What happened
On September 30, 2013, a Boeing 737-40 and departing from London Heathrow (EGLL) experienced a malfunction in the cabin pressurization system. Immediately after takeoff, the crew received an "AUTO FAIL" message, indicating that normal cabin pressurization was not being maintained. The crew responded by switching the pressurization control mode from AUTO to STANDBY, which engaged automatic control of the outflow valve and successfully restored proper cabin pressure. Because the crew identified and addressed the issue immediately, the cabin altitude did not reach unsafe levels.
Following the flight, the crew recorded the malfunction in the aircraft technical log. Maintenance personnel in Warsaw (WAW) subsequently cleared the aircraft for continued operation under the Minimum Equipment List (MEL 21-14-01), which allows for the operation of the aircraft with a malfunctioning AUTO mode until the next scheduled C-check (every 4,000 flight hours).
The investigation
The investigation, conducted by the operator, focused on the recurring nature of the malfunction. This was the second occurrence of the issue within a short period. Due to the aircraft's continuous operational schedule, performing the complex and time-consuming troubleshooting required for the B737 cabin pressure regulation system was difficult during short ground intervals.
On October 4, 2013, the malfunction recurred, at which point the aircraft was removed from service for technical inspection. The investigation established that the faulty outflow valve was the direct cause of the pressurization failure. Following the replacement of the valve, the issue was resolved. The defective component was sent to the manufacturer for repair, though the specific cause of the valve's failure remains unknown. The valve had been 2,299 flight hours into its 12,000-hour overhaul interval.