NTSB Identification: CHI08IA022
Scheduled 14 CFR Part 121: Air Carrier operation of Northwest Airlines (D.B.A. operation of Northwest Airlines)
Incident occurred Saturday, October 20, 2007 in Fargo, ND
Probable Cause Approval Date: 03/16/2011
Aircraft: Airbus Industrie A320-211, registration: N331NW
Injuries: 138 Uninjured.
NTSB investigators used data provided by various sources and may not have traveled in support of this investigation to prepare this aircraft incident report.
Shortly after departure, the flightcrew received a Brake and Steering Control Unit (BSCU) channel 2 fault message. Upon reaching 10,000 feet, the crew reset the BSCU per the cockpit operational manual (COM); however, the BSCU channel 2 fault returned and they could not turn off the auto brakes, nor could they change the brake setting from “Maximum.” The crew confirmed that they had “yellow” brake pressure (alternate brakes and parking brake), and believed that they would be landing with alternate brakes. Because turning the antiskid and nosewheel steering to the “Off” position would also turn off maximum braking, they discussed landing with the antiskid and nosewheel steering selected “Off” if necessary. When the crew extended the landing gear during the first approach, they received a landing gear shock absorber fault indication and the auto throttles and autopilot turned off. The crew then contacted air traffic control (ATC) and requested a climb to assess the situation. The captain stated that the landing gear remained down and the flaps were raised. The first officer continued to perform the procedures in the COM and contacted maintenance control to inform them of the landing gear shock absorber fault. The autopilot and auto throttles remained inoperative.
The flight crew then requested and received vectors to final approach. The captain stated that they set the flaps to full and completed the landing checklist. He stated that they planned to keep the brake pressure low during the landing and continue the landing roll with manual braking until they came to a stop. They then intended to turn the nosewheel steering on and taxi to the gate. The captain stated they anticipated an uneventful landing and did not advise ATC of the situation. After landing, the crew felt a vibration when the nosewheel touched down and ATC informed them of flames near the nose gear. The airplane came to rest on the runway with the nose gear strut turned 90 degrees to the direction of landing. The lower half of both the nose gear tire and wheel assembly was ground away due to runway contact. The nose gear was the only part of the airplane that sustained damage.
Postincident examination and testing of the nose landing gear systems revealed an unidentified and intermittent failure of an integrated circuit on the BSCU monitor board. This failure most likely resulted in frozen BSCU System 1 analog and discrete outputs. If the BSCU System 1 commands had frozen, the BSCU System 2 would have been able to detect the System 1 failure but would not have been able to take control. As a result of this incident, the aircraft and the nose gear manufacturers established a program intended to increase the reliability of the BSCU. Each issued a service bulletin for modification of the BSCU software and both the Federal Aviation Administration and European Aviation Safety Agency issued Airworthiness Directives that made compliance with the service bulletin mandatory.
The National Transportation Safety Board determines the probable cause(s) of this incident to be: The failure of the Brake and Steering Control Unit, which allowed the nose landing gear to turn to the 90-degree position. Full narrative available
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