Incident Overview
On 24 August 2015, a Boeing 737-400 operating from Warsaw Chopin Airport (EPWA) experienced a pressurization problem during climb. The aircraft, carrying 143 occupants, returned safely and no injuries were reported.
Sequence of Events
During takeoff with the air conditioning system configured for NO BLEED TAKE-OFF, the left BLEED TRIP OFF warning light illuminated. The crew continued the takeoff and, after retracting the flaps, restored the air conditioning configuration to the required post-takeoff NO BLEED TAKE-OFF setup. While climbing to flight level 120, the crew observed a sudden increase in cabin altitude due to a pressurization system malfunction, accompanied by the AUTO FAIL warning light. While performing the appropriate non-normal checklist, the cabin altitude exceeded 10,000 feet, triggering an audio warning. The captain requested descent to flight level 100 and a return to the departure airport. Air traffic control cleared the crew to descend to 7,000 feet and to enter a holding pattern. During holding, the crew restored proper air conditioning system operation and, after confirming that the landing weight would not exceed the maximum, decided to land.
Technical Findings
Technical personnel inspected the engine bleed air system and the pressurization system. The HOSE ASSY in both air conditioning packs and the left HIGH STAGE VALVE were replaced. Subsequent leak tests showed system integrity with no leaks. The aircraft was returned to service without restrictions, and the malfunction did not recur on subsequent flights.
Probable Cause
The investigation determined that the incident resulted from an air conditioning system malfunction due to a faulty HIGH STAGE VALVE and a leak in the engine bleed air line. Additionally, the illumination of the BLEED TRIP OFF warning light was caused by an overpressure condition in the bleed air line because the monitoring pilot did not follow operational recommendations contained in an informational bulletin for crews before proceeding with the non-normal checklist.