What happened
On 2 March 2009, a Boeing 757-236, registration G-LSAA, was performing a commercial passenger flight from Manchester to Taba, Egypt. While climbing from FL370 to FL390 near Athens, Greece, the flight crew heard a popping sound and felt a sudden pressure change. The co-pilot observed the cabin altitude rising rapidly, at an estimated rate of 4,000 feet per minute, and noted a cabin differential pressure of approximately 9 psi.
In response to the rising cabin altitude, the crew donned oxygen masks and transitioned the pressurisation system to manual mode, closing the outflow valve. The commander initiated an emergency descent to a lower altitude. During the event, the crew observed EICAS warning messages, including a cabin altitude alert. After the descent, the aircraft diverted to Athens Airport. Upon arrival, the crew found that the aircraft doors could not be opened because the internal and external pressures had not equalised, a result of the outflow valve remaining closed.
The investigation
The investigation examined flight data recorder (FDR) parameters, which confirmed that the cabin altitude exceeded 10,000 feet for 108 seconds between 38,700 and 33,700 feet. Post-flight maintenance inspections by the operator identified that a cabin positive pressure relief valve (CPRV) had opened.
Subsequent testing of removed components revealed that one CPRV operated at pressures significantly lower than specified. Additionally, a cabin pressure sensor on the active controller was found to be slightly out of tolerance. The investigation also noted that if the crew had followed the Quick Reference Handbook (QRH) checklists for the 'CABIN AUTO INOP' message, they would have depressurised the cabin before landing, allowing the doors to open easily.
Findings
- The primary cause of the pressure change was the premature opening of a cabin positive pressure relief valve.
- This opening caused a sudden rush of air from the cabin, leading to the rapid increase in cabin altitude.
- The combination of a faulty CPRV and an out-of-tolerance pressure sensor likely contributed to the valve opening at an unintended pressure differential.
- The difficulty in opening the aircraft doors upon landing was due to the crew not following the specific depressurisation steps outlined in the QRH for manual mode operations.