NTSB Identification: LAX98LA108.
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Accident occurred Saturday, March 07, 1998 in HOLLISTER, CA
Probable Cause Approval Date: 08/03/1999
Aircraft: Bell UH-1B, registration: N204DR
Injuries: 1 Serious.

NTSB investigators may not have traveled in support of this investigation and used data provided by various sources to prepare this aircraft accident report.

The pilot initiated a right turn, then he felt and heard a thump. He lowered the collective and attempted to straighten from the turn. He felt a strong vibration and the nose of the aircraft pitched down. A ground crewman who witnessed the accident reported that while the helicopter was about 35 feet agl, he saw a large piece of the tail and a rotor blade fly off the ship, followed by the tail rotor hub assembly, the remaining blade, and the 90-degree gearbox. The helicopter began a 360-degree right spiral, made 2 1/2 rotations, and then impacted the ground in a 30-degree nose-down attitude. He further noted that the engine still seemed to be producing about 60 percent power. The tail and tail rotor gearbox was found about 125 feet from the fuselage. Metallurgical examination of the vertical tail fin, tail rotor gearbox, and tail rotor blades disclosed an area of chafing which wore through the vertical fin's spar and initiated a fatigue crack. The fatigue crack initiated in an area where the vertical fin driveshaft cover fastened on the left side of the aircraft near the upper end. Chafing had occurred between the cover and the spar, wearing away the spar. According to Bell, the UH-1B requires a daily inspection to visually check the area of the upper vertical fin, per the Aviation Unit Technical Manual, where the fatigue cracking occurred.

The National Transportation Safety Board determines the probable cause(s) of this accident to be:

The failure of the maintenance personnel to recognize and repair the chafed area of the tail rotor driveshaft, which resulted in an in-flight separation of the tail fin and hub assembly.

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