What happened
On 29 April 2017, a Boeing 787-9 Dreamliner, registration G-ZBKF, was operating a commercial passenger flight from London Heathrow to New Delhi. The aircraft had been dispatched with the left air conditioning system disabled due to a previous component failure.
While cruising at 35,000 feet, the flight crew received an alert indicating that the cabin altitude was rising. As the cabin altitude reached 8,500 feet, the crew initiated a descent to stabilize the pressure. However, the cabin altitude continued to climb, eventually exceeding 10,000 feet. In response, the commander declared a MAYDAY and performed an emergency descent to 10,000 feet. During the descent, the cabin oxygen masks deployed automatically. The crew successfully managed the descent, and the aircraft eventually returned to Heathrow, where it landed without further incident. No injuries were reported among the 124 passengers or 13 crew members.
The investigation
The AAIB investigation focused on the cause of the depressurisation and the subsequent cabin response. Investigators examined the aircraft's air conditioning and pressurisation systems, finding that the right-hand inner recirculation duct had disconnected from the lower recirculation fan. This disconnection was attributed to an incorrect installation of the sleeve and coupling during recent maintenance.
Furthermore, the investigation looked into why the cabin crew and passengers were not immediately aware of the emergency. It was discovered that while the oxygen masks had deployed, the pre-recorded announcement (PRA) intended to alert the cabin was inaudible due to a software issue that reduced the announcement's volume. The investigation also reviewed the quality of the cockpit voice recorder (CVR) data and the maintenance history regarding the aircraft's air conditioning components.