Accident Overview
On May 26, 2017, at approximately 1044 mountain daylight time, a Bell 206B helicopter, registration N24FS, operated by Pauly's Helicopter Services Inc. of Ontario, Oregon, sustained substantial damage during a forced landing near Rockville, Idaho. The commercial pilot was seriously injured. The flight was being conducted under 14 CFR Part 137 as an aerial application flight. Visual meteorological conditions prevailed, and no flight plan was filed. The flight originated from a private staging area about five minutes prior to the accident.
Pilot Report
The pilot reported that after reloading the helicopter with chemical, he proceeded to a nearby field to begin application. After two passes, while performing a 180-degree turn at an altitude of about 40 to 50 feet above ground level, he heard a "loud growl type noise" and saw the gauges drop and go dead. Shortly thereafter, a "huge bang sound" jarred the helicopter, followed by a loss of power. The pilot lowered collective and attempted to land in a nearby open area, resulting in a hard landing in an open marshy field. He did not recall any warning sounds or lights.
Postaccident Examination
A Federal Aviation Administration inspector found that the tailboom and fuselage were structurally damaged. The tailboom had separated just aft of the attach point for wreckage transport. Flight control continuity was established from the cockpit to the main rotor and through the tailboom to the tail rotor. The airframe fuel filter was clear of debris. The main rotor rotated freely with no binding or noise from the transmission.
The engine-to-transmission output shaft (KAflex coupling type) had its middle portion separated from both ends, exhibiting mechanical damage. A large hole in the air separator housing was observed above the shaft. The transmission isolator mount bolts were separated, and the nut, cotter pin, and remaining bolt shanks of two bolts through bearing assemblies were not located. These components were retained for further examination.
The Rolls Royce M250-C20B engine remained attached to its mounts. Continuity was established when N1 was rotated. The 4th stage power turbine rotated with continuity. Chip plugs were free of debris. Fuel was present throughout the system to the nozzle, which was unremarkable. The engine compressor exhibited extensive damage, with varying amounts of damage opposite the direction of rotation. No associated damage was observed on the first gas generator turbine blades.
Removal of the outer combustion case revealed small amounts of metal debris flakes near the 1st-stage nozzle guide vane, likely from the compressor rotor damage. Combustion and turbine hardware showed no abnormal burner profile or heat distress. The turbine module assembly was removed, revealing additional metal debris in the exhaust collector support. Both N1 and N2 turbine rotors turned freely, but a slight scraping sound was noted when turning the power turbine. A borescope showed further metal debris throughout the turbine.
Drive Shaft and Isolation Mount Examination
Examination by an NTSB Materials Research Engineer of the isolation mount and drive shaft revealed that the isolation mount had multiple deep gouge marks on its external housing, consistent with repeated contact with an adjacent part after fracture. The transmission shaft flange had fractured between 0.5 and 1 inch from the flange radius, with circumferential wear and gouge marks consistent with post-fracture contact. The fracture surface was obliterated. Adjacent areas showed buckling and twisting consistent with torsional loading. Bolts were intact and connected to the flange. A flexure element attached to the flange had fractured in two locations, with cup-like fractures consistent with overstress failure.
The shaft body exhibited circumferential wear and gouge marks near the center flange. The forward side had inward buckling, deformation, and obliteration consistent with repeated impact and smearing. These marks matched those on the mating transmission flange. The aft portion had inward buckling and rotational wear marks matching the engine shaft flange. One flexure element had fractured along a bolt hole; the fractures were flat with no localized deformation, and the bolt hole wall had significant wear with material loss. The bolt and washer were not located. Other disassembled elements did not show similar wear. The circular depression on the fractured element was consistent with rotational contact with an adjacent washer.
The left fracture surface exhibited crack arrest marks and fatigue striations, consistent with progressive fatigue cracking from the lower portion. The exact initiation site could not be determined due to post-fracture damage, but it corresponded to the location of the washer-shaped depression. Plastically deformed material was present at the initiation site. The right fracture surface also had crack arrest marks and fatigue striations over much of the surface, with the remainder exhibiting dimple rupture (overstress). The right fatigue crack initiation site was partially damaged but showed no material discrepancies such as corrosion pits. Its location was at an area of local deformation consistent with the edge of the washer-shaped depression.
Maintenance Records
Review of records indicated that the Kamatics KAflex driveshaft assembly was installed in June 2001. The most recent maintenance was completed on April 28, 2017, at a HOBBS time of 1,503.2 hours and airframe total time of 15,446 hours. The logbook noted that the engine drive shaft was disconnected and the transmission removed to replace isolation mounts, drag pin assemblies, and repair transmission support links and desk mount fittings. At the accident time, the helicopter had accumulated 15.2 hours since that maintenance. The performing mechanic reported they did not loosen or replace any bolts on the KAflex assembly. A review of Kamatics Service Instruction SIN2348, Revision K, indicated that removal and installation of the KAflex shaft does not require loosening or removal of the flex frame bolts.