CHI08IA022

The failure of the Brake and Steering Control Unit, which allowed the nose landing gear to turn to the 90-degree position.

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.

HISTORY OF FLIGHT On October 20, 2007, about 2007 central daylight time, an Airbus A320-211, N331NW, landed on Runway 18 at the Hector International Airport (FAR), Fargo, North Dakota, with the nose gear turned 90 degrees from the direction of travel. The airplane sustained minor damage to the nose gear assembly. The captain, first officer, 2 flight attendants, and 134 passengers were not injured. The 14 Code of Federal Regulations Part 121 scheduled domestic passenger flight was being operated by Northwest Airlines as flight 1432. Visual meteorological conditions prevailed and an instrument flight rules flight plan was filed. The flight originated from the Minneapolis-St. Paul International Airport (MSP), Minneapolis, Minnesota, at 1911. Shortly after departing MSP the flight crew received a BRAKES BSCU CH2 FAULT caution display on the upper Electronic Centralized Aircraft Monitoring System (ECAM). The captain stated he noted that the autobrakes remained set to “maximum” after gear retraction. Upon reaching 10,000 feet, the flight crew reset the BSCU per the cockpit operational manual (COM); however, the BRAKES BSCU CH2 FAULT ECAM caution returned. The captain reported they could not turn off the autobrakes, nor could they change the brake setting to Low or Medium. The flight crew established they had yellow brake pressure (alternate brakes and parking brake), and given the indications they had in the cockpit, they believed they would be landing with alternate brakes. The crew then contacted the company maintenance control (MC) and advised them of the situation. The captain reported that when they turned the antiskid and nose wheel steering to the off position, maximum braking would also turn off. Based on that information, the crew discussed the possibility of having to land with the antiskid and nose wheel steering turned off. Although the first officer was the flying pilot for the flight, the captain took over the flying duties during the landing approach with the autopilot and autothrottles on. He reported that when the flaps were lowered to the number 3 position and the landing gear was extended, they received a landing gear (L/G) shock absorber fault indication and the autothrottles (A/THR) and autopilot turned off. The crew contacted the air traffic control tower (ATCT) and requested a climb so they could assess the situation. The captain stated the landing gear remained down and the flaps were raised to the number 2 position. The first officer continued to address the messages on the upper ECAM by performing the procedures in the COM. The first officer also contacted MC informing them of the L/G shock absorber fault. The first officer then informed the captain that he had finished the COM procedures and that the autopilot and A/THR were still inoperative. The flight crew then requested and received vectors to final approach. The captain stated the flaps were selected to full and they completed the landing checklist. He stated that they briefed keeping the brake pressure below 1,000 pounds per square inch (psi) during the landing and they would continue the landing roll with manual braking until they came to a stop. They would then turn the nose wheel steering on and taxi to the gate. The captain stated they anticipated an uneventful landing so they did not advise the ATCT of the situation. The captain stated the touchdown was normal, and the spoilers and reversers were deployed as the nose wheel was lowered to the runway. He stated they felt a vibration when the nose wheel touched down, and the ATCT informed them of flames near the nose gear. The ATCT called out the emergency equipment upon seeing the flames. The captain stated they continued the landing roll on the centerline with a brake pressure of 500 psi. He reported the vibration increased as they were slowing and that the air traffic controller informed them that they no longer could see flames. The airplane came to rest on Runway 18 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. PERSONNEL INFORMATION The captain, age 57, held an airline transport pilot (ATP) certificate. He held ratings for airplane single-engine land, airplane multi-engine land, and airplane single-engine sea ratings. The captain held a first-class medical certificate issued on June 5, 2007. The certificate contained a limitation requiring corrective lenses for near and distant vision. The captain had a total flight time of 18,440 hours, of which 6,309 hours were in Airbus A320 airplanes. The first officer, age 38, held ATP and flight instructor certificates. He held airplane single-engine land and airplane multi-engine land ratings. He also held a certified flight instructor certificate which contained airplane single-engine land, airplane multi-engine land, and instrument airplane ratings. The first officer held a first-class medical certificate issued on March 6, 2007. The medical certificate did not contain any limitations. The first officer had a total flight time at Northwest Airlines of 857 hours, of which 50 hours were in Airbus A320 airplanes. AIRCRAFT INFORMATION N331NW, manufacturer serial number 318, Northwest Airlines ship number 3231, was an Airbus A320-211 equipped with CFM56-5A1 engines. The maximum gross weight of the airplane was 166,400 pounds and the estimated gross weight at the time of the incident was 135,300 pounds. The nose landing gear was last serviced on April 12, 2007. At that time, a pressure check was accomplished and the nose gear strut was serviced with nitrogen and hydraulic fluid. The Northwest Airlines maintenance program called for the nose gear to be examined at each turnaround check and line check. On June 29, 2007, a pilot generated a maintenance log page stating that 1 – 1.5 inches of chrome was observed on the nose landing gear strut. There was no log entry that the nose landing gear strut was serviced at this time, only that the strut condition was within limits. METEOROLOGICAL INFORMATION The weather conditions reported at FAR at 1953 were: Wind 250 degrees at 4 knots; visibility 10 statute miles; few clouds at 10,000 feet; temperature 12 degrees Celsius; dew point 6 degrees Celsius; altimeter 29.55 inches of mercury. RECORDERS The airplane was equipped with a L-3 Communications Fairchild Model F100, 128 Word flight data recorder (FDR). The recorder was in good condition and the data were extracted normally. The FDR contained approximately 56 hours of data. According to the recorded data the Autobrake Decel Lo, Autobrake Decel Med, Autobrake Decel Max, Autobrake Fault, Autobrake Off, and the Brake Pedal and Brake Pressure parameters stopped registering valid data within 5 seconds after takeoff. Over the next 40 minutes, the flight climbed to 24,000 feet, cruised, and descended for the approach. At 19:54:24, the landing gear was selected down and the FDR recorded the gear down and locked at 19:54:37. Twenty seconds later, the landing gear Weight-on-Wheels (WOW) parameter for the nose transitioned from air to ground. At 19:55:28, the Autopilot 2, Autothrottle, and Flight Director parameters switched from engaged to disengaged, and a master Warning was recorded. The Gear WOW for Left and Right indicated ground at 20:06:46. The airplane was equipped with a Honeywell 6022 SSCVR 120 cockpit voice recorder (CVR). The recorder was in good condition and the audio information was extracted normally. A summary of the audio information on the CVR was prepared. The recording began at 18:28:13. The engine start and takeoff were normal. Shortly after takeoff the captain commented “we’ll deal with it later.” Several minutes later the captain stated