Hiller UH-12E N3324F accident near Naruna, Virginia

Probable cause

The fatigue failure of a main rotor blade drag strut.

NTSB analysis

While in straight and level flight, the pilot indicated that the helicopter began a "violent horizontal circular shake or wobble," and the main rotor rpm and began to decrease. The pilot entered an autorotation, turned the helicopter to the left to avoid trees, and touched down. During the forced landing, the tail rotor contacted the ground, and after the engine had been shut down, one of the main rotor blades contacted the tailboom. Post accident examination revealed that a main rotor blade drag strut had failed at the inboard threaded portion of the strut. Further examination revealed that a vibro-peen marked serial number was visible on the failed drag strut and that the drag strut had failed due to fatigue cracking that emanated from the root portion of the threads. The helicopter's maintenance records listed the failed drag strut as being a part number that should have had a life limit of 2,500 hours, but failed at 2,236 hours. Contrary to the helicopter manufacturer’s drawings, there was no part number on the failed drag strut, which put into doubt the actual hours of operation on the part.

Text from the NTSB. Open the NTSB docket for NYC08LA318 for the full record.