What happened
On 13 August 1998, a Boeing 737-204 ADV, registration G-SBEB, was operating a scheduled public transport flight from London Gatwick to Dubrovnik. While cruising at FL350 between Ostend and Dover, the aircraft suffered a rapid depressurisation. The first officer, noticing a significant rise in the cabin rate of climb indicator, attempted to switch to the standby pressurisation system, but the system failed to respond.
As the cabin altitude rose, the first officer donned an oxygen mask. The commander, attempting to don his own mask, accidentally knocked his spectacles to the floor; while attempting to retrieve them, he lost consciousness. The first officer, observing the commander' and the decreasing airspeed, initiated an emergency descent and issued multiple MAYDAY calls to Maastricht ATC.
In the cabin, the senior cabin crew member reported a loud bang and a rush of air. While passenger oxygen masks deployed automatically, the sudden pressure change caused the senior crew member to collapse on the flight deck after attempting to assist the commander. The first officer managed to provide oxygen to the commander, eventually regaining the commander's consciousness, though the pilot remained unaware of his period of unconsciousness until after landing. The aircraft successfully performed an emergency descent and landed at Gatwick Airport. Five people sustained minor injuries during the event.
The investigation
The AAIB investigation focused on the structural integrity of the aircraft's aft cargo door. Engineering examinations revealed a crack in the aft cargo door frame that had transitioned from fatigue to a fast tearing mechanism. Metallurgical analysis showed that cracks had also developed in the door's lower stop beam.
Investigators found that the cracks in the beam members had existed for approximately 17 years without detection. Although the aircraft had undergone previous inspections and modifications related to the cargo door, the specific location of the frame crack was obscured by the door's seal and carrier. Furthermore, the investigation noted that the presence of a dissimilar polysulphide compound in the joints suggested that cracks were already present when previous modifications were performed in 1981.
Findings
- The primary cause of the depressurisation was a crack in the aft cargo door frame that progressed via fast tearing.
- Fatigue cracks had also developed in the lower stop beam of the cargo door.
- Previous inspections failed to identify the structural defects, partly because the crack location was difficult to observe with the door installed.
- The commander became incapacitated due to hypoxia after failing to immediately secure his oxygen mask during the rapid pressure change.