What happened
On 12 June 2000, a Boeing 747-246B, registration TF-ATB, was performing a positioning flight from Luxembourg Airport to Manston Airport. The aircraft had recently undergone a scheduled A4 maintenance check. During the initial climb, passing approximately 1,500 feet, the flight engineer noticed the 'L WING ESCAPE DOOR' warning light illuminate on the control panel.
Following the Quick Reference Handbook (QRH) procedures, the crew continued the flight as the manual advised proceeding normally despite the warning. During the cruise at Flight Level 2/70, the warning light remained active. Upon landing at Manston, ground personnel alerted the crew that the left over-wing escape slide was missing. The slide was later seen floating in the sea before sinking.
The investigation
The investigation focused on the fairing-mounted escape slide system and the recent maintenance performed on the aircraft. While the maintenance check did not explicitly require work on the escape slide compartment, technicians had removed cabin sidewall panels to inspect door stop support fittings. This work required disconnecting cables related to the manual inflation handles.
Investigators examined the slide compartment and found the door was open. The slide had torn from its mountings, and the inflation gas feed pipe had fractured. The investigation focused on the integrator mechanism, which controls the door's latching. The integrator was found covered in a thick, waxy grease that obscured the alignment marks used to verify the latched position. Tests revealed that in its state prior to cleaning, the spring-loaded latch pin was unable to pull the forward piston back to a secure, dead-stop position.
Findings
- The Boeing 747-246B experienced a failure of the integrator mechanism, which allowed the slide compartment door latches to migrate toward an open position.
- Waxy, heavy grease on the integrator mechanism prevented the spring-loaded latch pin from functioning correctly and obscured visual indicators of the piston's position.
- It is likely that during the removal of cabin sidewall panels for other inspections, the manual release cable was inadvertently moved, pulling the integrator forward into an unsafe, unlatched state.
- The lack of a requirement to check the integrator position following maintenance meant the unsafe condition went undetected before takeoff.