What happened
On 15 October 2002, a Boeing 737-204ADV, registration EI-CJG, was operating a public transport flight from Prestwick to Beauvais. While cruising at FL330 near Birmingham Airport, the crew received a pressurisation warning horn. Upon investigation, the crew noted that the cabin altitude had risen above 10,000 feet and was climbing at a rate of approximately 400 feet per minute.
As a safety precaution, the crew donned oxygen masks and requested clearance from Air Traffic Control to descend to FL250. During the execution of emergency checklists, the crew discovered that the cabin pressure control valve was closed, yet the cabin altitude continued to increase. Consequently, the crew initiated an emergency descent, and ATC cleared the aircraft to FL100 while maintaining separation from other traffic.
Although the cabin altitude remained below the 14,000-foot threshold for automatic mask deployment, the senior cabin crew member manually deployed the passenger oxygen masks following company training protocols. During the descent, the commander observed the cabin pressure descending rapidly at 2,000 feet per minute and instructed the First Officer to switch the pressure control back to automatic mode. This stabilized the descent rate to between 500 and 700 feet per minute.
The flight diverted to Stansted, where the aircraft landed without further incident. There were no fatalities and no injuries reported among the 122 passengers or 5 crew members, though one passenger received medical attention for nausea.
The investigation
Following the incident, the aircraft underwent engineering inspections at Stansted, which initially yielded no findings. The aircraft was subsequently flown unpressurised and without passengers to the operator's primary engineering base for a more detailed examination. During these checks, investigators identified an intermittent fault involving the cabin pressure control valve. To rectify the issue, the pressure control panel and the cabin pressure controller were replaced before the aircraft returned to service.