What happened
On 25 May 2000, a Boeing 737-3Y0, registration G-IGOG, was performing a scheduled passenger flight from Venice to London Stansted. While cruising at FL 350 near Brussels, the crew noticed a rapid increase in cabin altitude, which was climbing at approximately 2,000 feet per minute. Although the crew attempted to use the standby pressurisation system and manually close the outflow valve, the cabin altitude continued to rise.
In response, the flight crew donned oxygen masks and initiated an emergency descent to FL 100. During the descent, the passenger oxygen masks deployed automatically. The cabin crew promptly secured the cabin and passengers, managing the situation through regular announcements despite some passenger confusion regarding the oxygen flow. The aircraft continued to its destination, Stansted, without further incident.
Two days later, during a return flight from Alicante to Stansted, a second event occurred. As the aircraft began its descent, engine power was reduced, causing the cabin altitude to climb at 4,000 feet per minute. The crew declared a PAN-PAN call and descended to FL 100. In this instance, the pressurisation system reverted to standby mode effectively, preventing the deployment of passenger oxygen masks.
The investigation
The investigation focused on identifying why the pressurisation system failed to maintain altitude during both incidents. Following the first event, the operator and manufacturer identified a fault in the feedback loop between the pressure controller and the outflow valve, leading to the replacement of the valve.
However, the second event prompted a deeper technical analysis. Investigators examined the electrical and pneumatic systems to determine why the cabin altitude climbed specifically when engine thrust was reduced. The investigation scrutinized the electrical relay components and the pneumatic air supply from the engines.