LOS ANGELES, CA, USA
N540UA
BOEING 757-222
The aircraft was climbing through 25,000 feet in light turbulence when the crew felt two jolts and heard a loud explosive noise followed by a sharp roll to the left. A visual inspection revealed that the left overwing emergency escape slide deployed and separated in flight. The crew returned to Los Angeles and made an uneventful landing. Post incident examination revealed that the slide compartment door was unlatched and open, and, the adjacent maintenance access door was open, with the latching handle in the unlocked position. The flight prior to the incident one had experienced two EICAS warning messages concerning the left overwing slide door. After landing, maintenance personnel accessed the compartment, cleaned a proximity switch then functionally tested the system. The flight was then dispatched with no open items. The maintenance closing procedure in effect at the time of the incident called for one mechanic to hold the bottom corners of the 33 inch wide slide door closed while manipulating the latching handle in a maintenance access door 12 inches aft of the slide door. The procedure has since been changed to require two mechanics, one to hold the door closed while the second manipulates the latching handle.
On June 8, 1993, at 1059 Pacific daylight time, a United Airlines Boeing 757-222 airplane received minor damage from the in flight deployment of a left overwing emergency slide. There were no injuries to the 184 passengers nor the crew of 13. The aircraft returned to the departure point, Los Angeles International Airport, and made an uneventful landing at 1117 hours. The aircraft had departed from Los Angeles as flight 382 to Denver Colorado. During the climb to cruise at 25,000 feet the captain stated that they experienced some light turbulence at which time they felt two hard jolts and heard an explosive noise followed by the aircraft rolling sharply to the left. A visual inspection by an observer member of the crew concluded that the slide had deployed and separated in flight. The captain of flight 382 reported that: "At takeoff rotation from LAX an EICAS advisory message 'L W SLIDE' appeared and remained on. As this had occurred and been investigated on previous flights with no discrepancy found, decided to continue on to Denver and log problem for maintenance." Post incident examination of the aircraft after landing at Los Angeles revealed that the left overwing slide door was unlatched and open with the slide missing. Further examination revealed that the nitrogen supply bottle for the slide was still full and the launching squids had not activated. There was minor damage to the composite fairing panel aft of the slide compartment. The external slide door ground/access door was noted to be open with the push latches unlatched. The maintenance operating handle inside was found in the unstowed position. Examination of the fuselage and the horizontal stabilizer revealed light scratching and scraping to: the fuselage paint; cabin windows; top and bottom of the horizontal stabilizer; and minor damage to the top side of the inboard wing flaps composite structure. Records from the previous flight were examined. They indicated that an intermittent EICAS message for the left rear overwing emergency exit light had illuminated twice during the descent into Los Angeles. United maintenance personnel at Los Angeles verified that the signal had come from the left rear overwing emergency exit. According to the maintenance records, mechanics disarmed the off wing emergency slide system, then accessed the slide compartment for inspection. The proximity switch that was faulted was cleaned of residue, lubricated and a functional system test satisfactorily completed. The aircraft was then dispatched as flight 382 to Denver with no open or deferred maintenance items. The off wing escape system on the Boeing 757 allows passengers to exit the airplane through four emergency exit hatches; two over each wing. When either the forward or the aft emergency hatch is opened from inside the airplane, an electrical circuit is completed from the 28 volt dc hot battery bus to the slide inflation system. The slide inflation system deploys the escape slide allowing passengers to safely reach the ground. If the electrical system fails to deploy the slide, the slide can be manually deployed by pulling a handle located in the sill of each emergency hatch. The slide inflation system uses pressurized nitrogen to inflate the slide with a 75% air and 25% nitrogen mixture. The off-wing escape slide door latch tube locks the slide compartment door closed. The latch tube is located along the lower edge of the slide compartment and extends through the bulkhead between the slide compartment and the pneumatic actuator, where it attaches to the crank-arm assembly. When the escape system is activated, the piston cap on the pneumatic actuator is driven forward, striking a roller cam on the crank-arm assembly. The crank arm is forced to rotate aft, pulling the attached door latch tube aft, and unlocking the slide compartment door. This same motion of the crank-arm rotates the carrier-lock bellcrank aft, and unlocks the carrier. The instructions for opening and closing the slide compartment door are only shown on the maintenance access door on a four by nine inch placard. The maintenance access door is located about 12 inches aft of the slide compartment door and houses the slide compartment latching handle. The slide compartment door is blindly latched with the use of a slide tube that captures two "mushroom" latches, one at each lower corner of the door. At present, both sides of the 33 inch wide slide compartment door must be held in place while the latching handle behind the maintenance access door is manipulated to capture both mushroom latches. Both the United and Boeing maintenance procedures call for this latching procedure to be accomplished by one mechanic. Repeated trials during the investigation revealed that, because of the size of the slide compartment door (33 inches wide) and it's flexibility, it is possible to not engage both latches upon closing when only one mechanic is attempting the procedure. Investigation of United and Boeing records revealed details of two other UAL 757 slide door incidents which occurred during the first flights following "A" checks. The incident reviews revealed that the occurrence of the two previous incidents did not result in slide deployment or loss. 1) UAL Flight 162, January 23, 1993, N523UA: Aircraft aborted takeoff at Seattle at approximately 120 knots following EICAS door warning. Seattle maintenance found the carrier incompletely latched. Investigation revealed that the compartment had been accessed prior to the flight by Seattle maintenance to document the installed slide part number. 2) UAL Flight 362, January 26, 1993, N559UA: Aircraft departed Portland following a maintenance check of slide compartment for documentation of slide installed part number and proceeded to Chicago uneventfully. After landing at Chicago, an EICAS door warning illuminated and maintenance found an incompletely latched carrier assembly. No scheduled maintenance in the overwing slide compartment is called out at "A" checks. The maintenance performed prior to the above incidents followed a non-routine company engineering order to record component part number information in the compartment. As a result of the three incidents, United has revised their off-wing escape slide compartment door closing procedure for the 757 aircraft, and created a one-time fleet inspection of the aft carrier lock fitting for signs of interference with the release pin. The new procedure will require two mechanics, one to hold the slide door in the closed position while the second manipulates the latching handle in the maintenance access door. According to Boeing representatives, the company has planned a revision to the maintenance manual with a proposed effective date of September 20, 1993. The revision will include documenting the door closing procedure which had only been covered by the four by nine inch placard on the maintenance access door. At present, Boeing plans to retain the single mechanic closing procedure currently in effect.
the inadvertent deployment of an overwing emergency excape slide due to the inadequate latching of the slide compartment door following access by maintenance personnel. A factor in the accident was the inadequate door closing procedure specified by the manufacturer and the airline in the maintenance instructions.
Source: NTSB Aviation Accident Database
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