What happened
On 22 May 1999, a Boeing 757-236, registration G-BIKL, was operating a scheduled public transport flight from Stockholm to London Heathrow. The aircraft was cruising at FL310 when the crew received an 'L ENG BLEED OFF' alert on the Engine Instrument Crew Alert System. This failure triggered a rapid increase in cabin altitude, which climbed at a rate of approximately 2,000 feet per minute.
In response to the depressurisation, the crew requested an immediate descent from Copenhagen ATC and subsequently declared a 'PAN' to request a further descent to FL100 and a diversion to Hamburg. During the descent, the crew performed the necessary non-normal checklists. The cabin altitude reached a maximum of 9,000 feet before the crew selected the 'Isolation Switch' to 'On', which restored pressurisation stability. The aircraft landed safely in Hamburg without any injuries to the 117 passengers or 8 crew members.
The investigation
Investigators examined the Flight Data Recorder (FDR) and the aircraft's technical logs. The FDR revealed that the descent was quite rapid, reaching 6,000 feet per minute. Data showed that the left environmental control system pack initially remained in 'hi-flow' but eventually switched to 'off' during the descent. The isolation valve eventually opened, allowing the left pack to return to 'hi-flow' mode.
Technical logs indicated a history of 'left bleed off' reports in cruise, though previous checks had failed to identify a specific fault. A recent replacement of a left engine speed card had been recorded on 18 May. While subsequent inspections of the left-hand system following the incident found no definitive defects, the pressure regulating and shut-off valve, along with the reverse flow check controller, were replaced as a precautionary measure.
Findings
- The aircraft was operating with a known unserviceability in the right air conditioning pack, which was permitted under the Dispatch Deviation Manual (DDM).
- The failure of the left engine bleed system caused a loss of air supply to the remaining functional pack, leading to the gradual depressurisation.
- Existing operator systems for detecting repeat defects were unable to identify the specific pattern of intermittent faults recorded in the aircraft's history.
- The crew successfully managed the emergency by following established procedures and diverting to the nearest suitable airport.