What happened
On January 15, 2007, a Learjet 60, registration OH-AEM, was performing a commercial flight when it experienced a landing gear malfunction upon arrival at Helsinki-Vantaa Airport. During the landing roll, the wheels on the left main landing gear locked completely, causing the outermost tire to puncture and deflate. The wheels on the right main landing gear also experienced partial locking.
As the pilot taxied the aircraft, the nose wheel steering system automatically disengaged. This loss of steering caused the aircraft to veer off the taxiway and onto a snow-covered safety area. During the excursion, the leading edge of the left wing struck a taxiway sign, resulting in structural deformation to the wing and the destruction of the sign.
The investigation
Investigators examined the aircraft's braking and steering systems, including testing conducted at the manufacturer's facility. The investigation focused on the anti-skid valves and the Digital Nose Wheel Steering Computer.
Testing of the anti-skid system's solenoid valves (part numbers P/N 39-309-2, serial numbers S/N 2585 and S/N 2584) revealed an electrical discontinuity. This electrical fault rendered the valves inoperable, directly allowing the wheels to lock during the landing roll.
Regarding the steering failure, analysis of the Digital Nose Wheel Steering Computer's fault memory identified four error logs related to the locked wheels during the taxi phase. These errors were triggered by wheel tachometer discrepancies. The system is designed to automatically disengage the nose wheel steering if the ground speed exceeds 10 knots and the two lower wheel speed readings drop below 60% of the highest reading, a safety feature intended to prevent steering conflicts during asymmetric braking.