What happened
On August 24, 2015, a Boeing 737-400 was performing a takeoff from Warsaw Chopin Airport (EPWA) using the NO BLEED TAKE-OFF configuration. During the initial climb, the crew observed a BLEED TRIP OFF warning for the left bleed air system. Following standard procedures, the crew retracted the flaps and subsequently restored the air conditioning configuration to the required post-takeoff settings.
As the aircraft climbed toward FL 120, the crew noticed a sudden increase in cabin altitude caused by a malfunction in the pressurization system, accompanied by an AUTO FAIL warning. During the execution of the necessary checklists, the cabin altitude exceeded 10,000 feet, triggering an audible warning. The commander requested descent to FL 100 and a return to the departure airport. Air Traffic Control authorized a descent to 7,000 feet and placed the aircraft in a holding pattern. While holding, the crew restored proper operation to the air conditioning system. After verifying that the aircraft remained within the maximum landing weight limits, the crew proceeded to land.
The investigation
Technical personnel inspected the engine bleed air system and the pressurization system. The investigation established that the maintenance team replaced the HOSE ASSY in both air conditioning systems and the left-side HIGH STAGE VALVE. Subsequent testing confirmed the system was leak-free and fully operational. The aircraft was returned to service without restrictions, and the malfunction did not recur in subsequent flights.
Findings
- The primary cause of the malfunction was a failure in the air conditioning system resulting from a faulty HIGH STAGE VALVE and a leak in the engine bleed air line.
- The BLEED TRIP OFF warning was triggered by an overpressure condition in the engine bleed air line.
- The overpressure occurred because the monitoring pilot failed to follow the operational instructions provided in a flight crew information bulletin prior to performing the NO-NORMAL CHECKLIST (NNC) procedures.