What happened
On 17 October 2023, a Boeing 737-8K5, registration G-TAWD, was operating a commercial passenger flight from Manchester to Kos, Greece. During the climb, as the aircraft passed FL130, a cabin altitude warning and associated horn were activated. This occurred because both engine bleed air systems had been left in the 'off' position.
Upon noticing the warning, the crew selected both bleed air systems to 'on' and leveled the aircraft at FL150. However, the crew did not perform the required Quick Reaction Handbook (QRH) memory items, which include donning oxygen masks. While attempting to continue the climb to FL280, a 'pack caution' light illuminated, indicating a fault in the right air conditioning pack. Following consultation with maintenance control, the commander decided to return to Manchester. The aircraft returned to the departure airfield after a period of fuel burning with landing gear extended to reduce weight.
The investigation
The AAIB examined the aircraft's flight data recorder and the circumstances surrounding the flight crew's actions. The investigation established that maintenance work performed the previous night on the air conditioning packs had resulted in the bleed air switches being left in the 'off' position. The engineer responsible believed the switches had been restored to their original configuration, and the subsequent crew did not detect the error during pre-flight or after-takeoff checks.
Data analysis revealed that the cabin altitude warning remained active for 4/3 minutes. Although the cabin altitude did not reach the 14,000 ft threshold required for automatic passenger oxygen mask deployment, the crew and passengers were exposed to a potential risk of hypoxia. The investigation also looked into crew fatigue, noting that the commander had experienced significant sleep disruption and a high workload in the preceding weeks.
Findings
- The engine bleed air switches were left in the 'off' position following maintenance work.
- The error was not identified during the crew's pre-flight or after-takeoff checklists.
- The crew failed to execute the prescribed QRH memory items, specifically the instruction to don oxygen masks, following the cabin altitude warning.
- The commander's decision-making may have been influenced by fatigue and a belief that the situation was under control once the bleed switches were activated.
- The crew's expectation that the system would rapidly rectify the cabin altitude may have led to a lack of urgency in following safety protocols.