowledge along with a logical thought process. Generally when a flight test is required it means there are no more parts to throw at the problem and no definitive abnormalities have been detected from at least a basic level of diagnostic techniques.

Have an objective and a checklist
As test flights can be both expensive and risky, a well-conceived plan should be created prior to the flight. The first determination should be the objective of the flight: will it involve only observations or will diagnostic actions be implemented? If so, a checklist should be created and reviewed by all parties involved. This should include considering the problem at hand and any possible conflict with aircraft limitations.

Assignment of crew member duties is integral to the success of any flight. One crew member should be tasked with flying the aircraft while a second pilot can assist with various flight deck selections to facilitate the required checks. Crew duties can vary depending on the qualifications of the technician accompanying the flight along with the airframe stipulation for minimum required flight crew.

Having participated in numerous airborne system evaluations in aircraft requiring two pilots it is my rule to never reach into the flight deck but to request the pilot not flying to make the needed control selections. As an example: if the discrepancy involves an auto flight system that could cause aerodynamic instability at high speed or high altitude, consideration should be given on how to proceed without putting the aircraft at risk. Some airframe manufacturers recommend using factory test pilots to conduct maneuvers or handle situations where the aircraft may be required to operate outside the normal performance envelope.

One of the first decisions should be exactly where the testing maneuvers will be accomplished. It may not be prudent to test an autopilot stability issue in an area of known turbulence or troubleshoot a magnetic compass problem in the Bermuda Triangle.

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