What happened
On November 3, 2011, at 18:15 UTC, a Boeing 737-400 began taxiing for runway 33 at Warsaw Chopin Airport (EPWA) for a flight to Brussels. During the takeoff roll, approximately 20 seconds after liftoff, the aircraft's MASTER CAUTION and EQUIP lights illuminated. Shortly thereafter, the left cockpit window opened unexpectedly.
The sudden opening of the window created extreme noise levels within the cockpit, which rendered radio communication with Air Traffic Control (ATC) and internal communication between the flight crew and cabin crew impossible. The crew initiated an immediate return to Warsaw, leveling off at approximately 2/2500 feet and performing a 180-degree turn.
To manage the emergency, the crew operated the aircraft with the landing gear extended and flaps set to position 5. During the approach, the captain successfully closed the window, restoring two-way radio communication. The aircraft landed at 18:47 UTC with the assistance of the Airport Fire Service. The landing was performed using flap 30 configuration under visibility conditions that were near the minimums for the airport due to patchy fog.
The investigation
The investigation focused on why the window opened and why the crew did not detect the underlying issue during pre-flight checks. Investigators found that maintenance personnel had been working on the aircraft prior to the flight to repair a malfunction in Autopilot 'B'. To perform this repair, the avionics bay had to be opened.
During the pre-flight taxi, the crew noticed the EQUIP and MASTER CAUTION lights but, after using the RECALL button and finding no further indication of a fault, dismissed the warnings as false. Post-flight inspection by maintenance staff revealed that the forward avionics bay door had been improperly closed and the monitoring system for the door's closure was malfunctioning.
Findings
- The primary cause of the window opening was the improperly closed avionics bay door.
- The failure to secure the avionics bay door was caused by maintenance personnel working in haste and fatigue during the 13th hour of their shift.
- The failure of the door closure warning system prevented the flight crew from detecting the unsecured bay during routine cockpit checks.
- The open avionics bay prevented the aircraft cabin from maintaining proper pressurization, which led to the window opening.
- The extreme noise generated by the open window necessitated the immediate emergency landing due to the loss of all vital communications.
- The specific cause for the window mechanism's failure to remain closed was not definitively established, though the window locks were subsequently lubricated.
Safety action
- The operator discussed the incident with flight crews during periodic training sessions.
- The investigation report was distributed to the operator's training and maintenance organizations for use in personnel training.
- The maintenance organization addressed the incident by providing additional human factors and MOE procedure training to the technicians involved.