Avionics Technology: The Test Flight

As part of the maintenance program


Another pre-departure discussion should involve the reactionary steps to be accomplished should the anomaly occur. The flight crew should be thoroughly briefed on what to expect and the order of steps to obtain the desired information.

In-flight diagnostics
Auto flight system issues are some of more challenging problems to resolve as ground testing can not simulate the effects of aerodynamic loads and airframe abnormalities. Recent technological advancements provide for high level maintenance diagnostics that can provide useful information in uncovering various malfunctions. In some cases diagnostic data can be retrieved or at least observed while in flight. Observation of flight deck indications along with awareness of what else is happening within the aircraft will often provide clues critical to problem resolution. Auto pilot disconnect, wing walking, and porpoise are some commonly reported discrepancies with numerous possible causes.

Aircraft speed, altitude, atmospheric conditions along with operation of peripheral systems such as telecommunications, WiFi, radio communications, and even lighting can be contributing factors to auto flight issues.

One example of observation providing a valuable clue to an auto pilot problem has to do with a report of an auto pilot disconnect at altitudes below 10,000 feet. Once the aircraft was on the ground the problem could not be duplicated. The aircraft was returned to service and flew two missions before the exact same problem reoccurred.

Again the maintenance investigation failed to reveal a smoking gun. As a precaution the auto pilot computer and an air data computer were replaced. After departure on the very next flight the crew could not get the auto pilot to engage. They did continue the flight and when level in cruise at FL330 they were able to get the auto pilot to come on line and it functioned flawlessly for the duration of the flight. After repeated ground testing and swapping of parts, it was finally decided that a test flight was needed.

The technician riding in the jump seat was instructed to be observant as the crew put the aircraft through a series of maneuvers. Everything went well and the auto pilot performed as it should throughout the flight and into the decent. Just as the aircraft descended through 10,000 feet the copilot reached for and actuated the switch illuminating exterior pulse lights. This was a common but not standard practice. Moments after the pulse lights began to function, the auto pilot disconnected and would not reengage. Once the switch was returned to the off position the autopilot was able to be reengaged and worked without incident. This phenomenon was later duplicated on the ground. The pulse light control box was replaced, electrical bonding was reinforced, and the problem, attributed to electro magnetic interference (EMI), never resurfaced.

Another example illustrating the value of a well-orchestrated test flight involved an autopilot slow oscillation about the roll axis. The initial maintenance actions were per the book and involved checking the autopilot servo along with cable tensions. The flight crew had been briefed on how to select various flight modes and substitute sensors. When nothing made a difference, a test flight was scheduled and resulted in the technician observing excessive movement of the turn and slip indicator during auto pilot commanded turns. This observation led technicians to finding a non-responsive yaw damper.

Test flights will no doubt continue to be a necessity even with the most sophisticated diagnostic systems and the key to success is effective planning. The utmost importance is thorough system knowledge, a pertinent script of the tests to accomplish, record keeping, and communication between all participants.

I have also learned that care and nurturing of gremlins will often lead to a less stressful workday.

Jim Sparks has been in aviation for 30 years and is a licensed A&P. His career began in general aviation as a mechanic, electrician, and avionics technician. Currently when not writing for AMT, 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. You can reach him at sparks-jim@sbcglobal.net.

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