Incident Overview
Shortly after the landing gear handle was moved to the up position during the initial climb, the flight crew observed an error message on the Electric Centralized Aircraft Monitoring (ECAM) system indicating a fault (L/G SHOCK ABSORBER FAULT) for the nose landing gear (NLG) shock absorber. The gear handle was subsequently moved to the down position, and the crew received another error message for a nose wheel steering fault (WHEEL N/W STRG FAULT). After determining that the nose landing gear was cocked 90 degrees from its normal orientation, the crew landed at an alternate airport. During the landing, the NLG tires and both wheels were worn down into the axle.
Cockpit Indications and Crew Actions
The initial ECAM fault occurred immediately after the gear retraction command. Upon gear extension, the nose landing gear achieved a down and locked position 1.5 seconds before the main landing gear and/or all landing gear doors closed. At that time, the nose wheel assembly was not centered. The Brake Steering Control Unit (BSCU) detected this off-center condition and attempted to re-center the nose wheel. However, due to the sequencing of the nose and main landing gear and their respective doors, hydraulic pressure was shut off to the NLG steering valve. This lack of hydraulic power to the servo valve resulted in a lack of position feedback to the BSCU. After a 0.5-second monitoring period, the BSCU detected this as a fault and deactivated the steering system, preventing the BSCU from returning the nose wheels to center. The failure of the nose wheels to center triggered the WHEEL N/W STRG FAULT caution message on the ECAM. The source notes that there were no approved procedures allowing the flight crew to attempt to reset the BSCU system, which could have re-enabled the hydraulic system and potentially allowed the system to re-center the nose wheels.
Post-Incident Examination
Post-incident examination of the nose gear assembly revealed that two of the four anti-rotation lugs on the NLG upper support assembly had fractured and separated. The fractures were attributed to induced fatigue from the steering system's programmed pre-landing dynamic steering tests, which repeatedly cycle pressure to the steering cylinders. The failed lugs allowed the NLG to deviate from its 0-degree position in the landing gear bay upon gear retraction during takeoff. This deviation resulted in the L/G SHOCK ABSORBER FAULT error message on the ECAM system.
System Operation
The sequence of events began with the fractured anti-rotation lugs, which permitted the NLG to rotate beyond its normal limits when the gear was retracted. This misalignment caused the shock absorber fault indication. When the gear was subsequently extended, the nose gear locked down earlier than the main gear and doors, leaving the nose wheel off-center. The BSCU’s attempt to correct the misalignment was foiled by the lack of hydraulic pressure due to the gear and door sequencing. The BSCU then detected the fault and deactivated the steering system, leaving the nose wheel in a cocked position. The source does not provide further analysis or causal links beyond these factual events.