What happened
On 17 July 2006, a Piper PA-23-160 Apache, registration G-MOLY, was involved in a ground accident at Little Shelford, Cambridge. The aircraft had completed a routine private flight from White Waltham and had landed on the easterly runway. After taxiing off the runway and coming to a halt, the pilot accidentally moved the landing gear retraction lever.
Upon moving the lever, the left landing gear collapsed. The pilot immediately reacted by shutting down both engines to prevent further damage and attempted to manually extend the gear using the emergency hand pump. While the right landing gear remained extended and locked, the collapse of the left side resulted in damage to the left wing tip, the left flap and its operating mechanism, the pitot mounting, and the left landing gear drag link. There were no injuries to the pilot, who exited the aircraft without difficulty.
The investigation
Investigators examined the aircraft's hydraulic and anti-retraction systems. The aircraft is equipped with a hydraulic system that uses an anti-retraction valve to prevent the gear from retracting while the aircraft is on the ground. This valve is designed to remain in a position that restricts hydraulic pressure from building up in the retraction system until the aircraft is airborne.
An examination of the anti-retraction valve revealed that the operating arm had become stuck in the closed position. This-stuck arm meant the valve remained in the 'FLIGHT' position even while the aircraft was stationary. This failure allowed the hydraulic system to actuate the gear retraction process despite the aircraft being on the ground.
Findings
- The pilot inadvertently operated the landing gear retraction lever while the aircraft was stationary.
- The anti-retraction valve operating arm was stuck in the closed position due to the accumulation of debris and corrosion.
- Because the valve was stuck in the 'FLIGHT' position, the landing gear was permitted to retract while the aircraft was on the ground.
- The failure of the anti-retraction valve was a dormant issue that had likely existed for some time prior to the accident.