Bell OH 58C N139RD accident near Cameron, North Carolina

Probable cause

An in-flight failure of a main rotor blade due to fatigue cracks. Contributing to the failure of the main rotor blade was the absence of guidance to the operator to inspect an area of the main rotor blade known to be susceptible to fatigue cracks and the accident helicopter type certificate holder’s lack of information of an existing inspection published by another restricted-category type certificate holder of the same model helicopter.

NTSB analysis

The commercial pilot was conducting a local aerial application flight in the restricted-category military surplus helicopter. After loading the helicopter with herbicide, the pilot departed from the staging area; however, during the initial climb, as the helicopter was about 50 ft above the ground, the outboard portion of one of the two main rotor blades separated. The main rotor blades then struck the tailboom, and the helicopter entered an uncontrolled descent to the ground. Metallurgical examination of the fractured main rotor blade revealed a crack with fatigue features emanating from multiple origins in the area of the inertia weight attachment hole.

The helicopter manufacturer, Bell, also a restricted-category type certificate holder for the same model helicopter as the accident helicopter, had published a military alert bulletin (MAB) about 30 years before the accident (revised about 9 years before the accident) and an operations safety notice (OSN) about 9 years before the accident applicable to the main rotor blade that fractured during the accident flight; operators were instructed to conduct initial visual inspections of the blades in the area of the inertia weight screw heads and subsequent inspections at intervals of 8 hours or 32 flights, whichever occurred first. However, the OSN only applied to the airframe serial numbers under the responsibility of Bell. Both the operator and the type certificate holder of the accident helicopter, Rotorcraft Development, indicated that they were not aware of the MAB or the OSN until after the accident. Additionally, a preventive maintenance services inspection program that was published by the previous type certificate holder for the accident helicopter (Garlick Helicopters) did not contain requirements to inspect the main rotor blade per the aforementioned MAB and OSN. Performance of the recurrent inspection required by the MAB and the OSN likely would have detected the fatigue cracks on the main rotor blade before its failure during the accident flight. The Federal Aviation Administration did not require sharing of safety information, such as the MAB and the OSN recurrent inspections, between restricted-category type certificate holders.

Text from the NTSB. Open the NTSB docket for ERA17LA032 for the full record.