What happened
On 8 October 2006, an Airbus A320-232, registration G-MIDW, was operating a commercial flight from Kos to Glasgow International Airport. While established in cruise at FL380, the crew received an ECAM warning for excessive cabin altitude. Although the system displayed normal pressurisation parameters, the crew donned oxygen masks as a precaution.
Approximately 18 minutes later, the passenger oxygen masks deployed, and the cabin crew reported changes in cabin lighting and seatbelt signs. The co-pilot noted a sensation of pressure change in his ears. The crew declared a mayday and initiated an emergency descent to FL100. During the descent, the commander switched the pressurisation control from System 1 to System 2, at which point a System 1 fault caption appeared on the display. The aircraft continued to its destination at FL100, landing without further incident. There were no injuries among the 156 passengers or 6 crew members.
The investigation
The AAIB examined the aircraft's Digital AIDS Recorder (DAR) and investigated the aircraft's pressurisation architecture. The data revealed that while the crew believed the initial warning was spurious, a significant step change in cabin altitude from 7,800 feet to 14,400 feet occurred during the flight. The investigation also involved a review of the manufacturer's findings regarding the System 1 Cabin Pressure Controller (CPC).
Findings
- The investigation established that the System 1 Cabin Pressure Controller (CPC) was providing incorrect altitude readings, displaying 7,800 feet while the actual cabin altitude was rising.
- The crew's decision not to descend immediately was influenced by the lack of physiological symptoms and the fact that the ECAM displayed normal parameters, leading them to believe the warning was a false alarm.
- A fault within the System 1 CPC prevented the automatic transfer of control to the standby system.
- The sudden jump in recorded altitude from 7,800 feet to 14,400 feet indicated that the nature of the fault changed during the event, eventually allowing the system to recognise the fault and trigger a System 1 fault caption.
Safety action
Following the incident, both the System 1 and System 2 CPCs, along with the single discharge valve, were replaced. The manufacturer reviewed the failure scenario through its Product Safety Committee to investigate how misleading information was displayed to the crew and to evaluate potential improvements to the system architecture.