N915BBell 206B · 1971

On June 24, 2022, at 0905 Pacific daylight time, a Bell 206 helicopter, N915B, sustained substantial damage when it was involved in an accident near Merced, California. The pilot was not injured. The helicopter was operated as a Title 14 Code of Federal Regulations Part 137 agricultural flight.

The pilot reported that he had just performed a 180° turn and was on the spray run, about six feet above ground level (agl), when he felt a vibration. He then felt a “snap” and the helicopter began to rotate left. He applied right pedal, entered an autorotation, and attempted to decrease the airspeed. He tried to perform a run-on landing in the 4-ft-tall corn, but upon touchdown, the helicopter rolled up on its nose and the main rotor blades severed the tail boom.

A Federal Aviation Administration inspector examined the wreckage. The KAflex driveshaft was separated and there were several sheared bolts and pieces of the KAflex located underneath the engine.

The KAflex shaft was manufactured by Kamatics Corporation. The part was shipped from their facility in October 2018 with all the bolts torqued within their specified range. The pilot reported that the last shaft inspection was done 98 flight hours before the accident in addition to the daily inspection check.

Review of the Kamatics RWG Service Instruction Number SIN2348, Revision K, Installation, Maintenance, and Repair of the KAflex driveshaft, revealed that a daily inspection was required of the shaft. It stated “Check for loose and missing hardware (bolts, nuts, washers)…” and “Inspect flex frame and mount bolt torque stripes for evidence of slippage.” There was also the following warning “DO NOT disturb or tighten flex frame nuts or bolts.”

The National Transportation Safety Board (NTSB) Materials Laboratory conducted a detailed examination of components within the KAflex coupling system. The analyzed elements included fractured flex plate remnants, attachment bolt fragments, and a separated outer shaft end fitting. Wear patterns on the cylindrical walls of the shaft end fitting were consistent with rotational movement between the shaft and interconnect fittings, with circumferential wear evident on the exterior surfaces.

The flex plates joined by attachment bolts were examined in the KAflex coupling. The bolts, marked with orange wax denoting their as-torqued position, were inconsistent as some were intact and others fractured, most indicative of impact damage. The separated outer shaft end fitting displayed streaks, scrapes, and gouge marks on flange surfaces. The presence of a remnant bolt within the fitting flanges and elongated holes was consistent with the force of impact with the circumferential orientation of wear and impact marks.

Detailed examination of the attachment bolts revealed fatigue striations on one bolt head fracture surface, showing crack initiation sites along the surface edge with arrest marks propagating inward from these locations. These initiation sites were located along an area of wear and material loss of the shank.

Several fractured flex plate remnants also exhibited fracture features consistent with fatigue, along with discoloration, reflective smoothing, and deformed bolt holes. The fracture surfaces revealed a faceted morphology with fatigue striations, indicative of fatigue crack propagation. The fatigue crack initiation sites on the flex plates were situated along the corner with the bolt hole surface, which exhibited washer-shaped depressions, deformation, impact marks, and smearing. Outside of the fatigue propagation regions, dimpled rupture was observed, indicative of subsequent overstress fracture.

Examination of other plates revealed thumbnail-shaped cracks at edges with ratchet marks, indicating fatigue cracking at multiple initiation sites. These features were consistent with fatigue cracks in multiple plates and at least one bolt before the accident.

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The pilot thought that if he had someone help him spin the blades slowly, he could have more thoroughly inspected the KAflex drive. He thought it would have also been helpful to have someone start and stop movement while reversing the blades so he could see if there is any movement in the plates.

A similar failure of a KAflex coupling occurred on a Bell UH-1H (N107CH) accident that occurred in Oakdale, CA on July 24, 2018 (NTSB # WPR18LA206). In that case, the engine-to-transmission driveshaft failed from a pair of fatigue cracks at a bolt hole in one element of the flexure coupling, where the bolt hole wear was severe enough to cause material loss on one face. This wear was consistent with repeated contact with an adjacent washer.

Another accident occurred on May 26, 2017 (NTSB # WPR17LA113) where there was an 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.

Another accident occurred on August 04, 2013 (NTSB # WPR13LA358) where maintenance personnel's failure to properly tighten the bolts securing the main driveshaft to the transmission, resulted in fatigue failure of the bolts and disconnection of the driveshaft during low-altitude flight.