Business aircraft are expensive to operate, but it is important to budget enough extra funds to allow for pilot proficiency and post-maintenance test flights. It would be embarrassing to roll the airplane out of the shop, load up the company officials and have to return immediately after takeoff because the cabin won’t pressurize. A return-to-service test flight may be required by regulation or operations manual, and it should be taken quite seriously.
Anytime one or more aircraft systems have been worked on, the subsequent flight needs to be conducted with jaundiced eye and attention to detail. Increase your vigilance as you complete each checklist item, more than just moving a control or confirming the absence of a light. Be sure the response occurred and was correct. Take extra time to run the checks.
“Here’s Your Aircraft”
When you arrive at the maintenance facility, have a discussion with the Director of Maintenance and the lead technician who actually did the work. Find out as much as possible about what was opened up on the aircraft, what discrepancies were found and how they were taken care of. Do not be accusatory with your probing. Treat the shop personnel as part of your team; to keep the aircraft airworthy, you need their input, and they need yours.
Your curiosity is not focused on learning how to be a mechanic. You simply want to gain insight about what to watch for during the test flight. If the brake components were changed, you may look for a different response to brake application, particularly if new disks or pads have to be broken in. Extra care is taxiing may be called for, until the brake performance is verified.
By knowing the specifics of where the aircraft systems have been disturbed, you’ll not just be on guard, you’ll also be ready to write down parameters and flight conditions during the flight. These “test card” notes are important to the technicians and can be valuable as a benchmark if readings change during service.
Of course, it helps to know what was worked on in case a vibration or instrument indication shows up. Over the years, I’ve seen a lot of maintenance-induced incidents, small matters caught before they resulted in an accident. On one test hop, accompanied by the mechanic, a burning smell showed up. My companion instantly knew the source; he had left a shop rag in the heater shroud when it was buttoned up after inspection. The smell went away before our hasty landing was completed.
On another test hop, after propeller governor adjustments had been made, smoke was observed streaming from the front of a cowling. All instrument indications were normal, but a precaution recovery was initiated. The “smoke” turned out to be powdered aluminum. A deice slinger ring was out of place and rubbing against the cowling, grinding away metal. Again, because we knew where the work area was, we were able to watch for abnormalities.
Remember, people are human, and humans make mistakes. And computers are built and programmed by humans, so they can make mistakes as well. Seldom is intentional sabotage a factor. Rather, it is haste and over-familiarity that leads to maintenance-induced errors. An initial or signature may show an item was completed and inspected, but you still need to watch that area during a test flight.
Once you’ve availed yourself of as much knowledge as you can about the work that was just done on the plane, conduct a preflight inspection armed with that knowledge. Check the cowlings that were opened, inspection covers that were disturbed, control surfaces that may have been re-rigged. Of course you look at these things before every flight; REALLY look at them this time.
Even an innocuous oil change can lead to disaster. A half-century ago, I picked up an airplane from the shop after an inspection, with a fresh fill of oil in the tank. As I climbed out from the airport, I scanned the instruments and noticed a low oil-pressure indication. It dropped further as I watched. I reversed course and reduced power for a descent to an immediate landing, and as the pressure dropped out of the bottom of its range I secured the engine. The landing was normal, but exiting after shutdown revealed an oil-covered engine compartment. The oil screen had been removed for cleaning and examination and its access plug was only replaced finger-tight, not torqued. Three sets of eyes failed to catch the blunder.
Once the preflight is done, consider the conditions before flying. I do not conduct post-maintenance flights into darkness or weather. I want to be unencumbered by flight tasks when I’m checking out the aircraft. Most test hops require only a half-hour of flight time, just long enough to bring up operating temperatures, climb to altitude, and cycle through the systems. But take time to stabilize and verify all parameters.
Add items to the before-takeoff checklist that may have been affected by the shop visit. Avionics, for instance, may not have been part of the squawk sheet, but settings, switches and wiring could have been moved, removed or wrongly reconnected. Does everything work normally? Better to catch it on the ground than in-flight. When the engines are started, take note of the starting sequences and temperature indications, and the battery and generator indications, for any abnormalities.
The Proving Flight
As power is applied for the takeoff run, see if both engines have similar responses, with the usual amount of trim and rigging irregularity. Takeoff acceleration needs to reflect power being developed and brake freedom; you may not normally gauge time on the roll or speed achieved by distance consumed, but this takeoff would be a good time to do that.
Sounds and feels are subjective test-pilot evaluations as we unstick and climb out, even to the smells coming from the environmental ductwork. You know your aircraft, so listen to what it has to say. Gear and flaps should cycle in the usual number of seconds, climb and acceleration rates should be normal, and trim should respond as usual. Hold off on the autopilot engagement until you manually check pitch and roll stability, and verify yaw response before you turn on the yaw damper.
Is the cabin pressure and temperature holding as it should, with no extra outflow noises and vents working normally? Is power responding as usual as altitude increases, windows staying clear, all engine parameters as expected? Engage autopilot modes incrementally; first switch on yaw damping, then turn on basic roll control to see if it works before using pitch hold, then altitude preselect, and finally nav tracking.
At level off, acceleration should be in line with previous observations, given the load and ISA; noise perception should be normal, fuel flows as expected. Stay close to the maintenance base until you’re satisfied that there are no glitches. Ask ATC for some maneuvering space if you need to cycle systems or check handling. Make notes of the stabilized performance and engine indications, as well as the day’s air conditions.
Finally, ask yourself, does this aircraft meet my expectations for a passenger-carrying trip across hostile terrain? Am I feeling or observing anything out of the ordinary that needs further attention? A descent for a non-passenger landing is a good time to see how the navigation and flight control systems handle a fully-coupled approach, despite the visual conditions; don’t waste the opportunity. Test your finely-tuned precision landing ability, apply maximum braking to see if it works, and note the fuel and engine readings at shutdown.
You are a vital part of the main-tenance team. Hopefully, your re-port to the shop is that all went well, with no residual write-ups. But don’t just say it was “A-ok”; give them some specific feedback, as they may have requested. How good did the engines run after their work, what was the time to climb, what did the environmental system do? Rather than send the plane back with complaints, give them some facts to work with.
The first flight after a maintenance visit needs to be conducted differently than a routine company trip. It’s an opportunity to contribute your skills as a pilot, and to act as the final inspector for the work that was done.