Hiller UH 12D N2297W accident near Mehama, Oregon

Probable cause

An in-flight separation of a tail rotor blade due to fatigue cracking and overstress, which resulted in a loss of directional control and subsequent ground impact.

NTSB analysis

The pilot was performing an aerial application flight. No radar information was available for the flight. The support truck operator reported that the helicopter departed from the truck and flew toward a field. Shortly after the helicopter departed, the truck operator heard the pilot report over the radio that he was “going down.” A witness reported that the engine noise changed pitch and became loud before impact. A postcrash fire ensued.

The helicopter struck several large trees and came to rest inverted at the base of a large tree. Postaccident examination of the engine revealed no anomalies that would have precluded normal operations. However, the tail rotor assembly was found fractured and separated from the tailboom with parts scattered in the initial part of the debris field. One tail rotor blade remained secured at the hub, and the other blade was fractured and separated.

The tension-torsion straps for that blade extended outward from the hub and were fractured and splayed. Metallurgical examination revealed that 10 of the 11 straps fractured from fatigue cracking followed by overstress. The fatigue cracking initiated at multiple locations along the bores in contact with the cylindrical plain bearing. Wear, in the form of smearing, gouging, spalling, and galling of the faying surfaces, led to the fatigue crack initiation. The fatigue cracks then propagated perpendicular to the area with the highest tensile stresses and at the thinnest locations along the bearing bore. No indications of corrosion were observed. Thus, the tail rotor blade separated during flight due to fatigue cracking and overstress.

According to the component replacement schedule, the tension-torsion strap bundles are to be replaced every 12,500 flight hours and were set to be removed in 1,383.3 flight hours from the time of the accident.

Text from the NTSB. Open the NTSB docket for WPR20LA224 for the full record.