What happened
Following the landing roll, the captain attempted to turn the aircraft off the runway. During this maneuver, the pilot reported extreme difficulty in executing the turn, noting that the aircraft reached a 45-degree angle relative to the runway and would not turn further.
A failure occurred within the steering system during the maneuver. In an attempt to regain control and stop the aircraft on the taxiway, the captain applied the brakes and utilized reverse thrust. Despite these efforts, the aircraft failed to stop and subsequently rolled off the taxiway onto the adjacent grass area.
The investigation
Investigators examined the brake system and identified that the housing of the hydraulic system's Power Transfer Unit (PTU) was cracked around its entire circumference.
According to a manufacturer's service bulletin, the PTU installed on this aircraft was an early-style unit. The bulletin had previously advised operators to replace the original aluminum housing with a cast iron version. Upon inspection of the fracture surfaces on the PTU housing, investigators discovered shrinkage cavities and large silicon inclusions.