On February 3, 2001, an Enstrom F28A (registration N98PM) was involved in an aviation accident near Napoleon, OH. Investigators recorded the probable cause as: The fatigue failure in the fillet radius of the mast, which resulted in the separation of the main rotor system. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
A helicopter experienced a main rotor mast fracture on final approach, causing the rotor system to separate. The pilot and two passengers were uninjured.
Incident Summary
While in flight, the helicopter developed a vibration. The pilot decided to return to the departure airport. During the final approach to landing, approximately 10 feet above the ground, the main rotor mast fractured. The main rotor system subsequently departed the helicopter. The pilot and two passengers were not injured.
Examination Findings
A metallurgist from the Safety Board examined the fracture surfaces of the mast. The examination determined that the mast had failed due to fatigue in the area of the fillet radius. Microscopic analysis revealed that a portion of the fatigue region had been painted over with paint matching the exterior of the mast. The mast installed in the accident helicopter had a narrower fillet radius compared to masts produced later by the manufacturer.
Regulatory Actions
An FAA Airworthiness Directive (AD) had been issued that required inspecting and strengthening of the fillet radius specifically for the narrower masts. Following this investigation, the FAA amended the AD. The amendment mandated the immediate retirement of certain transmission and mast combinations. Ultimately, the AD required the removal of all narrower masts from service.