Trim Malfunction

Fighting an out-of-trim aircraft will quickly wear you out

By Robert I. Snow

Compensating for a trim problem manually may seem easy at first, but the exercise can quickly become exhausting.

The mission was simple: Fly to East Hampton, New York; pick up three passengers; and fly them to La Guardia Airport in New York City. The weather was VFR, and there was no reason to anticipate delays or other problems en route. The equipment for the flight was an early 1970s-model Piper Aztec. The pilot in command was the company president, my boss. A milk flight.

I had been flying for the company for about a year. My background included several years as a full-time flight instructor and about two years flying charters in single-engine airplanes. Although I had plenty of hardball IFR experience in the demanding Northeast, I was low in multiengine time—just now getting close to the 100-hour minimum that my company's insurance required for pilot-in-command. The boss had started checking me out in the Aztec, and in the near future I would be taking my Part 135 (the regulations that govern charter flying) check ride. I was to fly left seat during the deadhead to East Hampton, then move to the right to act as copilot for the revenue part of the flight. The plan was that I would fly left seat again for the return to home base.

After conducting my walk-around, I let the boss know that we were ready, and we boarded. Engine start, taxi out, and runup were normal. Then we started on the before-takeoff checklist. When we got to "trim tab—set for takeoff," we encountered a problem. The electric trim failed to drive the trim tab indicator. Thinking that a connection or servo had failed, I reached down to actuate the trim by hand. To my surprise, it failed to move. It was completely jammed. Nothing we did would make it budge.

We had planned our takeoff so as to have an extra 10 minutes for some air work while en route. But taxiing back and readying another aircraft, assuming one was available, would still make us late. In charter flying, that's a no-no. I glanced down at the trim-tab indicator. It was slightly out of the takeoff range. Looking back today, it seems incredible that I even considered what we did next.

"Let's try it," I suggested. "It's only slightly out so maybe we can just hold forward pressure all the way. If it seems like it's getting out of hand, one of us can hold it while the other takes care of everything else." My boss seemed doubtful, but he finally agreed that we might as well go around the pattern once and see just how much of an effect the jammed trim might have. We called the tower and were cleared for takeoff.

At first the takeoff seemed normal. As we approached liftoff speed, however, I found myself pushing forward on the control wheel to prevent premature rotation. At liftoff I was holding an annoying amount of forward pressure to maintain the proper climb angle. It didn't seem excessive, but I was immediately aware that there was no way we could do this all the way to our destination. "This is no good," I said to the boss. He radioed to the tower that we needed to return to base. "Oh, well," I joked. "It was a nice try."

By midfield on the downwind leg, it was no longer funny. Even flying at reduced speed, the amount of forward pressure needed to maintain control, inconsequential at first, had become exhausting. I needed both hands to control the pitch. I thank my lucky stars that there were two of us aboard. The boss handled the throttles, landing gear, and radio. By the time we turned base, he was helping me on the yoke, taking off some of the pressure whenever he didn't need his hands for something else. Relief did not come until we started the flare. Finally, I was able to ease off forward pressure to raise the nose. As we rolled out, the boss said out loud what I had been thinking: "This was stupid."

There are several lessons here. First, trim position is on the pretakeoff checklist for a reason. Although I knew philosophically that the trim is a powerful part of the control system, I had never really experienced its potential before. Trimming away pressure had become such a reflex that I virtually never had to use more than minimal force on the yoke. If you are ever faced with a trim problem before takeoff, exercise better judgment than I did. Learn from my experience and taxi back to the shop.

If you are the kind who has to see things on your own, try it in flight some time. While in straight and level, roll in enough trim in either direction to force you to exert some effort to maintain pitch attitude, and then hold it for a while. You will quickly learn the same lesson I did. Don't try to prove that you're Superman by trimming to the limit of your strength. You could put yourself in very real danger if the trim system were to jam during your experiment. Likewise, stay near the airport so that if anything happens you can land quickly. Trim just enough to get the idea. Hold it long enough to get tired, then imagine that you have an hourlong flight ahead of you.

How about a trim failure in flight? Think through things that you could do to help reduce the loads. For example, if the nose wanted to go up, reducing speed would help because you would normally trim in up-elevator to maintain altitude anyway. In Cessna singles (which usually pitch up with flap extension) lowering the flaps at or below flap extension speed might help if the trim were jammed nose-low.

If your airplane is equipped with an autopilot or electric trim, know at least two methods to disable it. Knowing that you can pull the circuit breaker is not enough. Find the specific breaker and practice enough so that you can reach and pull it with minimal delay should your trim run away.

If you have an autopilot with pitch control, the unit will use the trim tab to relieve air loads just as you would. A modern autopilot should have at least three disconnects—the on-off switch, the circuit breaker, and a disconnect on the yoke. Never try to correct a pitch error by pushing or pulling on the yoke while the autopilot is on. The system will interpret your input as an air load and will fight you by trimming against the pressure you exert. If you keep at it, at some point the system will either go off-line by itself, or you will get concerned and switch it off. By then you could have a trim tab that is far out of position and a nose that wants to pitch abruptly as the auto-pilot disengages.

Finally, don't let the desire to fly overcome your suspicion that all is not well with the airplane. Think very carefully before you dismiss a problem as inconsequential. Treat any problem in the trim system as a no-go item.

The passengers at East Hampton weren't happy when I arrived in a single-engine Piper Lance. I was 40 minutes late, and they had contracted for a twin. They calmed down when I explained that the other aircraft had developed a mechanical problem and that, rather than take the slightest risk with their safety, I had brought another aircraft. The fact is that I was not as happy with my decision-making that day as I was pretending. For all pilots there are days like that. But I am thankful that I survived to take advantage of the lessons learned.