Incident Overview
On 21 November 2014, a commercial aircraft Airbus A330-343, registration HB-JHB, was involved in a serious incident approximately 110 nautical miles west-northwest of Basel, Switzerland. The Swiss Transportation Safety Investigation Board (STSB) conducted the investigation.
Sequence of Events
During the flight, the outflow valve became blocked, leading to a failure of the automatic cabin pressurization system. The flight crew attempted to control the cabin pressure manually but did not fully apply the prescribed manual control procedure. Subsequently, the crew initiated an emergency descent without first conducting a structured analysis of the situation. The aircraft's electronic instrument system masked the cabin pressure altitude and differential pressure displays when the cabin pressure altitude fell below -2060 feet.
Investigation Findings
The STSB determined that the serious incident resulted from the blocked outflow valve causing automatic regulation failure and the incomplete application of the manual control procedure. A causal factor was the crew's intuitive initiation of an emergency descent without a structured situation analysis. A contributing factor was the system's masking of the cabin pressure displays at low cabin pressure altitudes.
The investigation focused on safety and did not attribute liability.