SA-227 AC Metro III
Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Commercial; Flight Crew Flight Instructor
Flight Crew Last 90 Days 150; Flight Crew Total 1850; Flight Crew Type 450
Other / Unknown
Aircraft Equipment Problem Critical; Ground Excursion Taxiway; Inflight Event / Encounter Loss Of Aircraft Control
Flight Crew Regained Aircraft Control; General Maintenance Action
Aircraft; Human Factors
While taxiing out for departure when it appeared the nose wheel steering failed and turned to the left about 30-40 percent of full travel (estimated).  The aircraft was traveling at approximately 10-15 knots and the reported winds were 230 at 8 kts.  The aircraft veered sharply to the left at which time I placed both engines in full reverse and used full braking in an attempt to stop.  The aircraft continued forward and began to exit the taxiway into the grass at which point I pulled both engine stop and feather knobs and tried using braking to avoid runway and taxiway lights.  After coming to a stop ATC notified me that airport operations was en route at which time I powered down the plane.  After a thorough inspection from both myself and Maintenance it was determined that no obstacles (lights; signs; etc) were struck by the prop or the landing gear.  Airport operations reported no damage to airport property and the plane was then towed to the FBO and was turned over to Maintenance for further inspections.
SA-227 Captain experiences a sudden loss of nose wheel steering control during taxi out; resulting in a sharp swerve to the left and a taxiway excursion before the aircraft can be stopped.