Incident Details
On January 8, 2008, at approximately 1745 central standard time, N733TW, a Boeing 737-2H4 operated by Ameristar Air Cargo, Inc., experienced a loss of directional control while taxiing after landing at Lincoln Airport (LNK) near Lincoln, Nebraska. The aircraft exited the taxiway into mud and snow. The flight was a non-scheduled domestic passenger operation under 14 CFR Part 121. The aircraft sustained no damage. The airline transport rated pilot and co-pilot, 4 flight attendants, 1 jump seat occupant, and 31 passengers were uninjured and deplaned via the airplane's airstairs. Visual meteorological conditions prevailed. The flight originated at 1559 from Austin-Bergstrom International Airport (AUS) and was destined for LNK.
Hydraulic System Failure
According to the captain's statement, the landing and initial taxi were normal. After exiting Runway 36 onto Taxiway Juliet per tower instructions, the master caution "HYD" light illuminated, followed by an immediate loss of System B hydraulics. The captain attempted to stop the aircraft but found no braking response and no nose wheel steering via the tiller. He then tried to deploy thrust reversers without success. Within seconds, a total loss of System A hydraulics also occurred. The aircraft began a slight left turn; the captain and first officer applied right rudder with no effect, and the aircraft departed the taxiway to the left into mud and snow.
Maintenance Background
The operator reported that on January 6, 2008, the airplane underwent maintenance for a left main landing gear swivel valve leak. A mechanic removed both swivel valves on the left main landing gear: the aft valve was replaced with a new unit, and the forward valve was removed and reinstalled. The maintenance was checked for leaks and the aircraft was returned to service.
Examination after the incident revealed that both swivel valves were not installed in accordance with the maintenance manual. The manual (32-41-61) contains a warning stating that incorrect orientation of the swivel joint and hydraulic lines can prevent swivel operation, leading to hydraulic line breakage and loss of brake pressure during gear extension and retraction.