History of Flight
The aircraft, a Boeing 747-246B registered TF-ATB, was collected by the crew at Luxembourg Airport following completion of a scheduled A4 maintenance check. The Technical Log listed no unserviceabilities, and the external inspection and pre-start checks were normal. Prior to engine start, the flight engineer closed the aircraft doors and armed door L1. No abnormalities were observed until after initial climb. As the aircraft climbed through approximately 1,500 feet amsl, the flight engineer saw the 'L WING ESCAPE DOOR' light illuminate on his panel. The crew experienced no handling issues and verified that pressurisation was normal. The Quick Reference Handbook (QRH) directed that with this light illuminated, the crew should 'Proceed normally' and noted that the 'Affected Door 3 Off-wing slide May Not Deploy'. Following the QRH, the crew continued to Manston Airport. Throughout the cruise at Flight Level 270, the light remained on. A normal landing was made, and ground personnel informed the crew that the left over-wing escape slide was missing. The slide was later observed floating on the sea but sank before recovery.
Description of the Door 3 Escape Slide System
The passenger cabin over-wing (No.3) main doors on both sides were equipped with a two-piece off-wing escape slide system. The first part, housed in a pack at the base of the door interior trim, deployed aft alongside the fuselage to the flaps. The second part, located in the wing-to-fuselage fairing over the forward edge of the flaps, also deployed aft. Together they provided a means of descent during emergency evacuation. This incident involved only the fairing-mounted portion. The fairing-mounted slide was attached to the inside of the slide compartment door. Inflation was initiated when the door opened, causing an independent linkage to fire a cool gas generator that drove two injector pumps to fully inflate the slide in about ten seconds. The integrator mechanism, situated in a separate compartment forward of the slide, controlled the opening. Forward movement of the integrator rear piston unlatched the door and fired door deployment thrusters. The integrator also served as a safety dead stop. An interlock system prevented the integrator compartment door from closing if the integrator was not fully latched. The integrator was operated via a teleflex cable from a bellcrank in the main landing gear well. Initial movement of the bellcrank withdrew the spring-loaded integrator latch pin via a slack motion mechanism on the forward piston. Continued rotation drew the rear piston, which pulled four latch sliders forward to unlatch the door. The latch sliders had a neutral dwell area at each end of their stroke, requiring appreciable forward movement from the fully closed position before release began. Once in the releasing sector, outward forces on the door assisted opening. A microswitch near the rearmost latch operated the 'WING ESCAPE DOOR' indication in the cockpit.
Maintenance
The aircraft had just undergone an A4 check, scheduled at a maximum interval of 1,300 flight hours. This check included a test of the door warning system on all main cabin doors but did not require opening the fairing-mounted escape slide compartments or testing indications from the escape slide panel mechanisms. Since the slide compartment doors were not disturbed, there was no requirement to check the integrators. Additional work included Airworthiness Directive 98-26-13, which required inspection of the main door stop support fittings. To inspect those on the forward edges of the No.3 door apertures, the cabin sidewall lining panels just forward of these doors had to be removed. These panels were directly aft of the enclosure containing the fairing-mounted escape slide manual controls (manual deployment and ditching deactivation handles). The procedure for removing the lining panels requires disconnecting the cables to these handles. Work cards indicated the forward panels had been removed, but no specific mention was made of disconnecting or reconnecting the escape slide manual controls. Personnel who performed the maintenance reported no problems or discrepancies in that area.
Examination and Investigation
Examination of the aircraft revealed the fairing-mounted slide door, aft of main door 3L, was open. The slide had torn off its mountings and its inflation feed pipe had fractured at the bifurcation. The integrator mechanism was found fully unlatched, with both pistons well forward of the latched position and the four slide compartment door latches fully released. Neither door deployment thruster had fired, but the cool gas generator had discharged. There was no evidence of automatic or manual activation. The fairing door unlatching thruster had not fired, and the manual inflation handle appeared fully stowed with the lockout pin engaged. However, when the door was opened, the lockout pin had not released due to stiffness in the lever and cable mechanism. The operating cable from the manual inflation handle to the bellcrank was slack. Examination of the bellcrank in the left landing gear well showed the clevis pin of the fairing door unlatching thruster at about the mid position of the bellcrank slack motion slot, consistent with the bellcrank having rotated forward through about half its range. Undisturbed grease was present in the full length of the slack motion slot of the manual slide inflation mechanism.
The integrator was heavily covered with waxy grease that obscured the black lines marking the fully latched position of the forward piston. An attempt to reset the mechanism resulted in the spring-loaded latch pin not drawing the forward piston back to allow the interlock to achieve a safe condition. In the position reached, the door latch mechanism could be pushed open with relative ease. When the forward piston was manually pushed back to its correct latched position, the latch pin moved correctly and acted as a dead stop. After cleaning the integrator with solvent, it was found to be in very good condition and reset correctly, with the door latching mechanism becoming positively locked. The black lines indicating the latched position were no longer present. The setting of the door latch unsafe microswitch was established: the movement required to activate the warning light was considerably less than that of the latch sliders before the unlatching process began. Measurements showed that the forward piston could be drawn forward 0.115 inch before slack motion was consumed, and at that position the spring-loaded positive latch plunger was already sufficiently close to the top of its disengaging ramp to disable the integrator's dead stop function.