Canadian safety authorities have ordered modification of Bombardier Q400 nose-wheel steering systems after a latent design deficiency was discovered during a pushback incident.
The aircraft involved experienced an uncommanded, and unannunciated, steering of the nose wheel as it was pushed back from the gate.
Investigations revealed that a sustained open circuit within the steering control unit could result in uncommanded steering input, and possible runway excursion if this should occur during take-off or landing.
The modification, covering all Q400s up to serial number 4448, requires installation of new cable assemblies to ensure that nose-wheel steering reverts to a fail-safe free-castor mode should there be an open circuit in the system.
Regulator Transport Canada says the changes must be made within one year, or 2,000 cycles.