History of the Flight
The pilot planned a series of circuits and landings to refamiliarise with the aircraft after several months without flying and a prolonged maintenance input. Pre-flight checks included verifying the elevator trim was set to the takeoff position. Takeoff appeared normal until passing 200 ft, when the rate of climb seemed lower than expected. After reaching 1,000 ft, the pilot levelled and reduced power, but the aircraft tended to pitch nose down despite applying nose-up trim. Increasing power on the downwind leg alleviated the tendency. The pilot decided to terminate the flight and informed ATC. On base leg, reducing speed increased the nose-down forces, and adding power reduced them but increased ground speed. During final approach, the pitch-down tendency became uncontrollable, and the aircraft struck the ground approximately 15 metres from the runway threshold, then slid onto the paved surface. The aircraft sustained significant damage, and the pilot suffered minor injuries.
Elevator Trim System Description
The Cessna 172 has a movable trim tab on the right elevator. A cockpit trim wheel connects to cables and a chain driving the actuator. Turning the wheel for nose-up moves the tab downward, and nose-down moves the tab upward.
Aircraft Examination
Examination showed no defects in the aileron, rudder, or elevator control circuits. Operational testing revealed the elevator trim tab moved opposite to the trim wheel input: turning for nose-up caused the tab to move up, increasing nose-down tendency. The drive chain was correctly engaged on the sprocket and the chain guard fitted, but the cable and chain assembly in the horizontal stabiliser was rotated 180°, reversing the tab movement.
Maintenance History
Before the flight, the aircraft underwent extensive maintenance, including removal of the elevators, rudder, horizontal and vertical stabilisers to replace the horizontal stabiliser's forward spar. The Cessna 172 Maintenance Manual (AMM) includes a warning: "Be sure trim tab moves in correct direction. Nose down trim corresponds to tab up." Mechanics stated they referred to the AMM and found no abnormalities during post-installation checks. However, a duplicate inspection of the elevator trim system was not required under FAA regulations and therefore not performed.
Probable Cause
The elevator trim system was rigged incorrectly, causing the trim tab to move opposite to the pilot's control inputs, due to the cables and drive chain being rotated 180° from their normal position during maintenance.