Summary
On 30 September 2013, a Boeing 737-400 experienced a cabin pressurisation malfunction shortly after takeoff from London Heathrow Airport (EGLL). The flight crew noticed that normal cabin pressurisation was not occurring, indicated by an AUTO FAIL message. They switched the pressurisation control mode from AUTO to STANDBY, which restored normal pressurisation. The cabin altitude did not approach limits because the crew reacted promptly.
Sequence of Events
Immediately after departure from London Heathrow, the crew observed abnormal cabin pressurisation and received an AUTO FAIL message. Selecting STANDBY mode (which also automatically controls the outflow valve) corrected the issue. After landing, the crew recorded the malfunction in the aircraft technical log. Maintenance personnel at Warsaw (WAW) released the aircraft for further flights with the AUTO mode inoperative, in accordance with MEL 21-14-01, which permits operation until the next C check (every 4,000 flight hours).
Since this was the second occurrence of the fault, a troubleshooting plan was developed. However, execution was limited by aircraft availability, as the troubleshooting process for the Boeing 737 cabin pressure control system is lengthy and cannot be performed during short turnaround times. The aircraft continued in service until 4 October 2013, when the fault recurred, and the aircraft was handed over to technical services.
Analysis and inspections identified a faulty bleed valve as the direct cause. The valve was replaced, and the malfunction ceased. The removed valve was sent to the manufacturer for repair; no information is available on the root cause of its failure. After repair, it was reinstalled on 21 February 2014. The valve had accumulated 2,299 flight hours since its last overhaul; the scheduled overhaul interval is 12,000 flight hours. The valve had been repaired three times in the preceding three years.
Cause
The investigation determined that the incident was caused by a malfunction of the bleed valve in the cabin pressurisation system.