Incident Overview
On 3 November 2011 at 18:15 UTC, a BOEING 737-400 began taxiing to Runway 33 at Warsaw Chopin Airport (EPWA) for a scheduled flight to Brussels (WAW-BRU). During takeoff, at approximately 100 ft above ground, the left cockpit window (Captain's side) opened. The crew aborted the standard departure procedure, leveling off at about 2500 ft and executing a 180-degree left turn to a heading of approximately 155 degrees for an immediate landing back at Warsaw.
Noise from the open window prevented radio communication with air traffic control and cabin crew. The crew informed ATC that they could not receive transmissions. After turning onto final approach for Runway 33 and configuring the aircraft for landing, the Captain managed to close the window, restoring two-way radio communication. The entire flight was conducted with landing gear extended and flaps at 5. Landing occurred at 18:47 UTC with flaps 30. Airport fire services were alerted and provided assistance during landing. After taxiing to the parking stand, the crew handed the aircraft over to maintenance. Following identification of the causes, corrective actions, and tests, the aircraft was returned to service. The next flight departed at 19:54 UTC with the same aircraft and crew.
Findings
- Before the incident, during a walk-around inspection, the Captain found maintenance technicians addressing an autopilot 'B' discrepancy logged from the previous flight. To rectify this, a mechanic had opened the avionics bay door per AMM 22-11-34-501.
- During taxi, a momentary EQUIP and MASTER CAUTION warning appeared. The crew performed a RECALL check but, finding no confirmation of a fault, dismissed it as false.
- About 20 seconds after liftoff, a MASTER CAUTION illuminated and the Captain's window (Window No. 2) opened. The First Officer (pilot flying) observed an EQUIP light on the warning panel, indicating an open avionics bay door.
- The Captain, after an unsuccessful attempt to close the window, ordered the aircraft to level at 2500 ft and execute a circuit for an immediate landing. The approach was flown with gear down and flaps 5, in visual meteorological conditions.
- After the final turn and landing configuration, the Captain successfully closed the window, restoring two-way radio. The landing was uneventful with fire services standing by.
- Weather at EPWA at the time included variable wind at 2 knots, general visibility 200 m, visibility on Runway 33 between 200 and 475 m, patchy fog, and no significant clouds. These conditions were below the landing minima for Category C aircraft (cloud base 133 ft, visibility 400 m). However, the crew remained in visual conditions, and their decision to land was deemed correct given the serious malfunction requiring immediate landing.
- Post-landing inspection by mechanics revealed an improperly closed forward avionics bay door and a malfunctioning door warning system. The left cockpit window locking mechanism was lubricated and checked for proper operation. The door warning light system switch (S197) was cleaned per AMM 52-71-42P201-203 REV 74. The aircraft was cleared for flight under LOT MEL 52-31, which considered the door warning light system inoperative, requiring verification that all doors are closed and locked before departure.
Causes of the Incident
The investigation identified multiple contributing causes: 1. The cause of the cockpit window opening was not determined. Maintenance lubricated the window latches but found no defects or malfunctions. 2. The avionics bay door opened due to improper closure by a maintenance mechanic, attributed to haste and working after 13 hours of duty. 3. A fault in the door warning light system prevented the flight crew from detecting the incorrect closure during pre-flight checks. 4. The open avionics bay door prevented the cabin from pressurizing. 5. Noise from the open window made any radio communication or intercom impossible, forcing an immediate return to the departure airport.
Preventive Actions
The operator discussed the incident with flight crews during periodic training for the 2011/2012 season. The investigation report was forwarded to the operator's training department and maintenance organization for use during recurrent training for flight, cabin, and technical personnel in the 2012/2013 season. The maintenance organization issued a Technical Event Report recommending: discussing the event with involved personnel, sending the two involved employees for additional human factors and procedure training, and distributing information for inclusion in periodic training. The State Commission on Aircraft Accident Investigation did not issue safety recommendations.