Incident Overview
On 5 January 2014, an Airbus A380-800 aircraft operated by Singapore Airlines, registration 9V-SKE, diverted to Baku, Azerbaijan, after experiencing a gradual loss of cabin pressure. The flight was en route from London to Singapore with 494 persons on board. The aircraft landed without further incident, and no injuries occurred.
Sequence of Events
The aircraft had operated a flight from Singapore to London on 4 January 2014, during which a loud noise was heard from the third left door on the main deck (Door 3L). A flight crew member inspected the door but found nothing unusual other than the noise. Upon arrival in London, maintenance personnel visually inspected the door, including the door seal, seal guides, rollers, stops, coverplate, and exterior skin. No anomaly was found except a slightly worn seal, which had been noted previously. The defect was recorded in the Aircraft Deferred Defect (ADD) log.
On the return flight on 5 January 2014, the loud noise at Door 3L recurred after takeoff. The cabin crew member stationed at Door 3L felt around the door but did not detect an air leak or vibration. The In-flight Supervisor also checked and found no leak or vibration. The flight crew was informed when the aircraft reached cruise altitude of 37,000 feet; pressurization was normal with a cabin altitude of 6,000 feet. A flight crew member inspected the door and noted nothing unusual besides the noise. The crew sent an ACARS message to Singapore requesting rectification before the next flight. The noise was significant enough that passengers and crew near Door 3L could not hear public address announcements clearly, but the flight crew decided to continue and monitor the pressurization system.
Approximately five hours into the flight, the cabin altitude began to climb slowly from 6,000 feet, indicating a gradual loss of cabin pressure. As the cabin altitude approached 10,000 feet, the flight crew initiated an emergency descent. After the descent, they decided to divert to Baku, Azerbaijan.
Investigation Findings
Laboratory examination of the damaged door revealed that the coverplate (CP) on the upper edge of Door 3L had a fatigue crack originating from a spot weld used to attach the CP to the door skin. The crack propagated and resulted in the separation of the CP from the door. This separation allowed air to leak through the gap, leading to the gradual loss of cabin pressure. The CP was designed to cover the small gap between the door top edge and fuselage to prevent water trapping and icing, and to reduce aerodynamic noise.
Operational Factors
The aircraft had an existing ADD entry for a slightly worn door seal, and the noise issue had been reported on previous flights. Maintenance inspections did not identify the crack in the CP. The flight crew's decision to divert was based on the increasing cabin altitude.
Safety Recommendations
The Transport Safety Investigation Bureau issued safety recommendations aimed at improving detection of such cracks and reviewing procedures for en-route alternate aerodromes in the context of rescue and firefighting services.