What happened
On July 5, 2012, a Boeing 737-400 operating a flight from Madrid to Warsaw experienced a cabin altitude increase accompanied by an audible warning. The flight crew initiated a descent, though they did not declare an emergency procedure. During the descent, the cabin altitude reached a maximum of 13,0 and a total depressurization occurred at approximately 11,000 ft. Because passenger oxygen masks did not deploy, it was determined that the cabin altitude did not exceed 14,000 ft.
In addition to the pressure issues, the aircraft experienced malfunctions regarding pitot heating and the autothrottle system. The crew performed a normal landing at Geneva Airport. Following the incident, the aircraft was withdrawn from service. On July 7, 2012, the aircraft was ferried from Geneva to Warsaw. However, during this ferry flight, the technical malfunction recurred, and the aircraft was subsequently moved to a hangar for repairs.
The investigation
Following the landing in Geneva, a technical team performed system tests which initially yielded positive results, leading the operator's operations center to declare the aircraft airworthy. The investigation also examined the flight status of the Geneva-Warsaw leg, which was incorrectly logged as a "FERRY FLIGHT" instead of an operational repositioning flight. This administrative error was attributed to a mistake by the operator's operations center.
Upon the aircraft's return to Warsaw, a detailed analysis of the flight data recorder was conducted. This analysis revealed that the AIR/GROUND system was malfunctioning; specifically, while the aircraft was in flight (as confirmed by barometric altitude), the system incorrectly indicated a "GROUND" status. Analysis of the Merlin system showed that this was an isolated incident, as no similar malfunctions were recorded in the three months preceding or following the event.
Findings
- The primary cause of the depressurization and the secondary electrical/system malfunctions was a failure of the Main Landing Gear Ground Sensor (S105) and the Landing Gear Logic Shelf Printed Circuit Card (M990).
- The malfunction of the AIR/GROUND system directly caused the erroneous signals regarding pitot heating and autothrottle functionality.
- The technical issue was successfully resolved by replacing the faulty sensor and circuit card in accordance with the Aircraft Maintenance Manual.
Safety action
- The incident was documented in the B737 Fleet Safety Bulletin No. 7/12.
- The technical malfunction is to be discussed during periodic training sessions for maintenance personnel.