What happened
On 8 November 2004, a Boeing 737-204 ADV, registration EI-CJC, was operating a scheduled flight from Reus, Spain, to Dublin, Ireland. To accommodate a maximum takeoff weight on a limiting runway, the flight crew configured the aircraft for a "bleeds off" takeoff, utilizing the Auxiliary Power Unit (APU) to supply bleed air.
While cruising at FL320, the cabin altitude warning horn activated. The cabin altitude was rising rapidly, reaching approximately 15,000 feet. The captain initiated an emergency descent, bringing the aircraft down to FL100. During the descent, the crew identified that the engine bleed switches had been left in the "OFF" position. The aircraft diverted to Biarritz, France, where it landed safely. There were no fatalities and no injuries among the 111 passengers or 5 crew members.
The investigation
The AAIU investigation focused on why the pressurization system failed to maintain a stable cabin altitude. Investigators examined the takeoff configuration, the execution of post-takeoff checklists, and the crew's adherence to standard operating procedures. The inquiry looked into the specific settings of the engine bleed switches and the role of the APU in providing air supply during the initial climb.
Findings
- The crew did not conduct a formal briefing regarding the specific requirements of the "bleeds off" takeoff configuration.
- The "After Take-off" checklist was performed as a silent checklist rather than through a challenge-and-response method, which led to the failure to notice that the engine bleeds remained in the OFF position.
- The crew's scan of the pressurization parameters during the climb appeared normal because the APU was successfully supplying enough air to maintain pressure initially.
- The flight crew responded effectively to the depressurization by donning oxygen masks and initiating the emergency descent procedures.
Safety action
Following the investigation, the following safety recommendations were made to the operator:
- Transition the "After Take-off" checklist from a silent procedure to a formal Challenge and Response format to ensure critical items are verified.
- Amend the Standard Operating Procedures (SOPs) for the "Passing 10,000 ft" phase to include explicit checks of the pressurization system and the APU status.