AAIB investigation to Boeing 767-36N, G-POWD
Cabin depressurisation and emergency descent, 19 March 2017.
Summary:
Shortly after entering the cruise at FL370 the cabin altitude audio siren sounded and the CABIN ALTITUDE red warning caption illuminated. The crew carried out the depressurisation drills before commencing an emergency descent and subsequent diversion to Amsterdam Schiphol Airport. It was established that the aircraft failed to pressurise correctly due to a faulty positive pressure relief valve (PPRV) Both PPRVs had been replaced with overhauled valves during recent maintenance.
Testing and strip examination of the faulty valve proved inconclusive, with no obvious reason for the malfunction being found. The possibility of debris trapped in a metering section of the valve could not be ruled out, although no evidence of this was found.
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