What happened
On 8 December 2016, a DHC-8-402 Dash 8, registration G-FLBB, was operating a commercial passenger flight from Manchester to Jersey. While cruising at 25,000 feet, the aircraft experienced a loss of cabin pressurisation, triggering a master warning and a triple chime alert.
Upon receiving the warning, the co-pilot immediately donned an oxygen mask and initiated an emergency descent. The commander, who was initially preoccupied with a technical log entry, also donned an oxygen mask and declared a MAYDAY. During the descent, the co-pilot transitioned from autopilot to manual flight, increasing the descent rate to 3,500 fpm. The aircraft eventually stabilised at 10,000 feet, at which point the crew cancelled the emergency declaration and obtained clearance to continue to Jersey.
Despite the high workload and the commander experiencing slight lightheadedness, the crew completed the necessary emergency procedures and informed the 23 passengers of the situation. The flight landed in Jersey without further incident, and no injuries were reported among the 4 crew members or the passengers.
The investigation
The AAIB investigation examined the aircraft's flight data recorder (FDR) and cockpit voice recorder (CVR). The FDR confirmed that the cabin altitude warning remained active for nearly eight minutes until the aircraft reached 10,000 feet. However, the CVR did not capture the event because it was not secured following the incident, allowing the data to be overwritten.
The investigation also looked into the history of the aircraft's components. The outflow valve in question had a documented history of causing pressurisation issues on other aircraft within the operator's fleet. Furthermore, the investigation noted that the commander's performance may have been impacted by early signs of hypoxia, as he noted difficulty completing a technical log entry just prior to the warning.
Findings
- The primary cause of the depressurisation was a faulty outflow valve.
- The valve had a history of being associated with pressurisation faults in other aircraft.
- The flight crew experienced early symptoms of hypoxia, which likely hindered the commander's initial reaction speed.
- The loss of cabin pressure was likely a gradual process that reached warning levels after two minutes of cruise at 25,000 feet.
Safety action
- The operator has initiated a reliability investigation regarding the outflow valve in conjunction with the spare parts provider and the original equipment manufacturer.