31st of October 2014, Slide away, Blog #757
On 31 October 2014, a Boeing 757-3CQ, registration G‑JMAB, departed London Gatwick Airport on a scheduled passenger flight to Hurghada, On board the aircraft a crew of ten and 239 passengers. Egypt. Shortly after the take-off roll began, the crew received a RIGHT WING SLIDE advisory message on the aircraft's Engine Indication and Crew Alerting System (EICAS). At the time, there were no unusual aircraft handling characteristics and no reports from the cabin indicating anything abnormal.
The crew elected to continue the departure, troubleshoot the warning in accordance with the Quick Reference Handbook, and assess the situation once safely airborne. As no additional indications were present, the warning was considered likely to be spurious. After consultation with the airline’s operations, the decision was made to return to Gatwick.
As the aircraft was above its maximum landing weight, it entered a holding pattern to burn fuel before initiating an approach and landing.
After approximately 40 minutes in the hold enough fuel was burned, with the weight reduced sufficiently, the aircraft was vectored for an approach. During descent, several cabin crew members and passengers heard loud bangs and felt a brief shudder through the airframe. Some passengers also reported seeing a white object separate from the aircraft on the right-hand side.
As the crew configured the aircraft for landing, the captain noticed an increasing tendency for the aircraft to roll to the right. The autopilot was maintaining control, but once flap 30 was selected, a significant amount of left aileron input was required to keep the wings level. The captain disconnected the autopilot and manually flew the remainder of the approach. Despite the unusual handling characteristics, the aircraft landed safely without injury to the 239 passengers and 10 crew members on board.
After the aircraft was parked the airlines technicians found that the right over-wing escape slide had detached from the aircraft. The slide carrier had deployed from its stowed position, the access panel was open, and the slide itself had unfolded in the airflow before eventually separating completely. Damage was found to the slide carrier, actuator, pivot forging and surrounding fuselage structure.
The detached slide was later recovered from a tree and was found to be largely intact, apart from damage sustained during its separation from the aircraft.
The Air Accidents Investigation Branch (AAIB) was alerted and their investigators examined both the slide system and the maintenance history of the aircraft.
The escape slide installation includes multiple safeguards designed to prevent accidental deployment. Examination revealed that the slide panel latching system had not been correctly secured. Evidence showed that one of the latch tappets was not properly engaged, increasing the likelihood that the panel could open in flight.
A further and more significant finding involved the slide release crank handle. This handle was found in the release position, allowing the carrier locking mechanism to disengage. Investigators concluded that the handle had most likely migrated gradually towards the unsafe position over time rather than moving suddenly.
Complicating the investigation was the fact that maintenance personnel had replaced the slide system's inflation bottle the night before the incident. Although no evidence suggested that procedures had been intentionally bypassed, investigators could not rule out the possibility that the crank handle had been left slightly out of position during the work. The absence of alignment markings, intended to help engineers verify the correct handle position, meant this condition could easily go unnoticed.
The investigation identified another factor that may have played a role.Another factor that might have been a contributing factor was discovered during a subsequent maintenance check. Technicians discovered a loose flap-track attachment and a missing trunnion bush in the No. 6 flap screwjack assembly. For some time, crews had been reporting vibration and droning noises in the area around the right-over-wing slide compartment. The investigators believed these vibrations may have encouraged the partially unsecured crank handle to gradually move towards the release position until the slide carrier eventually deployed in flight.
Boeing, as the manufacturer, had previously received reports of similar events across the Boeing 757 fleet. Several operators had reported in-flight losses of over-wing escape slides caused by improperly secured slide compartment mechanisms.
As a result, Boeing had issued Service Bulletin 757-25-0298, later mandated through an FAA Airworthiness Directive. The modification introduced improved locking arrangements and more visible alignment markings. However, the modification had not yet been embodied on aircraft at the time of the occurrence.
The AAIB concluded that the right over-wing slide carrier deployed in flight, allowing the slide to unravel possibly because of the crank handle with a reduced breakout friction progressively moving, over an indeterminate period of time, to an unsafe position. A contributory factor was possibly the loose number 6 screw jack in the flap system which resulted in vibration in the area of the crank handle. The insecurity of the lever went undetected whilst the maintenance panel was open due to the lack of alignment marks and unfamiliarity of the observer(s) with how the crank handle should look when correctly positioned. SB 757-25-0298 addresses locking of the compartment door and provides revised and clearer alignment placards for the lever.
The AAIB report, which served as the source for this blog, can be accessed by clicking on the .pdf file below.
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