NTSB Final Report: Mehama, OR

Flight Conducted Under: Part 137: Agricultural

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.

Probable Cause and Findings

Findings

History of Flight

On July 17, 2020, about 0854 Pacific daylight time, a Hiller UH-12D helicopter, N2297W, was destroyed when it was involved in an accident near Mehama, Oregon. The pilot was fatally injured. The helicopter was operated as a Title 14 Code of Federal Regulations Part 137 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.” Witnesses reported that the engine noise changed pitch and became loud before impact and that there was heavy smoke and fire at the accident location.

Pilot Information

 The operator reported that the pilot had flown agricultural helicopters for about 50 years. The operator also reported that the pilot sprayed the fields where the accident occurred for many years and that he knew the fields well.

Meteorological Information and Flight Plan

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 postimpact fire, and the bottom of the transmission was exposed. The forward portion of the tailboom exhibited thermal damage, and the aft portion of the tailboom had fractured and separated just forward of the tail rotor assembly. A portion of a main rotor blade was located about 360 ft north of the main wreckage, and just northeast of the main rotor blade was a tail rotor blade; both came to rest in a tree field. The stabilizer and a tail rotor blade with its hub attached were located farther northeast in an area of large trees.

During a postaccident examination, significant crush damage was noted on the right side of the engine. Fuel was found 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 was unable to be established from the collective twist grip to the engine due to extensive cockpit and airframe damage. The control cables remained secured at their respective arms on the power turbine governor and fuel control units.

The six-stage axial compressor was seized; its housing exhibiting thermal and crush damage. Once the housing was removed, the compressor rotated smoothly. Some dirt and debris were noted on the compressor guide vanes. The gas producer and power turbine rotated smoothly. Light circumferential scoring was noted on the gas producer turbine housing. The accessory gearbox housing was removed; all components were oily and unremarkable. The fuel control unit, power turbine governor, bleed valve, fuel spray nozzle and fuel pump assembly were bench tested and found to function normally.

The tailboom had fractured and separated just forward of the tail rotor assembly. The tail rotor gearbox was found fractured and separated, and no anomalies were noted. The gearbox drive to the tail rotor hub remained intact, but its grooves displayed some rotational damage. No corresponding rotational damage was noted on the hub. One tail rotor blade remained attached to the tail rotor hub, and the other tail rotor blade was found fractured and separated. The tension-torsion straps for fractured/separated blade extended from the tail rotor hub and were fractured and splayed.

The tail rotor assembly was provided to the National Transportation Safety Board Materials Laboratory for further examination. The examination revealed that 10 of the 11 tension-torsion straps had fractured from fatigue cracking followed by overstress fracture. 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 found, and the fit tightness of the joint was unable to be determined.