Incident Overview
On 4 February 2021, VH-EBK, an Airbus A330-202 operated by Qantas Airways, departed Sydney, New South Wales, on a scheduled flight to Perth, Western Australia. About two hours into the flight, the Electronic Centralized Aircraft Monitor (ECAM) triggered a red CAB PR EXCESS CAB ALT warning alert due to the cabin altitude exceeding 9,550 ft. The alert’s procedure required the flight crew to don oxygen masks and initiate an emergency descent. However, the pressurisation data indicated normal operation, leading the crew to doubt the alert. After seeking additional information from the Flight Crew Techniques Manual (FCTM) and other sources, the crew delayed actioning the required response. Approximately seven minutes after the alert, they donned oxygen masks and commenced a diversion to Adelaide with a precautionary descent to 10,000 ft. Shortly after, the displayed cabin altitude suddenly increased, prompting an immediate emergency descent. The aircraft levelled at 10,000 ft and continued to Adelaide without further incident.
Findings
The aircraft was equipped with dual Cabin Pressure Controllers (CPCs) that automatically manage pressurisation: one CPC controls while the other serves as a backup. During the flight, a fault occurred in the pressure sensor of the controlling CPC (CPC2), but due to a design limitation, that CPC could not detect the fault. This resulted in loss of cabin pressure control and a slow depressurisation. The standby CPC detected the loss and triggered the CAB PR EXCESS CAB ALT alert when the cabin altitude exceeded 9,550 ft. However, the design limitation caused the systems display to continue showing pressurisation data from the faulty CPC, directly conflicting with the alert. The aircraft had not incorporated an Airbus service bulletin that would have corrected the design limitation and prevented the loss of control from the sensor fault.
The conflicting information led the flight crew to respond to the red warning by seeking evidence to verify the failure, delaying execution of the required procedure. The delay was compounded by uncertainty in the procedural guidance within the FCTM. The crew focused on obtaining data to support an emergency descent and did not consider the potential risk of depressurisation, delaying their use of oxygen to avoid hypoxia. Airbus had required flight crews to action the CAB PR EXCESS CAB ALT alert regardless of confirmatory data, but this requirement was in a preamble to the procedure, not in the ‘read and do’ steps, and relied on memory recall. The operator’s training system did not adequately cover the unique requirements of this alert procedure, increasing the risk of delayed or incorrect application.
Safety Actions
In response, the operator modified its training to ensure proper coverage of the CAB PR EXCESS CAB ALT red warning alert procedure. The operator and Airbus revised the FCTM content to reflect the preamble requirements. The operator inspected its fleet for similar CPC faults, finding none, and upgraded all applicable CPCs via the service bulletin. An enhanced CPC fault alert policy and monitoring system was established. Airbus advised that an evaluation of mitigations for the A330 cabin pressure control system design limitations was underway. As a proposed timeline was not provided, the ATSB issued a safety recommendation to support Airbus’ intended action.