What happened
On 2 August 2009, an Airbus A320-232, registration PH-AAY, was taxiing toward runway 21 at Maastricht Aachen Airport for a commercial passenger flight to Turkey. While performing a 90-degree right turn on taxiway W, the aircraft suddenly lost all nose wheel steering capability.
The flight crew attempted to reset the anti-skid and steering systems, but the failure persisted. In an effort to maintain the turn, the crew applied differential braking, applying more pressure to the left pedal. This maneuver caused the aircraft to veer left, exiting the paved surface at an angle. The left main gear sank approximately 20 cm into the soft ground, causing the aircraft to pivot until it came to a halt with the fuselage nearly perpendicular to the taxiway. There were no injuries among the 58 passengers or the six crew members, though a taxiway edge light was damaged and minor damage occurred to the aircraft's left main gear.
The investigation
The Dutch Safety Board (OVV) examined the aircraft's Braking and Steering Control Unit (BSCU) and the flight data recorder. The investigation revealed that the aircraft was operating under a Minimum Equipment List (MEL) condition, as a circuit breaker for BSCU system 1 had been pulled due to a previous fault.
Technical analysis of the landing gear proximity sensors was a primary focus. Investigators found that while standard resistance tests were within limits, inductance tests revealed that specific sensors were malfunctioning. Specifically, the investigation identified that the left-hand downlock proximity sensor (17GA) was failing due to moisture ingress into its ferrite core.
Findings
- The primary cause of the loss of steering was a faulty downlock proximity sensor that intermittently lost its "near" position.
- This sensor failure triggered a logic sequence where the landing gear doors selector valve was de-energized.
- The de-energization of this valve resulted in the loss of hydraulic supply to the nose wheel steering hydraulic module.
- The crew's use of differential braking with increased pressure on the left pedal contributed to the aircraft's departure from the taxiway.
- The limited time available between the steering failure and the aircraft leaving the pavement prevented the crew from regaining control via braking alone.