What happened
On October 4, 2013, a Boeing 737-400 was taxiing for departure at Tel Aviv (LLBG) when the crew identified an irregularity in the pressurization system. The aircraft was operating under a Minimum Equipment List (MEL) authorization that allowed for the automatic pressurization control mode to be inoperative.
After completing the before-taxi checklist, the crew moved the flight/ground switch to the flight position. Upon doing so, they observed that the outflow valve failed to close and remained in the open position, preventing the cabin from pressurizing. While the crew was able to close the valve by switching to manual DC mode, the valve reverted to the open position whenever the standby mode was engaged. Simultaneously, the cabin crew reported loud noises within the cabin. Due to the inability to operate the system in manual mode under the current flight authorization and the ongoing malfunction, the pilot decided to return to the parking stand and cancel the return flight to Warsaw.
The investigation
The investigation, conducted by the operator, focused on identifying the source of the valve failure and the recurring nature of the issue. Technical teams from Warsaw were dispatched to Tel Aviv on October 5, 2013, to perform repairs. The investigation established that the outflow valve had been replaced and the valve housing seal was installed according to the Aircraft Maintenance Manual (AMM).
Further analysis revealed that troubleshooting the B737 cabin pressure regulation system is a complex and time-consuming process, which had been difficult to perform during short ground intervals while the aircraft remained in active service. The investigation also noted that the faulty valve had been sent to the manufacturer for repair, though the specific cause of its internal failure remained unknown. Records indicated that similar outflow valve failures had occurred three times within the last three years for this specific component.
Findings
- The direct cause of the incident was a malfunctioning outflow valve within the cabin pressurization system.
- The valve in question had accumulated 2,299 flight hours since its last overhaul, whereas the maintenance program requires a full overhaul every 12,000 flight hours.
- The crew correctly identified the system failure during taxiing and prioritized safety by aborting the flight when the system could not be operated within the permitted MEL parameters.