What happened
On 21 September 2018, a DHC-8-402 Dash 8, registration G-PRPC, was operating a commercial passenger flight from Belfast City Airport to Edinburgh. During the previous flight sector, the crew had struggled with temperature regulation in the cabin and flight deck. During a pressured turnaround in Belfast, the commander attempted to reset the air conditioning packs using a supplemental ground guide to resolve these temperature issues.
Shortly after the aircraft reached its cruise altitude of FL170, an audio warning sounded and the cabin pressure warning light illuminated. The flight crew identified that the cabin altitude had risen above 10,000 ft. Following the Quick Reference Handbook (QRH) procedures for rapid depressurisation, the crew declared a MAYDAY and initiated an emergency descent. During the descent, the crew discovered that both air conditioning pack switches had been left in the off position. After re-engaging the packs, the aircraft began to pressurise. The flight continued to Edinburgh, leveling off at FL100, and arrived safely with no injuries to the 70 passengers or 4 crew members.
The investigation
The investigation looked into the high workload environment during the Belfast turnaround. Several factors contributed to the crew's distraction, including a lack of drinking water for the crew, a change in the flight's destination, and the need for the co-pilot to retrieve new flight plans from the terminal. These factors meant both pilots were away from the flight deck at various points.
Furthermore, the investigators found a discrepancy in maintenance documentation. An existing defect regarding automatic cabin temperature control had been recorded in the company's electronic system but was missing from the aircraft's physical Technical Log. Consequently, the crew was unaware of the deferred defect and attempted a manual reset of the system without full knowledge of the underlying technical status.
Findings
- The commander attempted to reset the air conditioning system during the turnaround but was interrupted, resulting in an incomplete and incorrect procedure.
- The air conditioning packs were left in the off position, which prevented the aircraft from pressurising at cruise altitude.
- High workload and operational pressures during the turnaround diminished the crew's ability to effectively monitor procedures and catch errors.
- A lack of communication between the electronic maintenance records and the physical Technical Log meant the crew was unaware of a previously deferred defect.