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Runaway Trim by Sean Ferguson

I had my first experience with an out of trim airplane in 1968, just a few days before my first solo. On a nice spring day I took off in a Piper Super Cub (PA18-150) to practice takeoffs and landings. When the airplane became airborne, I felt a tremendous back pressure on the stick. "Let the stick go!" I yelled at my instructor, who occupied the rear seat. "What do you mean by letting it go? I am not even touching it, he replied." This was when he realized that I took off with the elevator trim in a full up position. "You are out of trim," he said to me, "Just trim it and continue to climb." I tightened my right hand's grip on the stick as my left hand was reaching the crank type trim handle. Turning the handle forward caused only the steel cable to slip on the trim pulley without any effect on the trim control. At that point my right hand started to give in to the pressure. I told my instructor that the only way I could fly the plane was by holding the stick with both hands. We kept climbing to a safe altitude with the two of us holding the controls. I was flying while the instructor was correcting for the back pressure. Upon reaching a safe altitude, we initiated a glide. The decreased aerodynamic pressure enabled us to set the trim back to normal operation.

Ten years later, I was flying for a small air-taxi operator. One summer evening, I was assigned to fly five people in a twin engine Piper Seneca III. A comprehensive weather briefing revealed that Instrument Meteorological Conditions with scattered thunderstorms existed along the route of flight. These conditions were forecasted to continue for the entire duration of the relatively short flight. The preparations of the well-equipped Seneca included a careful weight and balance computation, proper fueling and the filing of an IFR plan.

At about 11 PM, I was cleared for takeoff under an overcast sky with the ceiling at 900 feet. At about 300 feet, I was instructed by the tower to contact the departure control. Shortly after I contacted the departure control, I engaged the autopilot. I was climbing through 600 feet when the flight director bar was deflected rapidly upwards. I immediately disconnected the autopilot, recycled its switch, and made a second attempt to engage it. The problem duplicated itself when I engaged the autopilot again. However, this time when I disconnected the autopilot, the airplane suddenly pitched up. It took both my hands and arms to counteract the pressure of the yoke. I was trying to trim the airplane with both the manual and the electrical trim when I realized that the elevator trim was stuck in full up position. The first question that came to mind was, "What am I doing here, is it all over?" There was no time for thoughts it was survival time. I had to keep the airplane out of the clouds and return to land. My arms started to ache badly as I had declared an emergency.

I reduced the power setting to low cruise in an attempt to relieve the control pressure, but the change was not significant. I had the airplane under control, but I knew that I could not hold the yoke like this for too long. The magic solution came when I decided to use my knees to counteract the pressure, and to use my hands to fly the airplane. The person next to me (at the co-pilot seat) was an elderly man who I did not think could help by holding the controls. While establishing myself on the downwind leg, I tried several things to ease the load. Among these were attempts to regain the trim, change the wing flaps position and recycle the landing gear. All my attempts to correct the problems had failed. I knew that there would be an accident. The only question was how bad of an accident would it be.

On the downwind leg I went back to the tower frequency. I advised the tower that I would extend the downwind leg to give me a long final approach. I needed enough time to stabilize the approach, thus minimizing the hazardous potential of the landing.

As I got closer to the runway, I reduced the power when I felt a light jolt. It felt as if something that was caught came loose. I immediately tried to trim the airplane. The trim was responsive, enabling me to trim the airplane shortly before touch down. I advised the tower about the change in conditions and made a normal landing. The blinking lights of the rescue vehicles could be seen at each intersection of the runway and they followed me back to the parking area.

A mechanic's inspection of the airplane revealed no irregularities. The airplane was returned back to service without any corrective action. Before my next flight, I felt a click while checking the elevator's freedom of movement. I decided to further investigate the click. I positioned the elevator trim in various positions and found that the click was subject to the elevator trim position. With this finding, I insisted that the trim mechanism be re-inspected. The second inspection revealed that the trim's plastic bushing thread was worn off, and that it jumped one tooth over the jack screw that operates the trim tab. For an unknown reason, caused most likely by a faulty circuit, the trim moved to the full up position and became stuck.

Pilots should be aware that an un-trimmed airplane may lead to a serious situation. Any irregularity that is found during the preflight inspection or during takeoff checks should be taken seriously and inspected by a mechanic.

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Last update May 15, 2005
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