Incident Details
At 1007 UTC on 29 April 2017, a Boeing 787-9 Dreamliner (registration G-ZBKF) departed London Heathrow for New Delhi with 13 crew and 124 passengers. The left air conditioning system had been disabled prior to departure due to a failed cabin air compressor shaft, as permitted by the Minimum Equipment List (MEL). The aircraft climbed to FL350, levelling off at 1032 UTC. Two minutes later, the cabin altitude exceeded 8,500 feet, triggering an EICAS warning. The crew observed the cabin altitude increasing at approximately 300 feet per minute, with both outflow valves indicating closed. The system status page showed a 'recirc fan lwr r' message. The crew began a descent but the cabin altitude continued to rise. At 10,000 feet, the crew declared a PAN and then a MAYDAY, performing a rapid descent to FL100 with speedbrakes. The cabin altitude peaked at 10,429 feet before decreasing. The aircraft eventually stabilised at about 3,700 feet and returned to Heathrow, jettisoning fuel and landing normally. No injuries or damage were reported.
Cabin Crew Actions
During the descent, the co-pilot contacted the cabin service director (CSD) to stop cabin service and advise passengers to be seated near oxygen masks. The CSD initially misunderstood, but the co-pilot clarified. The CSD later entered the cockpit wearing a portable oxygen system. The cabin crew reported that the pre-recorded announcement (PRA) accompanying the oxygen mask deployment was inaudible due to low volume, causing confusion. Many cabin crew did not follow depressurisation procedures immediately. Passengers were also unaware; some remained asleep or donned masks incorrectly. After the situation became clear, cabin crew shouted instructions. Several crew and passengers felt unwell afterward, but none required medical attention.
Investigation Findings
The AAIB determined that the loss of cabin pressurisation was caused by detachment of the lower right air conditioning recirculation fan duct. The left air conditioning system had been disabled before the flight. In response, the Aircraft Maintenance Manual was amended to alter the process of replacing the recirculation fan, and maintenance procedures for a related Maintenance Alert Message were changed. The investigation also identified a software problem affecting the volume of the cabin decompression PRA, which the operator addressed with safety action. Three Safety Recommendations were made concerning the testing of the installed performance of CVR systems.