NTSB Identification: IAD01LA031.
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Accident occurred Saturday, February 03, 2001 in Napoleon, OH
Probable Cause Approval Date: 05/13/2003
Aircraft: Enstrom F28A, registration: N98PM
Injuries: 3 Uninjured.

NTSB investigators may have traveled in support of this investigation and used data provided by various sources to prepare this aircraft accident report.

The helicopter developed a vibration, and the pilot elected to return to the departure airport. On final approach to landing, approximately 10 feet above the ground, the main rotor mast fractured, and the main rotor system departed the helicopter. The pilot and two passengers were not injured. Examination of the fracture surfaces by a Safety Board metallurgist revealed the mast failed in fatigue in the area of the fillet radius. Microscopic examination revealed that a portion of the fatigue region had been painted over with paint that matched the exterior of the mast. The mast installed in the accident helicopter had a narrower fillet radius than masts subsequently produced by the manufacturer. An FAA Airworthiness Directive (AD) that required inspecting and strengthening of the fillet radius affected the narrower masts. As a result of this investigation, the FAA amended the AD to immediately retire some transmission/mast combinations, and to ultimately remove all of the narrower masts from service.

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

The fatigue failure in the fillet radius of the mast, which resulted in the separation of the main rotor system.

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