What happened
On January 9, 2009, a Boeing 737-400, registration OK-VGZ, operating for ČSA, was en route from Amsterdam to Prague. While cruising at FL 370, the aircraft experienced a sudden malfunction in the cabin pressurization system. The crew observed a "MASTER CAUTION" and "AIR CON" alert, accompanied by a rapid increase in cabin altitude, which reached 10,000 feet within seconds, triggering the altitude warning.
The flight crew immediately initiated emergency procedures for rapid depressurization. The commander declared a "MAYDAY" to Maastricht ATC and initiated an emergency descent to FL 100. To manage the pressure loss, the first officer switched the pressure control to manual mode and closed the outflow valve. As the cabin altitude rose to 14,500 feet, the passenger oxygen masks deployed. The aircraft descended through heavy turbulence-like pressure changes, eventually stabilizing the cabin altitude below 10,000 feet as the crew reached the lower flight level.
In the cabin, the flight attendants managed the deployment of oxygen masks for passengers. Although the crew noted that the pilot's emergency announcement was difficult to hear, they successfully ensured all passengers were wearing masks and that no injuries occurred during the descent.
The investigation
The ÚZPLN investigation focused on the technical failure of the pressurization system and the adequacy of the cabin crew complement. Investigators examined the aircraft's flight data recorder (QAR) and interviewed the flight and cabin crews.
Technical analysis involved inspecting the analog cabin pressure controller and the cabin pressurization outflow valve (OFV). The investigation also looked into why the aircraft was operating with only three flight attendants, which was below the required minimum for a 162-seat configuration.
Findings
- The primary cause of the pressurization system failure was the blowing of a fuse (F3003) in the analog cabin pressure controller, likely caused by an increased current draw from the AC actuator in the outflow valve.
- Post-incident inspections of the decommissioned outflow valve revealed out-of-limit findings on the AC and DC actuators.
- The aircraft was operating with an insufficient number of cabin crew members. Due to a last-minute aircraft swap from a B737-500 to the B737-400, the flight was staffed with three attendants instead of the required four for the larger 162-seat configuration. This discrepancy occurred because the change in aircraft type was not properly registered by dispatch.
- The flight crew and cabin crew performed their emergency duties in accordance with established procedures.