What happened
On March 24, 2013, an AVRO 146-RJ100, registration HB-IYY, operated by Swiss European Air Lines, was performing a scheduled flight from London City to Geneva. While cruising at FL 270 near the LUSAR waypoint, the flight crew noticed a change in cabin pressure. Although the cabin altitude and differential pressure initially appeared normal, an error code appeared on the cabin pressurization controller.
In response to the pressure fluctuations, the captain switched the pressurization system to manual mode. During the subsequent descent, the cabin oxygen masks deployed spontaneously. The crew declared a PAN PAN emergency due to the loss of pressurization control. As the aircraft descended through FL 105, a "CABIN HI ALT" alarm briefly activated. The crew continued the descent to FL 100 and eventually landed safely at Geneva airport. There were no injuries to the passengers or crew.
The investigation
SUST examined the technical systems of the aircraft, including the pressurization regulator and outflow valves. The investigation revealed that the aircraft had experienced a separate, unrelated incident involving spontaneous mask deployment on March 8, 2013. During the investigation of the March 24 event, investigators found evidence of water in the pneumatic interconnection lines of the outflow valves and in the air supply lines for the jet pumps. Additionally, the investigation identified excessive friction affecting the movement of the secondary outflow valve. While the exact reason for the spontaneous deployment of the oxygen masks could not be definitively determined, post-flight tests of the deployment system were satisfactory.
Findings
- The loss of pressurization control was likely caused by ice formation in the pneumatic control lines for the cabin outflow valves.
- Excessive friction was present, which hindered the proper operation of the secondary outflow valve.
- The aircraft had been operating at high altitudes with temperatures well below freezing for approximately 90 minutes, which likely facilitated the icing of the pneumatic lines.
- Some oxygen masks had been improperly reinstalled or disconnected following maintenance performed after a previous incident in March 2013.