What happened
On March 14, 2012, a DO328-100, registration HB-AES, operated by SkyWork Airlines AG, was climbing through flight level 270 when the crew received a 'CAB ALT' warning accompanied by a triple chime. At that moment, the cabin altitude had already reached 9,500 feet and was continuing to rise. The crew immediately donned oxygen masks and initiated an emergency descent, broadcasting a 'Mayday' call. The aircraft was approximately 20 km south of Zurich Airport at the time of the warning. Following the incident, the crew diverted to Bern-Belp, managing the cabin pressure manually using checklists until landing.
The investigation
An investigation by the Swiss Transportation Safety Investigation Board (SUST) focused on the pressurization system and the crew's actions. Investigators examined the aircraft's technical configuration, specifically the forward outflow valve and the pressurization control mode. The inquiry also reviewed flight data, cockpit voice recordings, and the crew's use of emergency checklists. The investigation looked into why the cabin altitude rose so rapidly despite the aircraft being in a standard climb profile.
Findings
Technical analysis revealed that the forward outflow valve was not fully closed during takeoff. This occurred because a control element used to manage the cabin pressurization system was left in an incorrect position, a mistake that went unnoticed by the crew during pre-flight or departure preparations. Two specific design characteristics contributed to the severity of the incident:
- The control for the forward outflow valve, which is intended for manual use, also influences the valve's behavior during automatic operation.
- There is no cockpit indication available to inform the crew of the current position of the forward outflow valve.
Additionally, the investigation identified a risk factor where the captain and flight attendant were unable to communicate via the interphone during the event.
Safety action
Following the findings, the SUST issued Safety Recommendation No. 492, calling on the European Union Aviation Safety Agency (EASA) and the aircraft manufacturer to implement measures. The goal is to ensure that flight crews can more effectively detect if the forward outflow valve is not properly closed, particularly when the system is operating in automatic mode.