What happened
On 30 April 2018, a Cessna 172M Skyhawk II, registration N9085H, was performing a series of circuits and landings at Bermuda Airport to help the pilot refamiliarise with the aircraft following a period of extended maintenance. During the initial climb, the pilot noticed a lower-than-expected rate of climb. As the aircraft reached 1,0-00 ft, the pilot observed a persistent tendency for the aircraft to pitch nose down, even when applying nose-up trim.
To manage the flight, the pilot increased engine power on the downwind leg, which temporarily mitigated the pitch-down tendency. However, during the subsequent approach, the physical force required to maintain the glidepath increased significantly. Despite further attempts to apply nose-up trim, the nose-down pitch forces became too great to control. The aircraft struck the ground approximately 15 metres short of the runway threshold, causing significant damage to the aircraft. The pilot sustained minor injuries.
The investigation
The investigation focused on the aircraft's flight controls after the pilot reported the pitch issues. While the engine's control unit showed power and RPM exceedances due to the pilot's attempts to use engine power to assist with pitch control, no engine faults were found. Examination of the control surfaces revealed that the aileron, rudder, and elevator circuits were functioning correctly in terms of range and freedom of movement.
However, investigators discovered that the elevator trim tab was moving in the opposite direction to the cockpit trim wheel. Detailed inspection of the horizontal stabiliser revealed that the section of cables and the drive chain had been rotated 180 degrees, causing the cables to cross. This error caused the trim tab to move in reverse relative to the pilot's inputs.
Findings
- The elevator trim system was incorrectly rigged following a major maintenance period involving the replacement of the horizontal stabiliser's forward spar.
- The incorrect routing of the trim cables and drive chain caused the trim tab to move in the opposite sense to the cockpit control wheel.
- Although the aircraft maintenance manual contained a specific warning regarding the direction of trim tab movement during reinstallation, the error was not detected during post-maintenance function checks.
- The lack of a requirement for a duplicate inspection under FAA regulations meant there was no secondary check to identify the mis-assembly.
Safety action
- The maintenance organisation has implemented new procedures to ensure duplicate inspections of all flight-critical systems are performed after maintenance, regardless of the aircraft's state of registration.