What happened
On 10 August 2011, a Piper PA-28R-180 Cherokee Arrow, registration G-AVWO, was performing a private flight at Biggin Hill Airport. During the approach, the pilot extended the landing gear and observed that all three green 'down and locked' lights were illuminated on the instrument panel. Pre-landing checks confirmed the gear position.
Upon touchdown, the left main landing gear collapsed. While the green indication lights remained illuminated, the 'in transit' light also lit up. As the aircraft slowed, the left wing made contact with the runway. The pilot managed to lift the wing using aileron inputs while simultaneously shutting down the engine and electrical systems, bringing the aircraft to a halt. The pilot sustained no injuries and exited the aircraft normally.
The investigation
Investigators examined the aircraft's hydraulic and mechanical systems to determine why the gear failed. The investigation focused on the retractable landing gear mechanism, which utilizes hydraulic actuators and mechanical downlocks.
Maintenance records were reviewed, showing no recent removal of the main landing gear actuators for overhaul. Testing of the extension and retraction system confirmed the limit switches were correctly rigged and the system appeared to operate normally under standard conditions. However, a detailed inspection of the left main landing gear actuator revealed a small hydraulic leak occurring between the piston and the seal when the piston was fully extended. This leak was linked to specific wear on the chrome plating of the actuator piston.
Findings
- The primary cause of the gear collapse was wear on the left main landing gear actuator piston.
- This wear allowed a hydraulic leak to develop when the piston was in the extended position.
- The wear prevented the actuator piston from fully engaging the downlock hook on the lock-pin against flight loads.
- Because the hook partially engaged the limit switch, the cockpit indicators incorrectly showed the gear as 'down and locked' while the 'in transit' light was also active.
- The impact forces during touchdown caused the partially engaged downlock hook to disengage from the pin, leading to the collapse of the left main gear.