Cessna 560 N774SB incident near Dallas, Texas

Probable cause

The loss of aileron trim due to the lack of separation between the cannon head on the back of autopilot cannon plug and the aileron trim tab control gear. Contributing to the incident was the lack of guidance in the airplane maintenance manual to verify separation of the autopilot control head and aileron trim tab control gear before completing work on the control pedestal.

NTSB analysis

The airline transport pilot and the copilot, who was the pilot flying, were conducting an on-demand air taxi flight with three passengers onboard. The copilot reported that, during the takeoff rotation, he had difficulty maintaining the runway heading and that the airplane experienced a right rolling tendency. He noted that, while continuing to take off and as the airspeed was increased, the pressure required on the control yoke to maintain level attitude also increased. The copilot transferred control of the airplane to the pilot who then attempted to adjust the aileron trim; however, the trim control knob would not move, and the trim indicator appeared centered. The flight crew declared an emergency to air traffic control and requested to return to the airport. When the airspeed was decreased during the approach, the right roll control pressure decreased, and the flight crew was able to free the aileron trim control and land uneventfully.

Postincident examination of the aileron trim system revealed that the cannon plug on the back of the autopilot control head was contacting the aileron trim control gear within the cockpit pedestal. Additionally, the wires extending from the cannon plug exhibited chafing.

The day before the incident, maintenance personnel had inspected the cockpit pedestal, which required removal of the control heads for the autopilot from the pedestal. The pedestal was then cleaned and reassembled. Therefore, it is likely that, when maintenance personnel reassembled the pedestal, they did not ensure that the cannon head on the back of the autopilot control head and the aileron trim control gear were adequately separated, which resulted in binding of the aileron trim control gear and chafing of the wiring and led to the in-flight loss of aileron trim.

A review of the airplane maintenance manual revealed that the Control Pedestal Area General Inspection procedures did not include a step to ensure that sufficient separation existed between the autopilot control head/wiring harness and the aileron trim tab control gear. The lack of guidance in the maintenance manual to verify separation of the autopilot control head and aileron trim tab control gear before completing work on the control pedestal likely contributed to the incident. Following the incident, the airplane manufacturer implemented a change to the maintenance manual to include a step to wrap the wiring harness in a protective sleeve and verify separation between the autopilot control head/wiring harness and the aileron trim tab control. The manufacturer also issued a mandatory service letter that provided instructions to inspect for damage and correct wire harness positioning.

Text from the NTSB. Open the NTSB docket for CEN16IA103 for the full record.