What happened
On 14 August 2004, a Fokker F2lar F28 Mark 0100, registration G-BXWE, was operating a scheduled passenger flight from Brussels to London Heathrow. During the final approach, the crew observed that the nose landing gear 'down and locked' green light failed to illuminate, accompanied by visual and aural 'unsafe' warnings.
Following a go-around, the crew attempted to recycle the landing gear and performed the prescribed alternate landing gear lowering procedure, but the warnings persisted. After consulting with company engineers and notifying air traffic control, the crew declared a 'MAYDAY' and briefed the cabin crew for a potential landing with gear up.
As the aircraft descended through 1,000 feet, a 'LG NOT DOWN' message appeared. Despite the warnings, the aircraft landed at Heathrow without incident. During the landing roll, the nose landing gear indication transitioned to the safe position. The aircraft came to a stop using maximum reverse thrust, and passengers were evacuated via steps.
The investigation
The AAIB investigation examined the aircraft's flight data recorder and the mechanical components of the nose landing gear. While initial troubleshooting in the hangar failed to replicate the defect, a subsequent base maintenance check in September 2004 revealed that the nose landing gear downlock plunger clearance was outside the permitted limits.
Investigators found that the downlock plunger was misrigged, likely following a nose gear replacement in June 2003. The investigation focused on the Aircraft Maintenance Manual (AMM) procedure used to check the plunger clearance. It was determined that the manual was ambiguous regarding the necessity of applying a rearward force to the gear during the measurement process.
Findings
- The nose landing gear downlock plunger clearance was outside the required limits of 0.003 to 0.020 inches.
- The ambiguous AMM procedure did not specify that a rearward force must be applied when checking the downlock plunger clearance, leading to an incorrect measurement during maintenance.
- This lack of clarity resulted in the downlock plunger being misrigged, causing excessive friction that prevented the proximity sensors from triggering the 'down and locked' indication.
- The aircraft had previously experienced a similar nose gear unsafe condition in September 2003.