What happened
On 19 March 2017, a Boeing 767-36N, registration G-POWD, was operating a commercial passenger flight from London Stansted to Rzeszow-Jasionka, Poland. While cruising at 37,000 feet, the crew received a notification that the seat belt signs had illuminated, followed immediately by a cabin altitude warning and a red warning caption.
Upon investigation, the crew found the cabin altitude had risen to approximately 10,000 feet. The crew and a maintenance engineer on board donned oxygen masks to follow emergency depressurisation procedures. The co-pilot experienced some difficulty with his mask and suspected the onset of hypoxia. The crew declared a MAYDAY to Maastricht ATC and initiated an emergency descent, descending through 20,000 feet and 10/000 feet before leveling at 7,000 feet. The aircraft diverted to Amsterdam Schiphol Airport, where it landed safely without further incident. There were no fatalities and no injuries among the 262 passengers or 9 crew members.
The investigation
AAIB investigators examined the aircraft's flight data and cockpit voice recordings. An external inspection of the aircraft revealed that the indicator flag on the upper positive pressure relief valve (PPRV) had deployed, confirming the valve had operated in flight and allowed air to escape.
Testing of the faulty valve in a simulated environment showed that it allowed air to flow at a pressure differential of only 3.36 psi, far below the intended threshold of 8.95 psi. This low-pressure leakage caused the gradual loss of cabin pressure. The investigation also looked into the maintenance history, noting that both PPRVs had been replaced with overhauled units during recent heavy maintenance. The specific valve in question had been in storage since 2012. While the manufacturer suggested that a tiny piece of debris could have prevented the valve from sealing, a strip examination of the component was inconclusive.
Findings
- The primary cause of the failure to maintain cabin pressure was a faulty positive pressure relief valve.
- The valve had been installed during recent maintenance, and existing leak-check procedures failed to detect the defect.
- The faulty valve had been in storage for approximately five years; although maintenance manuals suggest re-testing valves after 12 months of storage, this requirement was not included in the release documentation provided to the installing technicians.
- It is possible that the leak rate was not significant enough to trigger warnings during a previous check flight that only reached 35,000 feet.