What happened
On 22 July 2019, a DHC-8-402 Dash 8, registration G-PRPK, was performing a scheduled passenger flight from Edinburgh to London City. While cruising at 25,000 ft near Manchester, the cabin pressure warning light illuminated. The flight crew observed that the cabin altitude had risen to 10,000 ft and was increasing slowly, though no fault lights were visible on the pressurisation control panel.
Following established checklists, the pilot initiated an emergency descent. The crew donned oxygen masks and declared a MAYDAY, receiving clearance to descend to 10,000 ft. During the descent, the warning light extinguished and pressure indications returned to normal. The aircraft diverted to Birmingham, where it landed without further incident. There were no injuries among the 56 passengers or 4 crew members.
The investigation
The AAIB investigation revealed that the initial event was not an isolated incident. Between late July and mid-September 2019, the aircraft experienced nine additional pressurisation events over 260 flight sectors. While maintenance actions were performed following entries in the technical log, the aircraft continued to operate.
Engineers found no faults on the control panel or diagnostic systems during initial checks. However, a second emergency descent occurred on 17 September 2019, prompting the operator to withdraw the aircraft from service for an intensive engineering investigation. This process involved replacing several components within the bleed air and pressurisation systems. Following these repairs, no further issues were reported.
Findings
- The primary cause of the emergency descent was a failure in the aircraft pressurisation system.
- The operator' failed to identify the pattern of recurring defects in its maintenance database (AMOS) until late in the period of occurrences.
- There was an inconsistency in safety reporting, as the crew did not submit Air Safety Reports (ASRs) for several of the pressurisation events.
- The operator's reliability programme did not specifically address the ongoing pressurisation issue between July and October 2019.
Safety action
- The operator issued a notice to all flight crews providing guidance on the mandatory reporting of safety events.
- The operator conducted a series of briefings at all bases to encourage both crews and engineers to submit ASRs.
- A review of the operator's reliability programme was initiated to improve the monitoring of recurring aircraft defects.