ERA16FA043 · Beech A36TC

The private pilot was beginning a cross-country flight and was instructed by the ground controller to remain clear of Class B airspace. After an uneventful takeoff, while in Class B airspace, the pilot was instructed by the local controller to remain outside the Class B airspace, and the pilot advised the controller that the flight was descending. However, the airplane climbed briefly, followed by a series of descents and climbs with varying airspeeds that continued for about 5 minutes. The pilot informed the controller that he could not disengage the autopilot and requested radar vectors to return to the departure airport. While returning, the pilot informed the controller that it took full forward and back control pressure to descend and climb, respectively, and he solicited and received assistance from another pilot on how to turn off the autopilot. The advice included pulling the autopilot circuit breaker, which the pilot said he did. The pilot apparently did not consult the emergency procedures for an autopilot pitch trim malfunction, which included a step to manually retrim the airplane. The other pilot then suggested powering down the airplane, intending for the pilot to turn off the electrical power. However, the accident pilot reduced the airplane's power setting based on his misinterpretation of the advice from the other pilot. About 9 minutes after takeoff, the airplane slowed to within 12 knots of its stall speed and continued to slow. As the airplane turned to join the final approach leg of the airport traffic pattern, witnesses saw the airplane enter a vertical descent and impact a lake. Airplane performance studies showed that, during the turn, the airplane was just 2 knots above its stall speed. Based on the low airspeed and the witness observations, it is likely that the airplane's wing exceeded its critical angle of attack and experienced an aerodynamic stall.

Postaccident examination of the airplane found the pitch trim in the full airplane-nose-up position. In the pilot's haste to mitigate an airspace violation while climbing with the autopilot engaged, he likely pushed and held the control yoke to arrest the climb. This action would have resulted in the autotrim running in the opposite (airplane-nose-up) direction to reduce the force on the pitch servo. Although the pilot's comment about pushing as hard as he could on the control yoke to descend was consistent with the full airplane-nose-up trim found during the investigation, his subsequent comment about using full force to climb was not consistent with the trim position. It could not be determined if the autotrim changed the trim position between the time the pilot first reported the autopilot issue and the time he reported that he had pulled the circuit breaker; however, it likely did not change after he reported pulling the circuit breaker. The electrical connection between the pitch servo and the airframe wiring harness was found not fully seated, but this likely occurred during recovery. Postaccident examination and testing of the autopilot system and components of the electric and manual trim systems revealed no evidence of preimpact failure or malfunction that would have caused an uncommanded full airplane-nose-up trim or would have prevented application of manual or electric trim to correct the mis-set trim condition.

Although testing of the maximum torque of the trim servo could not be performed because the circuit was damaged during postaccident operational testing, the as-found position of the pitch trim in the full airplane-nose-up direction indicates that the trim servo motor had adequate torque to overcome the aerodynamic force of full trailing-edge-tab deflection and would have been capable of moving the trim tab toward a neutral position if this had been selected by the pilot. It is likely that the pilot could have corrected the full airplane-nose-up trim by applying either manual or electric trim.

Although it could not be determined why the pilot in this accident did not promptly recognize and take corrective action regarding the mis-set trim, his lack of recognition was likely because he misinterpreted the cues as an autopilot failure. The excessive control forces required to maintain control and pilot muscle fatigue caused by prolonged operation of the airplane with mis-set pitch trim likely contributed to the pilot's failure to maintain control of the airplane.

Verbatim NTSB analysis and probable cause from the NTSB dataset