What happened
Approximately six minutes after takeoff, at 2138:31 CDT, the pilot of N8CC reported reaching an altitude of 9000 feet mean sea level. Two minutes later, at 2140:17, he transmitted that he was experiencing difficulties and stated he could neither control nor disconnect the autopilot, which was causing the aircraft to descend. At 2141:14, the pilot indicated a descent rate of 6000 feet per minute. Another company pilot in a different aircraft asked if he could locate the circuit breaker for the autopilot system. At 2141:27, the pilot of N8CC replied that he would call back. Seconds later, radio and radar contact with the aircraft were lost. A search was initiated, but the wreckage was not found until the following morning.
The investigation
An examination revealed that the aircraft impacted the ground in a steep, nose-down, inverted attitude while at high speed. Much of the wreckage was buried. No preimpact part failure or malfunction was found. With the autopilot engaged, back pressure on the control column would have caused the system to trim the nose down.
Safety message
Subsequently, the manufacturer issued a precautionary advisory to all Mu-2 owner/operators regarding proper operation of the autopilot and the various ways in which it can be disengaged.