History
Following an uneventful positioning flight, the aircraft landed on the tarmac portion of Runway 22 at Stapleford Airfield. The landing was performed with full flaps and full aft elevator. After the main wheels touched down softly, the nosewheel was lowered to the ground. The pilot applied gradual braking while reapplying full back elevator. Despite this, the nose continued to lower until it made contact with the runway. The pilot immediately shut down both engines and the electrical system, closed the fuel cocks, and exited the aircraft after it slid to a stop on the tarmac.
Examination
The aircraft was recovered and the nose landing gear was pulled down. Engineers discovered that the upper, adjustable eye end of the lower downlock link assembly had broken in its threaded portion. The nose gear was subsequently made safe by bracing the drag link into the over-centre position. Further examination by the AAIB revealed that the upper eye end of the sprung lower downlock link had failed in a single bending overload event at the point where its adjustment threads entered the lock-nut at the top of the link body. The cross-pin and the slot in the skirt of the link body exhibited evidence of high compressive axial loading, including severe bearing crushing of the upper edge of the aperture and bending of the cross-pin. Wear marks indicated that the link had been extended to its maximum length in service, but it had been fitted new only five flights prior to the accident. There was also evidence of distortion of the upper downlock link, consistent with severe upward loading of the downlock link centre pivot.
Analysis of Damage
The observed damage to the nose landing gear down-locking system components was consistent with a high compressive load having been applied to the downlock link. Under normal static conditions, with the mechanism free of significant wear and correctly adjusted, the downlock is required to sustain only a light compressive load. However, larger compressive loads can be generated either through misrigging of the lower downlock link length or via shock loads transmitted from the oleo strut. The report noted two potential rigging sources of high compressive loads: if the lower link is adjusted too long, the actuator driving the drag link into the locked position will put compressive load into the downlock links after the stops contact; if the lower link is too short, the drag link assembly may move into an under-centre position, allowing the gear to collapse under weight. Because of the distortions and failures, it was not possible to determine the pre-collapse rigging of the gear.
Conclusion
The accident investigation did not state a probable cause for the failure.