What happened
On February 11, 2006, an Air One Boeing 737-400, registration EI-COK, was performing a scheduled passenger flight from Bari to Venice. During the climb phase, the crew received an "Altitude Horn" alert, indicating that the cabin altitude had exceeded 10,000 feet. The rate of cabin altitude increase was approximately 1,500 feet per minute.
In response to the rapid change in pressure, the flight crew leveled the aircraft at 26,500 feet. After performing necessary checks and determining that the pressurization system could not be restored, the crew initiated the "Cabin altitude warning or rapid depressurization" abnormal procedure. This necessitated an emergency descent. The crew requested and received authorization from air traffic control to descend to flight level 130. During the descent, the crew deployed the speed brakes before disabling the auto-throttle, which resulted in a temporary reduction in engine thrust. The aircraft subsequently returned to Bari and landed safely. There were no injuries among the 60 passengers and 6 crew members on board.
The investigation
Following the incident, maintenance personnel conducted inspections in accordance with the Aircraft Maintenance Manual. As a precaution, the Cabin Pressure Controller, Pressure Control Panel, and the outflow valve were replaced. Data retrieved from the Quick Access Recorder (QAR) provided no specific information regarding the origin of the malfunction.
Technical examinations were performed on the removed components by a certified facility. The investigation revealed that the Cabin Pressure Controller was out of calibration due to electrical tolerances being outside acceptable limits, though it did not directly cause the event. However, the Pressure Control Panel was found to be inefficient, and the outflow valve showed significant wear, specifically regarding an inefficient AC actuator.