Brantly B-2 N5901X accident near Midland, Michigan

Probable cause

The in-flight separation of the tail rotor blade due to fatigue, which resulted in the subsequent emergency landing and dynamic rollover.

NTSB analysis

After departing from the airport, the helicopter was in a level flight profile about 2,000 ft above ground level, about 1/4 mile south of the airport. The pilot stated that, when the helicopter was about 60 kts and on a heading of about 270°, he felt a "strong flutter" emit from the helicopter and heard a subsequent "bang" from the rear of the helicopter. The helicopter immediately turned 180° to the right, and the pilot reduced collective and maintained control of the helicopter. The pilot increased the airspeed and maneuvered the helicopter for an emergency landing to the departure airport. During the landing sequence, the helicopter experienced a dynamic rollover and impacted terrain.

A postaccident examination revealed that one of the tail rotor blades fractured and separated at the spar. Further, one of the bosses of the tail rotor hub fractured from fatigue. The fatigue crack initiated at multiple sites near the shoulder of the hub shank. These smaller cracks coalesced and propagated inward through over half the hub cross-section. During this propagation, as reverse bending stresses increased, a series of smaller fatigue cracks initiated on the opposite surface, coalescing and propagating inward. When the cracked hub could no longer withstand the stress, the remaining cross-section fractured from overstress. The tail rotor blade affixed to this portion of the hub liberated when the final fracture of the hub boss occurred.

Text from the NTSB. Open the NTSB docket for CEN18LA313 for the full record.