What happened
On 14 March 2001, a Boeing 757-236, registration G-BIKW, was conducting a scheduled passenger flight from London Heathrow to Edinburgh. During the approach to Edinburgh, the flight crew initiated a go-around at approximately 500 feet due to an aircraft back-tracking on the runway.
As the crew selected the landing gear to the up position and adjusted the flaps, the left hydraulic system fluid quantity dropped rapidly to zero. This was immediately followed by a low-pressure warning and warnings indicating a disagreement between the selected and actual positions of the flaps and landing gear. Upon reselecting the landing gear to the down position, the crew noted that while the nose and left main landing gear lights were illuminated, the right main landing gear light remained extinguished.
Despite performing Quick Reference Handbook (QRH) procedures and attempting to replace a filament in the indicator, the light remained off. After receiving air traffic control clearance, the aircraft overflew the runway at 3,000 feet, where ATC reported the right gear appeared to be down and locked. The aircraft landed safely, but because the loss of the left hydraulic system rendered nose wheel steering inoperative, the crew stopped the aircraft on the runway and the passengers were disembarked. The aircraft was subsequently towed from the runway.
The investigation
The investigation examined the hydraulic system failure and the landing gear indication discrepancy. Analysis of the Flight Data Recorder (FDR) confirmed that all landing gears were in the down and locked position prior to landing and that the hydraulic pressure loss occurred 30 seconds after the gear was selected up.
Engineers discovered that the source of the hydraulic leak was a crack in the nose landing gear-operated sequence valve body. This valve had accumulated over 25,000 hours of service without a recent overhaul. Investigation into the indicator light revealed that the light cap for the right main landing gear had separated from the assembly due to failed hinge lugs. While the left filament in the assembly had shorted and caused electrical arcing, the cause for the simultaneous failure of both filaments could not be determined.
Findings
- The left hydraulic system failure was caused by a fatigue failure of the nose landing gear sequence valve body across the 'UP' port, which occurred when pressure was applied during the go-around.
- The operator had not implemented a previously issued Boeing Service Bulletin which provided a redesigned valve with a thicker casting to prevent such fatigue failures.
- The failure of the landing gear indicator filaments was coincidental to the hydraulic failure.
- The flight crew's ability to verify the gear position was complicated by the fact that the existing QRH procedures did not specifically account for the simultaneous occurrence of hydraulic failure and gear door light indications.