History of the Flight
The aircraft, a Boeing 757-236 registered G-BIKW, was operating a scheduled passenger flight from London Heathrow to Edinburgh. On approach, the landing gear was selected down and three green 'down and locked' lights illuminated. A go-around was initiated from 500 feet agl due to an aircraft backtracking the runway. During the go-around, the flaps were retracted from 30° to 20° and the landing gear was selected up. Immediately, the left hydraulic system fluid quantity indication dropped to zero, followed by a left hydraulic system low pressure warning and flap and landing gear position disagree warnings. When the landing gear lever was reselected down, the nose landing gear and left main landing gear green lights illuminated, but the right main landing gear light remained off. Cycling the lever several times did not resolve the issue. The flight crew performed Quick Reference Handbook procedures for left hydraulic system failure and alternate landing gear lowering, but the right main landing gear green light remained off. The flap and landing gear disagree warnings later extinguished. After replacing one of the two filaments in the right main landing gear indicator with a spare, the light still remained off. ATC visually confirmed from a flyover at 3,000 feet that the right main landing gear appeared fully down. A flaps 20° approach was flown and the aircraft landed safely. Nose wheel steering was inoperative due to the hydraulic loss, so the crew stopped the aircraft on the runway and shut down engines. Passengers were disembarked, and the aircraft was towed. An engineer on the runway confirmed all landing gear were locked down by inserting locking pins without difficulty.
Landing Gear System Operation and Indication
The left hydraulic system powers landing gear and gear door operation. If it fails, the gear cannot be raised and doors cannot close; the gear can be lowered using the alternate system, which releases uplocks and depressurizes the hydraulic circuit, allowing free-fall. Proximity sensors on locking links and doors send signals to the Proximity Switch Electronic Unit, which drives the green 'down and locked' lights and amber 'gear doors open' lights. Each indicator light has dual filaments from independent power sources.
Flight Data Recorder Information
FDR data showed all landing gears were down and locked, and doors were open prior to landing. The left hydraulic low pressure warning occurred 30 seconds after selecting the gear up.
Engineering Investigation
The hydraulic leak originated from a fatigue crack in the nose landing gear gear-operated sequence valve body across the 'UP' port. This valve, with 25,985 hours and 28,234 cycles, had not been overhauled. A Service Bulletin issued in 1988 introduced a redesigned valve with thicker casting to address fatigue failures; the operator had not adopted it due to no prior significant problems. The right main landing gear indicator light cap was found protruding with neither filament illuminated. Pressing it back lit the right filament. The light cap had separated due to failed hinge lugs. Replacing the cap and filaments only lit the right filament; full assembly replacement rectified the defect. The base showed the left filament power contact pin loose, with evidence of arcing, indicating a short that welded the spring. However, why both filaments failed remained unclear. The operator had previously experienced ten similar dual-filament failures in five years, sometimes prompting go-arounds. A more reliable LED light cap is available.
Discussion
The left hydraulic system loss resulted from fatigue failure of the sequence valve body, a known issue addressed by the Service Bulletin. The two right main landing gear indicator failures were coincidental and unrelated. The QRH procedure for missing green lights assumes no EICAS messages or door lights, but here amber door lights were on, creating a multiple-failure scenario not covered. Once hydraulic pressure was lost, cycling the gear lever had no effect; the gear would have fallen to the down and locked position. The operator continues to monitor the valve and plans to replace indicator filaments with LED light caps.