Rockville, ID, USA
N24FS
BELL 206B
The commercial pilot reported that, while conducting an aerial application flight, he performed a 180° turn at an altitude of about 40 to 50 ft above ground level and heard a growling noise, which was followed by the sound of a loud bang and a subsequent loss of power. The pilot lowered the collective, likely to initiate an autorotation to a nearby open field, and the helicopter landed hard. Postaccident examination of the helicopter airframe and engine revealed that the middle portion of the engine-to-transmission drive shaft was separated between the engine and transmission KAflex couplings. No additional evidence of any preexisting mechanical malfunction was observed. Metallurgical examination of the shaft and couplings revealed that the shaft had failed from a pair of fatigue cracks at a bolt hole in one element of the flexure coupling (flex frame). This bolt hole exhibited rotational wear severe enough to cause material loss on one face of the bolt hole wall. This circular-wear depression was consistent with repeated rotational contact with an adjacent washer. It is likely that, once the fatigue cracks at the bolt hole propagated deeply enough, the remaining flexure element cross-section fractured from overstress. The failure of this flexure element led to multiple and subsequent overstress failures in other flexure elements, eventually compromising the entire drive shaft assembly. The circular-wear depression and the longitudinal wear inside the flexure element bolt hole were both consistent with a loose bolt assembly. Because the bolt assembly was loose, the washer would have been able to rub on the flexure element. In addition, the bolt shaft would have been able to rub or gouge the inner wall surfaces of the bolt hole. The bolt and associated washer were not located in the wreckage. None of the maintenance performed on the helicopter about 1 month before the accident involved the flex frame bolt assemblies, so the investigation could not determine how one of the bolts became loose.
On May 26, 2017, about 1044 mountain daylight time, a Bell 206B helicopter, N24FS, sustained substantial damage during a forced landing near Rockville, Idaho. The commercial pilot was seriously injured. The helicopter was registered to and operated by Pauly's Helicopter Services Inc., Ontario, Oregon, as a Title 14 Code of Federal Regulations Part 137 aerial application flight. Visual meteorological conditions prevailed, and no flight plan was filed for the local flight, which originated from a private staging area about 5 minutes prior to the accident.The pilot reported that after the helicopter was reloaded with chemical, he proceeded to a nearby field to begin an aerial application. The pilot said that he performed two application passes, one to the north, and the second to the south. As he performed a 180o turn at an altitude of about 40 to 50 ft above ground level (agl), he heard a "loud growl type noise" and saw the "…gauges drop then go dead." Shortly thereafter, the pilot heard a 'huge bang sound" that jarred the helicopter, which was followed by a loss of power. The pilot lowered collective and attempted to land in a nearby open area. Subsequently, the helicopter landed hard in an open marshy field. The pilot added that he did not recall any warning sounds or lights throughout the accident sequence. Examination of the helicopter by a Federal Aviation Administration inspector revealed that the tailboom and fuselage were structurally damaged. Postaccident examination of the wreckage revealed that the tailboom was separated just aft of the attach point to facilitate wreckage transport. Flight control continuity was established from the right seat cockpit controls to the main rotor and aft to the area where the tailboom was separated. Flight control continuity continued throughout the remaining portions of the separated tailboom to the tail rotor. The airframe fuel filter was removed and examined. The filter assembly and fuel within the bowl was free of debris and clear in appearance. The main rotor was rotated by hand and rotated freely with no binding or noise from the transmission. The engine-to-transmission output shaft was of a KAflex coupling type. Both the forward and aft portions of the drive shaft remained attached to the engine and transmission. However, the middle portion of the shaft was separated from both ends and exhibited mechanical damage. Associated mechanical damage to the surrounding area was observed. A large hole in the air separator housing was observed directly above the shaft. The transmission isolator mount bolts were separated and the nut, cotter pin, and remaining bolt shanks of the two bolts that are installed through the bearing assemblies were not located. The remains of the transmission isolator mount and located hardware, engine-to-transmission shaft with KAflex couplings were retained for further examination. The Rolls Royce M250-C20B engine remained attached to its mounts, which were secure to the airframe. All engine accessories appeared attached to their respective mounts. Continuity was established throughout the engine when N1 was rotated. The 4th stage power turbine was rotated with continuity noted throughout. The chip plugs were removed and found free of debris. Fuel was located throughout the system to the fuel nozzle. The fuel nozzle was unremarkable. The engine was removed, and one compressor case half was removed to facilitate further examination of the compressor. Extensive damage was noted throughout the compressor. In addition, varying amounts of damage opposite in direction of rotation was observed throughout the compressor. No associated damage was observed throughout the first gas generator turbine blades; a lighted borescope was used to examine the blades. The outer combustion case (OCC) was removed to visually assess the condition of the 1st-stage nozzle guide vane (NGV) and 1st-stage turbine wheel blades, as well as the combustion liner. Small amounts of metal debris flakes were noted in the cavity between the outside diameter of the NGV and gas generator support at the 6 o'clock position. It was concluded that the most likely source of the debris was from the compressor rotor damage. The associated combustion and turbine hardware exhibited no sign of abnormal burner profile, heat distress, or thermal deterioration. To facilitate the compressor replacement, the entire turbine module assembly was removed from the gearbox. Following removal, additional bits of metal debris flakes were found in the exhaust collector support. Both N1 & N2 turbine rotors turned freely, but a slight scraping sound was noted when turning the power turbine. A borescope was inserted into one of the thermocouple ports which showed further evidence of metal debris throughout the turbine. Examination of the isolation mount and engine-to-transmission drive shaft by a National Transportation Safety Board Materials Research Engineer revealed that the isolation mount exhibited multiple, repeated deep gouge marks on the external housing surface. This damage was consistent with repeated contact with an adjacent part after it had fractured. The shaft portion section of the transmission shaft flange, facing the flex-coupling, had fractured between 0.5 and 1 inches from the flange radius. The shaft exhibited circumferential wear and gouge marks, consistent with contact with an adjacent component, post-fracture. The fracture surface was obliterated by this post-fracture contact. The areas adjacent the fracture surface exhibited buckling and twisting consistent with rotational or torsional loading. The bolts, as received, were intact and remained connected to the flange. Of note was an affixed flexure element (flex frame) that had fractured in two locations, still attached to the flange. The fractures exhibited a general cup-like profile, with a general rough texture and dull luster, consistent with overstress failure. The shaft body itself exhibited circumferential wear and gouge marks near the center flange on the bottom of the marking label. The forward side of the shaft body exhibited inward buckling, deformation, and obliteration consistent with repeated impact and smearing with an adjacent component. These marks were consistent with those present on the mating transmission flange. The aft portion of the shaft body exhibited inward buckling deformation, as well as interior and exterior rotational wear marks. These marks were consistent with those on the aft engine shaft flange. The aft shaft body flange had exhibited deformation forward. The attachment bolts on all the shaft body flanges were still fastened. The flexure element remnants had fractured away from the bolt holes. These fracture surfaces exhibited features that were similar to those of the flexure element remnants on the transmission shaft flange. In addition, on the aft side of the shaft body, the element remnant fractures mated with those on the engine shaft flange. These four fracture surfaces were located away from the bolt holes and exhibited similar features. The fracture surfaces were rough textured, with either a cup or cone shape, or a 45° angle orientation, consistent with overstress fracture. A flexure element had fractured along a bolt hole. The fractures were flat, with no indications of localized material deformation or necking. The flat surface exhibited a smooth, circular-shaped depression. The inner surface of the bolt hole also exhibited wear on a portion of the wall. This wear was significant enough to create material loss on the bolt hole wall. The bolt and associated washer were not located. Several of the other flexure elements that were still fastened, as received, were disassembled to inspect for similar wear. The type wear leading to material loss was not observed about the other disassembled element bolt holes. The circular-shaped depression of the fractured flexure element was compared with a washer from a disassembled bolt. The peripheral wear marks in this depression were consistent with rotational contact with an adjacent washer. The flexure element exhibited two fractures about the bolt hole and were labeled "left" and "right" for purposes of identification. The left fracture surface exhibited crack arrest marks consistent with progressive cracking from the lower portion, extending upward. A closer examination of this fracture surface found fatigue striations, consistent with fatigue fracture. The exact initiation site(s) could not be determined, due to post-fracture damage. However, the fatigue crack initiation site corresponded to the location of the washer-shaped depression. Plastically deformed material was also present at the fatigue crack initiation site on the left fracture. The right fracture surface was more damaged than the left; prominent crack arrest marks were present on this fracture surface. As with the left fracture surface, fatigue striations were prevalent over much of the right fracture surface. Outside of the fatigue crack regions, the rest of the undamaged right fracture surface exhibited dimple rupture, which is consistent with subsequent overstress fracture. The fatigue crack initiation site of the right fracture was partially damaged. However, there were no indications that the initiation site contained any material discrepancies such as corrosion pits or oxide inclusions. The location of the right fatigue crack initiation site was at an area of local deformation, consistent with the edge of the washer-shaped depression. (For further information, refer to the NTSB Materials Lab factual report within the public docket.) Review of airframe and engine maintenance records revealed that the Kamatics KAflex driveshaft assembly was installed in June of 2001. The most recent maintenance performed on the helicopter was completed on April 28, 2017, at a HOBBS time of 1,503.2 hours, and an airframe total time of 15,446 hours. The logbook entry noted that the engine drive shaft was disconnected, and the transmission was removed from the airframe to facilitate replacement of the new isolation mounts, drag pin assemblies, and repair of transmission support links and desk mount transmission fittings. At the time of the accident, the helicopter had accumulated 15.2 hours since its most recent maintenance. The mechanic that performed the maintenance on the helicopter reported that they did not loosen or replace any bolts on the KAflex assembly. Review of the KamaticsRWG Service Instruction Number SIN2348, Revision K, Installation, Maintenance, and Repair of the KAflex driveshaft, revealed that removal and installation of the KAflex shaft does not require loosening or removal of the flex frame bolts.
The in-flight failure of the engine-to-transmission drive shaft due to a fatigue fracture of one of the KAflex flex frames caused by a loose bolt, which resulted in a total loss of engine power and a subsequent hard landing.
Source: NTSB Aviation Accident Database
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