Incident Overview
On 2 December 2014, a Bombardier DHC-8-400 aircraft was taxiing at Warsaw Chopin Airport (EPWA) when a sudden, spontaneous change in the nose wheel steering angle occurred. The crew observed unintended movements of the steering handwheel and the nose wheel oscillating without any input to the tiller.
Immediate Actions
The crew returned the aircraft to its parking stand and summoned technical services. Maintenance personnel removed the steering tiller handwheel and replaced it with one from another aircraft. The steering system was then subjected to functional tests. After satisfactory results, the aircraft was returned to line service.
Investigation Findings
The investigation determined that the incident was caused by incorrect lubrication of the spring inside the nose wheel steering tiller mechanism, resulting in excessive spring stiffness. This condition led to a shift in the potentiometer output limits for left and right handwheel rotation.
Preventive Measures
Because a similar issue had previously occurred on another aircraft (registration SP-EQI, incident number 1279/14), the operator requested a defect report from the component manufacturer. No safety recommendations were issued by the investigating body.