What happened
On 13 August 2017, a Boeing 767-332ER, registration N1608, was departing London Heathrow Airport on a commercial passenger flight. During the climb, the aircraft's cabin altitude rose sharply, eventually reaching 15,900 ft. This rapid increase in cabin altitude triggered the automatic deployment of passenger oxygen masks and prompted the flight crew to initiate an emergency descent.
As the aircraft climbed, the crew followed the Quick Reference Handbook (QRH) procedures. After reaching their planned cruise altitude of FL320, the crew noted that the cabin pressure was unstable. The situation escalated when a master warning light illuminated and the cabin altitude control panel indicated a failure. The commander observed that the cabin pressure was rising rapidly and noted that the outflow valve appeared to be nearly closed.
Following the emergency, the crew requested a descent to FL100 and coordinated with London ATC to establish a holding pattern. This was necessary to jettison fuel from the centre tank, as the aircraft was prepared to land at Heathrow at a weight approximately 21,900 lbs above the maximum allowable landing weight. During the event, one passenger fainted and required medical attention upon arrival. The aircraft landed safely at Heathrow.
The investigation
The AAIB investigation examined the aircraft's maintenance history and flight data. It was noted that prior to this flight, maintenance personnel had replaced a worn right engine pressure regulating valve (PRV) following an advisory message on the inbound flight from Atlanta.
Flight Data Recorder (DFDR) analysis revealed that the cabin altitude climbed to 15,900 ft before the descent began, contradicting crew reports that the pressure had been steady between 7,000 and 7,500 ft. The investigation also looked into the engine systems, noting that the right Environmental Control System (ECS) pack had been switched off during the climb, which caused the left pack to switch to high-flow mode.
Post-flight inspections at Heathrow and Atlanta involved replacing several components, including the right engine PRV controller, both cabin pressurisation controllers, and the outflow valve. Testing of the removed right engine PRV controller revealed a blown internal diaphragm, which explained the initial engine PRV advisory message.
Findings
- The rapid cabin altitude increase was the primary event, though the specific cause for the failure to maintain pressure could not be definitively established.
- The right engine PRV controller fault was responsible for the engine-related advisory messages.
- While the air conditioning system functioned correctly, the investigation could not identify a specific fault that caused the excessive leakage or insufficient airflow required to trigger such a significant pressure loss.
- Two passenger oxygen masks failed to deploy automatically, though these were later found to be due to faulty oxygen generators which were subsequently replaced.