What happened
During an approach under instrument meteorological conditions, the pilot was utilizing the autopilot for an ILS localized timed approach. While the pilot was monitoring the time and searching for the runway, the autopilot inadvertently dispend, causing the Mitsubishi MU-2 to turn to the left. The pilot managed to right the aircraft just before touchdown; however, the airplane landed on its main gear with the nose slightly high. This hard landing resulted in the front wheel and strut breaking, causing substantial damage to the aircraft. There were no injuries to the pilot or the three passengers.
During a subsequent ferry flight intended for repairs, the pilot experienced a similar issue after engaging the autopilot at 2,500 feet. Upon engagement, the airplane pitched down and turned sharply to the right, forcing the pilot to use opposing control inputs and differential engine power to maintain control. The pilot completed a no-flap landing at an en route airport featuring a runway length of 2,497 feet.
The investigation
An examination of the aircraft revealed that when the autopilot computer was moved within its shock mount, the yoke moved violently forward in the pitch axis and to the right in the roll axis. The flight manual for the aircraft noted that while the autopilot can be manually overpowered to correct attitude, it must be disengaged immediately. If left engaged, the system will trim the airplane to oppose pilot inputs, potentially resulting in a severely out-of-trim condition.