What happened
On 17 February 2003, a Boeing 757-28A, registration G-OOOD, was performing a positioning flight from Cardiff to Rome with a crew of eight and no passengers. During the later stages of takeoff, the crew received an Engine Indication and Crew Alerting System (EICAS) message indicating a high stage issue on the right engine. Following standard procedures, the takeoff was continued, and once the aircraft reached a safe altitude, the crew executed the relevant drills, which involved switching off the right engine bleed system. This action restricted the aircraft's maximum operating altitude to FL350.
After climbing to FL350 and consulting with company engineers, the crew elected to proceed to the destination. However, approximately one minute after leveling off, a cabin altitude warning was triggered. The crew donned oxygen masks and initiated an emergency descent. During the initial descent, the crew observed that the cabin altitude continued to rise through 10,500 feet, despite the cabin outflow valve being fully closed. Once the aircraft was established in the descent, the cabin altitude began to decrease. The commander, noting the absence of passengers, opted not to deploy the emergency oxygen system. A MAYDAY call was issued, and the flight diverted to London Gatwick, where the aircraft landed safely.
The investigation
The investigation focused on the sequence of events leading to the cabin altitude warning and the subsequent inability to maintain pressure at higher altitudes. The AAIB examined the impact of operating the aircraft with a single functioning bleed system. It was noted that the investigation considered the technical limitations of a lightweight B757 when one bleed system is inoperative.
Findings
- The initial engine issue necessitated the deactivation of the right engine bleed system, which limited the aircraft's service ceiling to FL350.
- The cabin altitude warning was triggered shortly after reaching the new altitude limit.
- There is a known phenomenon where lightweight B757 aircraft operating with only one active bleed system may struggle to maintain a cabin altitude below 10,000 feet.
- Boeing had recently issued Service Letter 75lag-SL-21-055 to provide ground procedures for determining if an aircraft can maintain cabin pressure on a single air conditioning pack.