LAX95LA238

The instructor was performing a low RPM recovery when the aircraft began an uncommanded yaw to the right. He attempted to control the aircraft by applying full left anti-torque pedal, but it had no effect on the yaw. He lowered collective and the aircraft touched down hard, rebounded, and continued to yaw right. He applied left cyclic and rolled off the throttle and touched down again. An inspection revealed that the left anti-torque control cable had separated. A laboratory examination revealed that failure was due to severe wear. The cable had chafed near the bottom of the starter. The cable was in an area that could be inspected during a preflight inspection of the aircraft.

On July 3, 1995, at 1145 Pacific daylight time, a Hiller UH-12C, N1348Z, made a hard landing from a hover at Rio Vista Municipal Airport, Rio Vista, California. Neither the flight instructor nor his student were injured; however, the aircraft sustained substantial damage. The aircraft was operated by Whirlybirds, Inc., as a dual instructional flight when the accident occurred. The flight originated from Buchanan Field, Concord, California, at 1135. Visual meteorological conditions prevailed at the time and no flight plan had been filed.

The student terminated his approach at a hover after overshooting the helipad. The instructor requested that he make a left pedal turn and hover back to the pad. While executing the pedal turn at a 4-foot hover, the student allowed the engine and rotor rpm to drop below the green arc. The instructor took the controls and brought the rpm back into the green range. He completed the procedure and was proceeding to repeat a low rpm recovery when the aircraft began an uncommanded yaw to the right.

The instructor attempted to control the aircraft by applying full left anti-torque pedal; however, when he applied the left pedal, it traveled all the way to the stop without any effect on the yawing movement. He then attempted to land by lowering the collective. As he did so the aircraft settled, touched down hard, rebounded, and became airborne while continuing to yaw to the right. In response, he applied left cyclic and rolled off the throttle.

When the aircraft touched down for the second time, he applied forward cyclic and confirmed that the throttle was at idle. After allowing the engine to run for about 1.5 minutes, the instructor performed a normal shutdown procedure.

A postaccident inspection of the aircraft revealed that the left anti-torque control cable had separated. Both ends of the separated cable were examined by materials engineers at the National Transportation Safety Board (NTSB) Materials Laboratory. Their examination revealed that failure resulted from severe wear damage that occurred along a 4-inch section of the cable.

The operator reported that the cable had chafed near the bottom of the starter. The chafing and separation occurred in an area that could be viewed and felt during a preflight inspection. The manufacturer reported that the cable is supported by a series of phenolic guides and grommets and should not come into contact with aircraft components or structure. The starter has a phenolic scuff plate in the event the cable should contact that component as a result of engine torque; however, continuous contact in that area is not expected.

The aircraft had undergone a 100-hour inspection on March 6, 1995, and had flown 70.8 hours since that inspection.