Sleuthing a maintenance issue
I finally narrowed down the source of the problem when I was about 2,000 feet into the climb.
It had been nagging me for weeks. The autopilot's auto-trim function, which controls pitch trim when the autopilot is engaged, had stopped working, and for the life of me I could not figure out why. I thought it was something in the trim amplifier, a black box (a separate avionics component mounted remotely in the airframe) that sends an electrical signal to the servo motor telling it to adjust the trim tab on the stabilator (a horizontal stabilizer that pivots on the pitch axis, thus serving as the elevator, too).
I had offered that hypothesis to the avionics shop, and their response was "Bring the airplane by and we'll take a look." That sounded like an open-checkbook opportunity for the shop to spend hours troubleshooting the problem, so I resolved to try and do some sleuthing myself.
I turned off the autopilot and then engaged it one mode at a time to make sure I wasn't missing any of the symptoms. That's when I discovered that the auto-trim worked in the climb--I watched as the trim wheel slowly rotated, adjusting the tab on the stabilator for more nose-up trim--but it did not, or could not, move the trim tab in level flight or in a descent. The point was that the auto-trim was working at least some of the time, so the trim amplifier must not be the culprit.
I thought about it for a moment, and realized that the problem appeared to be a function of airspeed. In a climb, when indicated airspeed is relatively low, the auto-trim worked. But when I reached cruise altitude and selected the Altitude Hold function on the autopilot, which caused the airplane to level off and indicated airspeed to increase, the auto-trim did not adjust the stabilator tab for more nose-down trim.
Apparently, it could not overpower the higher slipstream forces to adjust the tab. The same was true in the descent, when indicated airspeed is at its highest.
Armed with this new evidence, I called the avionics shop and provided a revised, more detailed account of the problem. "Ah, sounds like the servo motor," the technician said. "Could be carbon buildup on the motor, a slipping clutch, or maybe the motor is just going bad. Bring it by and we'll take a look at the servo motor." I would still be on the hook for some troubleshooting labor, but at least now the shop knew where to look and should be able to identify and correct the problem quickly.
Identifying the source of an insipid mechanical or electrical problem can be a head-scratching experience, especially when you're trying to fly the airplane. A problem crops up during a flight--a needle on an instrument begins to waver, an amber caution light flickers, a radio appears to be intermittent. Nothing has failed--yet--but clearly something is amiss. You look around the panel and try flipping a few switches or punching some buttons, but with inconclusive results. At that point you wisely decide to abandon the distraction of troubleshooting and concentrate instead on flying the airplane.
Back at home base you stop by the shop with a general description of the ailment, leaving the technician with very little to go on to sort out the problem. He'll have to chase gremlins.
Mechanics and avionics technicians appreciate detailed information about a problem. It makes their job easier, which means you, the customer, will be happier, too. Sometimes, however, that detailed information can be hard to come by. Ironically, the simpler the airplane, the more difficult it can be to identify the source of a problem. Whereas a turbine-powered business aircraft has an extensive annunciator panel to warn of problems or failures in every electrical, hydraulic, mechanical, and other system and component, light general aviation aircraft may have but a single gauge for an entire system.
For example, it's common in fixed-gear singles for the electrical system to be controlled and monitored by an alternator switch, ammeter or voltmeter, and possibly a low-voltage light. An abnormally high reading on an ammeter would indicate an electrical overload possibly caused by a short circuit, but there's not much that can be done to identify the exact source of the problem. The recommended action would be to turn the battery (Master) switch off and check alternator output on the ammeter.
If the ammeter reading decreases, reduce electrical load and land as soon as practical. If the ammeter reading stays high, turn off the alternator switch, use the battery sparingly to power essential electrical equipment, and land as soon as practical. In either case, you don't have many clues to carry to the shop. About all you'll be able to report is the major symptom--the ammeter readings.
Clues can be vital to understanding and zeroing in on a problem, as I learned dealing with a flap issue. The flaps and landing gear on the airplane I fly are hydraulically actuated. When I began flying the airplane, I would partially extend the flaps on approach followed by the landing gear and then, depending on conditions, select full flaps for landing. Everything worked fine, or so I thought.
One one flight I happened to glance at the flap indicator a couple of minutes after I had selected one-quarter extension, and was surprised to see that the flaps had bled up some and now were at about one-eighth extension. I pushed the flap selector lever down until the indicator showed one-half extension, then watched the indicator needle slowly creep back up toward the retracted position. No doubt about it, the hydraulic system had some sort of problem that was allowing the flaps to bleed up.
I tried full flaps, and was mildly surprised to see that the flaps stayed fully extended. I was confused, but when I reported the problem--and the clues I had gathered--to the shop, the mechanic was intrigued. "That's good information," he said. I had armed him with useful observations about the functioning of the flaps at various settings. Now he could dig into the system with a good idea of what to look for and where.
Ask your child how his or her school day went and no doubt the answer will be "OK." How is a parent to interpret that? Does "OK" mean horrible, wonderful, or just OK? There's little information to go on in an "OK" answer.
If you think the airplane you are flying is afflicted with some sort of inscrutable electrical or mechanical problem, don't tell the shop "It's not OK." Give the technician something more to go on. Observe the symptoms, work the checklists, gather the clues. You never know which one might be the piece that helps solve the puzzle
Mark Twombly is a writer and editor who has been flying since 1968. He is a commercial pilot with instrument and multiengine ratings and flies a Piper Aztec.
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