COLUMBUS, OH, USA
N628AW
Airbus Industrie A-320-231
When the landing gear was lowered, the flight crew received landing gear control and interface unit (LGCIU) faults. A visual fly-by revealed the nose wheels were rotated 90 degrees from the desired direction for landing. A normal touchdown was made, after which, the captain commanded an emergency evacuation using the overwing exits. Examination of the airplane revealed the external 'O' rings in the steering control valve had extruded and by-passed pressurized hydraulic fluid to rotate the nose wheels. This event had occurred before, and the manufacturer had issued a service bulletin. The operator had not complied with the service bulletin, nor were they required to comply with it.
On February 16, 1999, at 1602 Eastern Standard Time, an Airbus A-320-231, N628AW, operated by America West Airlines as flight 2811, received minor damage when it landed at Port Columbus International Airport (CMH), Columbus, Ohio, with the nose wheels rotated 90 degrees. There were no injuries to the 2 certificated pilots, 3 flight attendants and 26 passengers. Visual meteorological conditions prevailed for the scheduled passenger flight which had departed from Newark (EWR), New Jersey, about 1404. Flight 2811 was operated on an instrument flight rules flight plan conducted under 14 CFR Part 121. According to statements from the flight crew, flight 2811 was uneventful until the landing gear was lowered prior to landing at CMH. After the landing gear was extended to the down-and-locked position, the flight crew received indications of dual landing gear control and interface unit (LGCIU) faults. The flight crew entered into a holding pattern and attempted to troubleshoot the faults; however, they were unable to determine the source of the problem. The flight crew then prepared for a landing at CMH, with nosewheel steering and thrust reversers inoperative due to the faults. During the final approach, at the flight crew's request, the control tower performed a visual check of the landing gear, which revealed that the nosewheels were rotated about 90 degrees. The flight crew then initiated a missed approach and declared an emergency. The cabin crew was notified of an impending emergency landing, and the cabin and passengers were prepared for the landing. The captain initiated the approach, and described the touchdown as soft. The airplane stopped on the 10,250-foot-long runway with about 2,500 feet of runway remaining. Damage was limited to the nose landing gear tires and rims. The captain reported that after landing, he noticed smoke was drifting up on the right side of the airplane. He said he attempted to contact the control tower and confirm if a fire was present, but was unable due to frequency congestion. He then initiated an emergency evacuation using the left and right side overwing exits. A review of the air/ground communications, as recorded by the Columbus Air Traffic Control Tower, did not reveal a congested frequency when the emergency evacuation was initiated. According to Airbus, nose wheel steering was hydraulically actuated through either the cockpit tiller and/or the rudder pedals. A post-incident visual inspection of the nose landing gear assembly revealed no anomalies. The steering control module was replaced, and a subsequent functional check of the nosewheel steering was successful. The steering control module was a sealed unit, opened only during overhaul, with no specified overhaul time, and had accumulated 3,860 hours since last overhauled on March 3, 1998. It was shipped to Messier-Bugatti, the manufacturer, and examined under the supervision of the French Bureau Enquetes Accidents (BEA). The examination revealed that the external hydraulic O-ring seals on the steering control module's selector valve were extruded (distorted out of the seal's groove). A small offset was found in the steering control valve. Airbus further reported that while the offset would have been measurable, it would not have been noticeable under normal operations. Additionally, during landing gear extension, the brake and steering control unit (BSCU) would have been energized and hydraulic pressure would have been directed toward the steering servo valve. The BSCU would have then commanded a small rotation of the nose wheel to check for proper movement. Any disagreement between the commanded position and actual position of the nose wheel would have deactivated the nose wheel steering. However, if hydraulic pressure had bypassed the steering control valve, there would have been continued pressurization to the servo valve, and because of the servo valve's inherent offset, in-flight rotation of the nose wheels. Procedures existed for removal of hydraulic pressure from the steering control module. However, once the nosewheel strut had deflected 90 degrees, the centering cam would have been rotated to a flat area, and would have been incapable of overriding the 3,000 PSI hydraulic system, and returning the nose wheels to a centered position. Documents from Airbus indicated there have been three similar incidents in which A320 airplanes landed with the nose wheels rotated about 90 degrees. Examination of the steering control modules on two of the airplanes revealed extrusion of the selector valve's external seals similar to that found on N628AW. Airbus had attributed the extrusion failures to the lack of a backup seal or the effects of aging on the seals. As a result of these incidents, Airbus issued Service Bulletin (SB) A320-32-1197 on October 8, 1998, to recommend replacement of the external seals on the steering control module's selector valve on A320 and A321 airplanes within 18 months of the SB's issuance. At the time of the incident, neither the French Direction General de l'Aviation Civile (DGAC), or the Federal Aviation Administration (FAA), had adopted the service bulletin as an airworthiness directive. The operator was not required to comply with the service bulletin, and had not complied with it. On March 24, 1999, the DGAC issued Airworthiness Directive (AD) 1999-124-129(B) to require compliance with the SB. On December 17, 1999, the FAA issued AD 99-23-09 which was based upon the French AD, with a 12 month time of compliance for modification of the nose wheel steering control valve.
a failure of the external o-rings in the nose landing gear steering module.
Source: NTSB Aviation Accident Database
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