History of Flight
On July 17, 2020, at about 0854 Pacific daylight time, a Hiller UH-12D helicopter, registration N2297W, was destroyed during an aerial application flight conducted under 14 CFR Part 137. The accident occurred near Mehama, Oregon. The pilot sustained fatal injuries.
The support truck operator reported that the helicopter departed from the truck and flew toward a field. Shortly after takeoff, the truck operator heard the pilot radio that he was "going down." Witnesses described a change in engine noise, which became loud before impact, and observed heavy smoke and fire at the accident site. No radar data were available for the flight.
Personnel Information
According to the operator, the pilot had approximately 50 years of experience flying agricultural helicopters. The operator also noted that the pilot had sprayed the fields where the accident occurred for many years and was familiar with the area.
Wreckage and Impact Information
The helicopter struck several large trees and came to rest inverted at the base of a large tree. The cabin area sustained extensive crush damage and was consumed by postimpact fire. The fuel tank was also consumed by fire, and the bottom of the transmission was exposed. The forward portion of the tailboom showed thermal damage, while the aft portion had fractured and separated just forward of the tail rotor assembly. A section of a main rotor blade was found about 360 ft north of the main wreckage, with a tail rotor blade nearby, both in a tree field. Another tail rotor blade with its hub attached was located farther northeast among large trees, along with the stabilizer.
A postaccident examination revealed significant crush damage on the right side of the engine. Fuel was present in the line to the fuel nozzle and within the fuel control unit. The fuel control arm/lever was found in the "off" position. Engine control continuity from the collective twist grip to the engine could not be established due to extensive cockpit and airframe damage. Control cables remained secured at their respective arms on the power turbine governor and fuel control units.
The six-stage axial compressor was seized, with its housing showing thermal and crush damage. After housing removal, the compressor rotated smoothly. Some dirt and debris were noted on the compressor guide vanes. The gas producer and power turbine rotated smoothly, and light circumferential scoring was observed on the gas producer turbine housing. The accessory gearbox components were oily and unremarkable. Bench testing of the fuel control unit, power turbine governor, bleed valve, fuel spray nozzle, and fuel pump assembly indicated normal function.
The tailboom had fractured and separated just forward of the tail rotor assembly. The tail rotor gearbox was fractured and separated, with no anomalies noted. The gearbox drive to the tail rotor hub remained intact, but its grooves showed rotational damage. No corresponding rotational damage was found on the hub. One tail rotor blade remained attached to the hub; the other was fractured and separated. The tension-torsion straps for the separated blade extended from the hub and were fractured and splayed.
Examination Findings
The tail rotor assembly was examined at the National Transportation Safety Board Materials Laboratory. The examination determined that 10 of the 11 tension-torsion straps had fractured from fatigue cracking followed by overstress fracture. The fatigue cracks initiated at multiple locations along the bores in contact with the cylindrical plain bearing. Wear, including smearing, gouging, spalling, and galling of the faying surfaces, led to the fatigue crack initiation. The cracks then propagated perpendicular to the area with the highest tensile stresses and at the thinnest locations along the bearing bore. No corrosion was found, and the fit tightness of the joint could not be determined.