Incident Overview
On 4 October 2013, a Boeing 737-400 operating at Tel Aviv Ben Gurion Airport (LLBG) experienced a pressurization system malfunction during taxi. The aircraft had been flying with a Minimum Equipment List (MEL) dispatch for an inoperative AUTO pressurization control mode. After performing the BEFORE TAXI checklist, the crew moved the FLIGHT/GROUND switch to the FLIGHT position. They observed that the outflow valve did not close and remained in the OPEN position, preventing cabin pressurization. When the valve was set to MANUAL DC, it closed, but upon returning to STBY mode, it reopened. Cabin crew reported loud noises in the cabin. The crew decided to return to the parking stand because there was no dispatch approval for MANUAL mode and because of an abnormality during the descent into Tel Aviv: at approximately FL70, a L BLEED TRIP OFF message appeared and then disappeared, accompanied by a bang-like sound.
Technical Findings
The return flight to Warsaw (WAW) was cancelled. On 5 October 2013, a technical team from WAW arrived and replaced the outflow valve in accordance with AMM 21-31-11 PAGE 401 REV77, and also installed a valve housing seal. The aircraft returned to WAW that same day as an operational flight. After a second occurrence of the fault, a troubleshooting plan was developed to locate the issue. Troubleshooting the cabin pressure control system on the B737 is time-consuming and cannot be done during short turnarounds. The aircraft remained in continuous operation until the fault recurred on 4 October 2013, when technical services were dispatched. Analysis and work on the aircraft revealed that the direct cause of the pressurization fault was a faulty bleed valve. After replacement, the fault was eliminated. The faulty valve was sent to the manufacturer for repair; no information on the cause of the failure was provided. After repair, it was reinstalled on 21 February 2014.
Maintenance Actions
Bleed valve faults occur frequently. This particular valve had been sent for repair three times in the previous three years. According to the maintenance program, a full overhaul is performed every 12,000 flight hours. The valve in question had accumulated 2,299 flight hours since its last overhaul.