History of the Flight
On 5 January 2010, a DHC-8-402 Dash 8, registration G-JECN, departed from Southampton Airport on a commercial passenger flight to Dublin Airport, Ireland. The takeoff was uneventful, and the aircraft was cleared to climb to FL240. During the climb, at approximately FL230, the pilot not flying observed an excessive climb rate on the pressurisation system, shortly followed by the illumination of the pressurisation fault annunciator.
Crew Actions
The pilot flying attempted to rectify the fault by selecting the pressurisation controller to manual and then back to automatic, but the fault persisted. Both flight crew members immediately donned oxygen masks. The pilot not flying declared a MAYDAY, and the pilot flying executed a standard operating procedure emergency descent, levelling off at FL100. The MAYDAY was subsequently downgraded to a PAN, and the aircraft returned to Southampton without further incident. The flight crew later reported that they did not recall hearing the call bell from the cabin crew.
Cabin Crew and Passenger Observations
According to air safety reports, the cabin crew noticed that sandwich packets and coffee cup foils were bursting, and one crew member experienced ear popping, shortness of breath, and leg weakness. They used oxygen bottles and attempted to contact the flight deck but received no response. An announcement from the flight deck over the PA system stated that the emergency descent was complete. Several passengers complained of sore ears.
Engineering Examination
Post-incident, the aircraft's Central Diagnostic System identified the aft pressure outflow valve as the failure mode. The suspected faulty valve was replaced, and a functional test confirmed normal operation. Additional inspections of all aircraft door seals and air conditioning ducts revealed no defects. A full operational test of the pressurisation system was successfully completed, and the aircraft returned to service. The removed valve, fitted at build in 2005 with 7,493 hours and 8,649 cycles, was returned to the manufacturer for investigation. The aircraft operator noted a perceived reliability concern with the outflow valve on this aircraft type, though the aircraft manufacturer stated it was within tolerable limits. The valve fitted at the time was not the most recent revision; two subsequent revisions had been introduced, and a service bulletin (SB84-21-09) for a further revised valve was released.
Analysis
The root cause of the outflow valve failure had not been determined by the component manufacturer at the time of reporting. However, based on evidence from the Central Diagnostic System and the uneventful return to service after valve replacement, it is probable that the aft outflow valve was the source of the depressurisation. The passenger address and interphone system tested satisfactorily, and review of the CVR revealed that the call bell was audible in the cockpit but went unnoticed by the flight crew during a busy period. One flight crew member mentioned experiencing ear pain, which may have contributed to not hearing the call.
Probable Cause
The investigation concluded that a faulty aft pressure outflow valve was the probable cause of the pressurisation failure.