What happened
On 22 February 2003, a Piper PA-34-200T Seneca II, registration G-BNEN, was performing a passenger flight from White Waltham Airfield to Cambridge. During the takeoff roll on the grass runway, the aircraft reached a speed of approximately 70 kt when the nose section suddenly dropped. The pilot was unable to arrest the nose-down pitch using the elevator controls.
As the nose descended, both propellers struck the runway surface, causing severe damage to the propeller blades and shock-loading the engines. The aircraft slid along the ground, supported by the main landing gear, while the underside of the nose, engine cowlings, and landing gear doors sustained significant damage. There were no fatalities and no injuries to the crew or the single passenger on board.
The investigation
The AAIB investigation focused on the mechanical integrity of the nose landing gear assembly. Physical examination of the aircraft revealed that the nose landing gear doors and the forward cabin structure had been distorted.
Upon inspection of the downlock link, investigators found that the threaded stem of the upper eye-end had fractured. The investigation also identified significant wear within the mechanism. Specifically, the bronze bush within the upper link showed heavy, non-uniform wear, with the bore becoming bell-mouthed. Furthermore, the investigation looked into recent maintenance performed on the aircraft. Approximately 200 flight hours prior to the accident, the maintenance organisation had adjusted the length of the downlock link to address a known issue where the gear would not lock during a free-fall extension. This adjustment involved shortening the link to assist the retraction spring in pulling the linkage into position.
Findings
- The primary cause of the collapse was the failure of the nose landing gear to remain in the locked position due to the drag link moving into an 'under-centre' condition.
- This instability was triggered by a recent maintenance adjustment that shortened the downlock link, which reduced the 'over-centre' dimension.
- Significant wear in the upper and centre pivots of the drag brace increased the amount of free play within the mechanism.
- The aircraft maintenance manual lacked sufficient guidance for correcting the inability of the nose gear to lock via free-fall.
- The structural flexibility of the aircraft and the dynamic loads applied during takeoff allowed the linkage to move out of its secure, locked state.