Pressurisation Loss Incident During a Boeing 757-236 Diversion (G-BIKL)

Overview and Service Context On 22 May 1999, a Boeing 757-236, registration G-BIKL, departed Stockholm for London Heathrow as a public transport flight. The aircraft was operated with its right air-conditioning pack unserviceable. Following previous checks, the carrier permitted the flight under its Dispatch Deviation Manual, with instructions to limit cruise altitude to FL350 or below. A new crew had just taken control of the aircraft for the sector, and the reported pre-flight status showed no additional restrictions beyond the one unserviceable item. ## Event Sequence After reaching FL310, the crew received an Engine Bleed Off indication on the EICAS panel and observed cabin altitude climbing at approximately 2,000 feet/min. The commander requested an immediate descent from Copenhagen ATC, and the aircraft was initially cleared to FL250. As the descent continued, the crew declared a PAN and requested FL100, with a diversion to Hamburg. The aircraft continued to descend until pressurisation recovered, which the crew noted occurred around FL140. At the time, the maximum cabin altitude displayed was 9,000 feet. Under Bremen ATC control, the PAN was cancelled while the diversion to Hamburg was continued. ## Recorded Data and Engineering Findings The FDR showed the event occurred 49 minutes after reaching cruise altitude. The aircraft was level at FL310, at 290 kt IAS, with the left ECS pack in high flow and the right pack off. A two-VHF transmission pattern identified the onset, followed by a 15-second pause and then a descent to FL100 at 6,000 feet/min. The left pack moved out of high flow and then off approximately one minute after the descent began. Two minutes after reaching FL100, the isolation valve opened and the left pack returned to high flow. The FDR recorded no warnings, and the cabin altitude 10,000 feet discrete did not activate. The cockpit voice recorder was not removed for replay because its 30-minute duration was insufficient to retain the relevant segment. ## Operational Implications The technical log contained multiple reports of left bleed off indications in cruise without faults found on checks. A later self-test result on 18 May indicated a fault in the left engine speed card, which was replaced. The investigation concluded that failure of the left engine bleed system caused loss of air supply to the remaining pack, resulting in gradual depressurisation. The operator noted that its repeat-defect detection systems would not have flagged the pattern seen in G-BIKL’s technical logs. In response, changes to the DDM were drafted to emphasise review of defect history.