History of Flight
On June 24, 2022, at 0905 Pacific daylight time, a Bell 206 helicopter, registration N915B, sustained substantial damage in an accident near Merced, California. The helicopter was operating as a Title 14 Code of Federal Regulations Part 137 agricultural flight. The pilot reported that after performing a 180° turn and while on the spray run at about six feet above ground level, he felt a vibration followed by a "snap." The helicopter began to rotate left. He applied right pedal, entered an autorotation, and attempted to decrease airspeed. He tried to perform a run-on landing in 4-ft-tall corn, but upon touchdown, the helicopter rolled up on its nose and the main rotor blades severed the tail boom. The pilot was not injured.
Damage and Examination
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.
KAflex Driveshaft History and Inspection
The KAflex shaft was manufactured by Kamatics Corporation and shipped in October 2018 with bolts torqued within specified range. The pilot reported that the last shaft inspection was conducted 98 flight hours before the accident, in addition to daily inspections. Review of Kamatics Service Instruction SIN2348, Revision K, required a daily inspection to check for loose and missing hardware and to inspect flex frame and mount bolt torque stripes for evidence of slippage. The instruction also warned not to disturb or tighten flex frame nuts or bolts.
NTSB Materials Laboratory Examination
The NTSB Materials Laboratory examined components of the KAflex coupling system, including 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. Circumferential wear was evident on exterior surfaces.
Examination of flex plates and attachment bolts revealed that bolts marked with orange wax denoting as-torqued position were inconsistent; some intact, others fractured, most indicative of impact damage. The separated outer shaft end fitting displayed streaks, scrapes, and gouge marks on flange surfaces. A remnant bolt within the fitting flanges and elongated holes was consistent with impact force and circumferential orientation of wear and impact marks.
Fatigue striations were found on one bolt head fracture surface, with crack initiation sites along the surface edge and arrest marks propagating inward. Initiation sites were located along an area of wear and material loss of the shank. Several fractured flex plate remnants exhibited fatigue fracture features, including discoloration, reflective smoothing, and deformed bolt holes. Fracture surfaces showed faceted morphology with fatigue striations, indicative of fatigue crack propagation. Fatigue crack initiation sites on flex plates were along the corner with the bolt hole surface, which exhibited washer-shaped depressions, deformation, impact marks, and smearing. Outside fatigue propagation regions, dimpled rupture was observed, indicating subsequent overstress fracture. Other plates showed 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.
Pilot's Observation
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 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.
Similar Accidents
A similar failure of a KAflex coupling occurred on a Bell UH-1H (N107CH) in Oakdale, California 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 bolt hole wear was severe enough to cause material loss on one face, consistent with repeated contact with an adjacent washer.
Another accident occurred on May 26, 2017 (NTSB # WPR17LA113) involving 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, resulting in a total loss of engine power and a subsequent hard landing.
Another accident occurred on August 4, 2013 (NTSB # WPR13LA358) where maintenance personnel's failure to properly tighten bolts securing the main driveshaft to the transmission resulted in fatigue failure of the bolts and disconnection of the driveshaft during low-altitude flight.