What happened
On March 1, 2014, a Boeing 737-400 was cruising at FL330, approximately 10 nautical miles north of the VELAX waypoint, when the cabin altitude rose to 10,000 feet. This triggered a CABIN ALTITUDE/CONFIGURATION audio warning. In response to the depressurization, the crew donned oxygen masks and attempted to manually control the pressurization system to restore the proper cabin altitude.
Following standard procedures, the crew notified air traffic control of the need to descend due to the depressurization. The ACC EPWW controller authorized a descent to FL100 and instructed a 150-degree left turn for separation from other traffic. After approximately five minutes of unsuccessful manual pressure regulation, the crew requested a return to Warsaw Chopin Airport (EPWA). Although the crew did not declare an emergency, they requested time to burn off fuel. An uncertainty status was declared at EPWA around 12:53 UTC, and the aircraft landed safely on runway 11 at approximately 13:18 UTC.
The investigation
The investigation focused on the failure of the pressurization system. It was noted that the automatic pressure control system was already inoperative and had been permitted to fly under the Minimum Equipment List (MEL) reference 2114-1.
Technical analysis of the aircraft's maintenance history revealed that the failure was traced to the Cabin Pressure Control Selector on Panel P5-6. The investigation examined the maintenance of this specific component, noting that the selector had been replaced twice previously (once in February 2014 and again in March 2014) following repairs at a specialized workshop. The investigation also reviewed the aircraft's maintenance program, confirming that pressurization system operational tests were performed during scheduled 'C' checks, with the most recent test completed in September 2013.
Findings
- The direct cause of the cabin altitude deviation was the malfunction of the Cabin Pressure Control Selector located on Panel P5-6.
- The failure was attributed to the natural wear and tear of spare parts, specifically the degradation of components that transmit the rotation of the knobs to the switches.
- The aircraft was operating under an MEL authorization that allowed for the flight despite the automatic pressure control system being inoperative.