Incident Overview
The pilot of a general aviation airplane reported that the autopilot was inadvertently engaged while taxiing to the departure runway. In an attempt to disengage the system, the pilot and a passenger pressed the "A/P" button on the control panel. Warning buzzers sounded during these efforts. Eventually, the pilot turned off the avionics master switch to completely power down the autopilot. After restoring power, the autopilot appeared to be disengaged.
Takeoff and Abort
The pilot then contacted the control tower for takeoff clearance. During the takeoff roll, the airplane "prematurely" rotated and became airborne. The pilot immediately aborted the takeoff but reported struggling with elevator control due to a significant nose-up pressure. The aircraft departed the left side of the runway, struck a snow bank and another runway, and came to rest nose down.
Post-Accident Examination
Examination revealed that the nose landing gear had collapsed, resulting in substantial damage to the engine firewall. The airplane was equipped with a two-axis autopilot system with servos controlling the elevator and elevator trim positions. The elevator trim was not set to the takeoff position; the indicator showed a nose-up trim setting of approximately 8/10 of its total travel. An operational test of the autopilot system and a download of its fault log revealed no anomalies. The autopilot was installed with the correct software version and complied with all applicable airworthiness directives.
Pilot Experience
The pilot reported having minimal experience with autopilot usage and stated that he had not received training on the autopilot system installed in the accident airplane.