2011-09 · NASA ASRS report 971435
The flight crew of a King Air was unable to disconnect the autopilot when it failed to level at their cleared altitude. Control was regained 1;000 FT below their cleared altitude after they pulled the autopilot circuit breaker.
As we approached our destination we were given an initial descent and then a continuing descent to 4;000 FT. I was looking at the approach plate for the expected approach when I looked at the altitude and noticed that the rate of descent was not slowing to capture our cleared altitude. I said something to the pilot flying and he pressed the disconnect for the autopilot. At the time we thought the autopilot disconnected. As he tried to pull back on the controls; I noticed the trim start to move. I thought we had a problem with the trim. At this time we descended through our assigned altitude. I advised ATC we had a problem and to standby. We continued to descend slightly below 3;000 FT with both of us trying to override the trim. I then noticed the autopilot had not disconnected. We both tried to disconnect without success and the other pilot tried the pedestal disconnect without success. I then pulled the circuit breaker for the autopilot and at that time we regained control of the aircraft and advised ATC we had the situation under control and could continue.
At the time of recovery we noticed that we were 1000 feet below our assigned altitude. We contacted ATC at this time and said we were experiencing a problem with a runaway trim and were working on it.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.
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