What happened
On 5 February 2021, an Airbus A330, registered VH-EBK, was cruising at approximately 40,000 ft during a flight from Sydney to Perth when the crew received an excess cabin altitude warning. The aircraft had been experiencing a gradual loss of cabin pressure for several minutes prior to the alert. Upon receiving the warning, the flight crew donned oxygen masks and initiated an emergency descent, eventually diverting the aircraft to Adelaide, where the landing was completed without further incident.
The investigation
The investigation focused on why the cabin pressure controller (CPC2) failed to maintain the correct altitude and why the crew was presented with conflicting cockpit information. It was determined that the controlling pressure controller experienced an intermittent fault in a pressure sensor. Because the sensor continued to provide data that appeared credible, the system did not automatically detect the error or transfer control to the secondary, functional controller (CPC1).
As the actual cabin altitude rose, the primary controller (CPC2) still believed the cabin was at 7,100 ft. This prevented the initial 'CAB ALT' advisory message from triggering. When the altitude eventually exceeded 9,550 ft, a red 'CAB PR EXCESS CAB ALT' alert was activated by the secondary controller. However, because the controlling unit (CPC2) still reported normal parameters, the crew's system displays showed normal cabin pressure, creating a discrepancy between the warning light and the digital readouts.
Findings
- An intermittent sensor fault in the controlling cabin pressure controller caused inaccurate outflow valve regulation.
- The lack of a specific software service bulletin meant the aircraft could not automatically switch to the more accurate sensor when a large pressure difference was detected.
- The flight crew's response was complicated by a discrepancy between the ECAM alert and the system display data.
- While Airbus had introduced a manual preamble requiring immediate action regardless of data, this instruction was not integrated into the ECAM 'read & do' checklist, making it dependent on crew memory.