What happened
On 21 November 2014, an Airbus A3/330-343, registration HB-JHB, was operating in the airspace approximately 110 NM west-northwest of Basel when a serious incident occurred involving the aircraft's cabin pressurization. The aircraft experienced a failure in its automatic pressure regulation system due to a blocked outflow valve. In response to the developing situation, the flight crew initiated an emergency descent.
The investigation
The Swiss Transportation Safety Investigation Board (STSB) examined the technical and operational aspects of the event. The investigation focused on the mechanical failure of the outflow valve and the subsequent actions taken by the crew. Investigators reviewed the aircraft's electronic flight instrument system (EFIS) and the centralized fault display system to understand how the crew perceived the cabin altitude and differential pressure during the descent. The probe also looked into the cockpit's automated display logic and the crew's application of manual pressure control procedures.
Findings
Technical analysis revealed that a blocked outflow valve caused the automatic cabin pressure regulation to fail. A significant contributing factor was a system-related limitation where the cockpit displays for cabin altitude and differential pressure were automatically hidden when the cabin altitude dropped below -2,060 ft.
Regarding the crew's response, the investigation determined that the crew initiated an emergency descent intuitively without first performing a structured analysis of the situation. Furthermore, the procedures required for managing the cabin pressure via manual control were not fully implemented by the flight crew during the event.