What happened
On December 12, 2006, an AVRO 146-RJ100, registration HB-IXU, operated by Swiss European Airlines, was performing a scheduled flight from Munich to Zurich. Following engine start, the flight crew neglected to engage the aircraft's air conditioning and cabin pressurization packs. As the aircraft climbed, the cabin altitude rose steadily without the crew noticing the omission.
While the aircraft was cruising near FL 200, approximately 20 km north of Kempten, Germany, the cabin altitude exceeded 13,000 feet. This triggered the automatic deployment of oxygen masks for the passengers and cabin crew. The cabin chief, noticing the masks had dropped, contacted the cockpit via the interphone to report the situation. The captain, initially unaware of the pressure loss, realized the cabin altitude had reached approximately 10,000 feet. The flight crew donned their own oxygen masks and declared an emergency, initiating an emergency descent through FL 150, FL 120, and eventually FL 80. Once the packs were correctly switched to the ON position, cabin pressure stabilized, and the aircraft proceeded to land safely in Zurich.
The investigation
The Swiss Transportation Safety Investigation Board (SUST) examined flight data recorder (DFDR) archives, radio communications, radar data, and crew statements. The investigation focused on why the crew failed to notice the pressurization failure and why the onboard warning systems did not alert them to the rising cabin altitude.
Findings
- The primary cause of the incident was that the crew failed to activate the air conditioning and cabin pressurization systems after engine start and did not detect the error during the climb.
- A defective cabin altitude warning switch prevented the 'CABIN HI ALT' alert from triggering when the cabin altitude reached 9,300 feet.
- Existing procedures and checklists were found to be insufficiently adapted to the complexity of the aircraft's pneumatic system.
Safety action
Following the investigation, several safety recommendations were issued to address the identified deficiencies, including the need to explicitly highlight vital points in operational checklists and the necessity of using written checklists during flight operations. Additionally, recommendations were made to update crew procedures to ensure that cabin crew members immediately notify the flight deck if oxygen masks deploy without prior cockpit notification.