Incident
After arrival, while taxiing up to a hangar under the direction of a lineman, the flight experienced a loss of hydraulic pressure and brakes. The pilot could not activate the emergency brakes prior to rolling into the hangar and colliding with two aircraft before coming to rest.
Examination Findings
Examination of the aircraft revealed that the spoileron control valve assembly suffered fracture of a NAS1351-4H-24P screw due to hydrogen embrittlement cracking. This cracking caused the screw head to separate from the screw shank. The screw is one of three that retains the selector valve to the manifold assembly. When the screw failed, the valve body separated, allowing loss of all usable hydraulic fluid.
The valve assembly had been installed in the aircraft since its manufacture in 1977 and had accumulated 5,642 hours. The valve is maintained on condition.
Probable Cause
The probable cause was the fracture of a NAS1351-4H-24P screw due to hydrogen embrittlement cracking, which caused the screw head to separate from the screw shank. This screw is one of three retaining the selector valve to the manifold assembly. When the screw failed, the valve body separated, allowing loss of all usable hydraulic fluid. The source of the migrating elemental hydrogen into the fracture area could not be determined.