What happened
On 5 January 2010, a DHC-8-402 Dash 8, registration G-JECN, was conducting a commercial passenger flight from Southampton Airport to Dublin. While climbing to FL240, the pilot not flying noticed an abnormal increase in the cabin climb rate, which was immediately followed by a pressurisation fault warning.
In an attempt to resolve the issue, the pilot flying toggled the pressurisation controller from manual back to automatic, but the fault persisted. Following standard emergency procedures, the flight crew donned oxygen masks and initiated an emergency descent. During the descent, the MAYDAY call was downgraded to a PAN. The aircraft eventually levelled off at FL100 and returned to Southampton. While the crew and 23 passengers escaped without injury, several passengers reported ear pain, and the cabin crew experienced physical symptoms such as weakness and shortness of breath due to the depressurisation.
The investigation
The AAIB examined the aircraft's Central Diagnostic System, which identified the aft pressure outflow valve as the source of the failure. Following the replacement of this valve, functional tests confirmed the pressurisation system was operating normally. Inspections of the air conditioning ducts and door seals revealed no other defects.
Investigators also reviewed the flight crew's response and the aircraft's technical history. While the cabin crew attempted to contact the cockpit via the call bell, the flight crew did not respond, as they were focused on managing the emergency and were wearing oxygen masks. Analysis of the cockpit voice recorder indicated that one crew member was experiencing ear pain, which may have hindered their awareness of the call bell.
Findings
- The primary cause of the pressurisation failure was a faulty aft pressure outflow valve.
- The aircraft operator had previously noted reliability concerns regarding this specific valve component.
- The specific valve installed on G-JECN was an older version that had not yet received the manufacturer's software noise filter update, a feature intended to improve reliability.
- The flight crew's focus on the emergency descent and the physical effects of depressurisation contributed to them not noticing the cabin crew's attempt to communicate via the call bell.