What happened
On 10 June 1999, a Boeing 757-236, registration G-BIKG, was conducting a scheduled public transport flight from London Heathrow to Edinburgh. While cruising at FL 350, approximately 55 nm southwest of Newcastle, the flight crew was alerted by an audio warning siren and a 'Cabin Altitude' alert on the Engine Indication and Crew Alerting System (EICAS).
Upon noticing the warning, the commander ordered the flight deck crew to don oxygen masks. The cabin altitude was observed to be exceeding 15,000 feet, and the outflow valve indicator showed the valve was fully closed. To manage the depressurization, the crew switched the passenger oxygen to 'ON' and the second pilot manually operated the cabin altitude control to confirm the valve position. The commander initiated an emergency descent and issued a 'MAYDAY' call to Air Traffic Control, who cleared the aircraft to descend to FL 100.
During the descent, the crew utilized the Quick Reference Handbook (QRH) to verify memory drills. Once the aircraft leveled at FL 100, the senior cabin crew member reported no injuries to passengers and no visible damage to the cabin. As the cabin altitude stabilized between 4,000 and 5,000 feet, the crew removed their masks, downgraded the distress call to a 'PAN', and proceeded to the destination.
The investigation
Investigators examined the aircraft's recent maintenance history and found that the Cabin Altitude Control System had previously struggled to maintain pressure at FL 370. While leaks at several main entry doors had been repaired, the No 2 air conditioning pack had been 'locked out' prior to this flight due to a heat exchanger leak, leaving the aircraft operating with only the No 1 pack in high flow mode.
Subsequent inspections of G-BIKG following the incident revealed that the equipment cooling overboard exhaust valve was stuck in the 'smoke' position. Further investigation identified several additional airframe pressure leaks that caused the cabin pressure to drop at a rate exceeding 2,000 feet per minute when the aircraft was at 4 psi differential pressure.
Findings
- The primary cause was the inability of the single operational air conditioning pack to maintain cabin pressure due to excessive air leakage through the airframe.
- The aircraft was dispatched with one air conditioning pack inoperative, a configuration permitted by the Despatch Deviation Manual (DDM), but the remaining pack could not compensate for the existing leaks.
- A defect in the equipment cooling overboard exhaust valve contributed to the loss of pressure.