Incident Overview
Following a landing roll, the captain reported that it was "extremely difficult to turn" the airplane off the runway. During the turn, the airplane reached a 45-degree angle relative to the runway and could not turn further. The steering system had failed. The captain then attempted to stop the airplane on the taxiway using brakes and reverse thrust, but the airplane did not stop and rolled off onto the grass area adjacent to the taxiway.
Examination Findings
Post-incident examination of the brake system revealed that the hydraulic system power transfer unit (PTU) housing was cracked around its entire circumference. The PTU installed on the incident airplane was identified as an early style unit according to the manufacturer’s service bulletin. That service bulletin advised owners of the airplane to replace the original aluminum housing with a cast iron type.
Examination of the PTU's housing fracture surfaces disclosed shrinkage cavities and large silicon inclusions.
Relevant Directives
No additional directives or recommendations were stated in the source material.