History of the Flight
The handling pilot was undergoing training for a type conversion and renewal of his multi-engine instrument rating. He was in the left-hand seat, and the commander (instructor) was in the right-hand seat. After completing power and pre-takeoff checks, the handling pilot taxied the aircraft past the holding point to line up on Runway 29. At a reported groundspeed of approximately 5 to 7 kt, the nose gear leg suddenly collapsed. The aircraft slid approximately 30 feet and came to rest with its nose and both propellers touching the ground. The commander reported that before shutting down, both he and the handling pilot confirmed the gear selector was in the down position and the main gear showed '2 greens'. The aircraft was shut down, and both occupants evacuated via the main door.
Maintenance History
The aircraft had a previous accident on 2 December 2001 when it overran a runway, causing a nose gear collapse. Temporary repairs allowed it to be flown to Sherburn in September 2002 for major repair and an annual inspection. The nose gear repair included reassembling the drag link and replacing the downlock spring link with a serviceable second-hand part. The annual inspection included functional checks of the landing gear, which were found satisfactory, and was completed in April 2003. On 30 April 2003, a test flight revealed problems with the gear free-fall test: the nose gear locked down on the first attempt but failed on two subsequent attempts, while the left main gear did not lock down during any of the three attempts. The maintenance organisation lubricated the left main and nose gear linkages but did not perform subsequent free-fall tests. On 3 May 2003, during a flight to Guernsey, the gear in-transit light illuminated. The pilot cycled the gear approximately five times, but only two 'greens' appeared; the right main gear indicator did not show a 'green'. The aircraft diverted to Bournemouth, where a broken wire in the right main gear down-lock system was found and replaced. Free-fall tests on the ground at Bournemouth showed the left main gear would not lock down, attributed to a bent landing gear actuator. No work was done on the nose gear at Bournemouth. The aircraft flew to Guernsey and then returned to Sherburn two days later. The maintenance organisation at Sherburn was advised of the left main gear problem; a new actuator was ordered but no work was done before the accident flight.
Nose Gear Examination
After the accident, extension and retraction tests were carried out under supervision on behalf of the AAIB. When the nose gear was extended, the drag link did not move to the over-centre position; it was under-centre with a 0.10 inch gap at the centre pivot point. The microswitch indicated 'down and locked' despite the under-centre condition. The downlock spring link slot was enlarged, and the lock-nut was loose. The drag link and downlock spring link were removed for further examination. The downlock spring link was slightly bent, and the slot enlargement appeared to be from overload crushing damage, possibly sustained during the collapse, though pre-existing wear may have been masked. The drag link required some force to open and close; the torque on the nut attaching the upper and lower drag links was measured at approximately 50 in-lb, which prevented free movement. A drag link from another Seneca produced the same restriction above 20 in-lb. No other anomalies were found.
Investigation Findings
The investigation considered possible causes. Both pilots reported no hand near the gear selector at the time of collapse. The squat switch, which isolates landing gear hydraulics on the ground, operated correctly. The two main gears remained down and locked, ruling out inadvertent gear-up selection. The aircraft had recently undergone extensive repair involving the drag link and downlock spring link. The drag link was over-tightened, resisting movement into the over-centre position. The manufacturer opined that excessive play in the downlock spring link could allow the drag link to move out of over-centre before the downlock link moved out of its own over-centre position, resulting in an excessive load and possible collapse.
Probable Cause and Safety Action
The probable cause of the nose gear collapse was a mis-rigged nose gear downlock spring link. One safety recommendation concerning the Seneca Maintenance Manual was made to the Federal Aviation Administration of the USA.