What happened
On 16 April 2016, a Piel CP3 and 01B Emeraude, registration G-AZGY, was taxiing from its hangar at RAF Henlow, Bedfordshire, for a private flight. Shortly after the aircraft transitioned from a paved surface onto a grass taxiway, the pilot heard a loud popping sound. As the pilot attempted to stop the aircraft, a second loud pop occurred, and the left wing began to sink toward the ground as the left landing gear leg folded forward. The pilot, who sustained no injuries, was able to exit the aircraft without assistance. The incident resulted in damage to the left landing gear, the wing skin, and one of the wooden propeller blades.
The investigation
Investigators examined the aircraft's main landing gear, which consists of a tubular steel leg attached to the wing main spar via steel plates and three long bolts. The investigation established that the threads on the inboard forward attachment bolt had been stripped, causing the nut to pull off the end of the bolt. This loss of restraint allowed the lower attachment plate to bend, which subsequently caused the landing gear leg to twist outboard and fail just above the attachment plate.
Further inspection of the hardware revealed that the failed bolt appeared to be a poorly manufactured, homemade component. The threads were of poor quality and occupied only a small portion of the bolt's length. Additionally, the nut used was only half-thickness, providing insufficient thread engagement. The assembly was secured using a method of staking the nut and bolt threads with centre punch strikes rather than using washers. The investigation also noted that the aircraft's wing skin lacked the standard access panels usually found on this model, which likely prevented the pilot from performing more thorough inspections of the attachment points.
Findings
- The primary cause of the landing gear collapse was the failure of the threads on a landing gear attachment bolt due to poor manufacturing.
- The use of a half-thickness nut provided inadequate thread engagement.
- The locking method, involving staking the threads with a centre punch, prevented effective tightening and obscured the condition of the hardware.
- The absence of access panels on the wing skin hindered the ability to inspect the landing gear attachments.
- A previous landing on a rutted runway may have contributed to the fatigue of the bolt, though it was not considered the primary cause.