What happened
On 23 February 2017, an Airbus A340-642, registration G-VGAS, was performing a commercial passenger flight from London Heathrow to New York. The aircraft was carrying 198 passengers and 14 crew members. While cruising at FL 400, approximately 200 nm northeast of Boston, the crew received a 'CAB PR EXCESS CAB ALT' warning on the ECAM.
During this period, the pilots and two other crew members experienced symptoms consistent with hypoxia. In response to the warning, the crew initiated a descent. As the aircraft passed FL 260, the warning disappeared. The crew chose to level off at FL 2D50 to proceed to the destination; however, the warning reappeared roughly 30 minutes later. Despite the pressurisation system displays showing normal parameters, the crew descended further to 11,000 ft. The flight continued to New York, landing without further incident and with no injuries reported.
The investigation
The investigation focused on the simultaneous occurrence of the cabin altitude warning and a recorded fault in Cabin Pressure Controller 1 (CPC1). Investigators examined the aircraft's data, which confirmed that the CPC1 fault coincided with the initial warning. It was suspected that this malfunction caused the cabin outflow valves to open, leading to the rise in cabin altitude.
Upon physical inspection of the hardware, engineers identified five defective components within the CPC. These components were found to have suffered from wear, though the specific failure mode that led to the valves opening could not be definitively determined.