What happened
On 20 April 2025, an Airbus A320-251N, registration G-UZEF, was operating a scheduled passenger flight from Belfast Aldergrove to Palma de Mallorca. The flight was also serving as a training session for the co-pilot. Prior to departure, the crew had prepared the aircraft for potential de-icing due to visible frost on the wings and fuselage. Although the de-icing crew eventually determined that no fluid spraying was necessary, the crew proceeded to reconfigure the aircraft using a supplementary checklist.
During the climb to a cruising altitude of FL390, the crew observed a rising cabin altitude indication. As the cabin altitude climbed past 9,000 ft, the crew donned oxygen masks and initiated an emergency descent. The commander, acting as the pilot monitoring, eventually took control of the aircraft to manage the descent. While descending, the commander switched the cockpit annunciator lights from dim to bright, which allowed him to notice that the ditching pushbutton remained in the selected position. Once the button was deselected, the aircraft's pressurisation returned to normal. After leveling off at FL200 to assess the situation, the crew determined they could safely continue the flight, eventually climbing back to FL370 to complete the journey to Spain.
The investigation
The AAIB examined the flight data and cockpit voice recordings, which confirmed that the selection of the ditching pushbutton had caused the pack control valves to close, preventing air from entering the packs and thus inhibiting pressurisation. The investigation found that the crew had omitted the step to return the ditching pushbutton to the 'off' position during the reconfiguration process. The investigation also noted that the commander felt pressure to avoid delays and may have performed the reconfiguration with minimal reference to the checklist. Furthermore, the dark, dawn light conditions and the dim setting of the annunciator lights made the illuminated pushbutton difficult to detect from the left seat.
Findings
- The crew failed to deselect the ditching pushbutton during the aircraft reconfiguration following the de-icing assessment.
- The failure to return the ditching pushbutton to the 'off' position caused the aircraft to lose pressurisation capability during flight.
- The commander's desire to minimize departure delays likely contributed to an incomplete checklist execution.
- The low-light conditions and the dim setting of the cockpit annunciator lights hindered the visibility of the illuminated pushbutton.
- The co-pilot, being in training, was unfamiliar with the specific de-icing and reconfiguration procedures.
Safety action
- The operator published details of the event in their pilot safety magazine to raise awareness.
- The manufacturer updated the de-icing checklist to include specific warnings regarding the ditching pushbutton to prevent future pressurisation issues.