# NTSB WPR22LA236: Bell 206B, N915B, Merced, CA, June 24, 2022 - Type: NTSB accident investigation - Classification: Accident · No injuries · Class 3 · Limited · Final report - Date: June 24, 2022, 09:05 local (16:05 UTC) - Place: Merced, CA - Coordinates: 37.286657, -120.59379 (estimated) - Nearby events: https://openairdocs.org/map.md?lat=37.2867&lon=-120.5938&mi=10 - Phase of flight: [Maneuvering](https://openairdocs.org/ntsb-phases/450) (OpenAirDocs' grouping) - Defining event: [Maneuvering-low-alt flying: Sys/Comp malf/fail (non-power)](https://openairdocs.org/ntsb-events/330) - Highest injury: No injuries - Injuries: 1 uninjured - Status: Completed - Report: Approved by Office Director - Investigation class: Class 3 - NTSB office: Western Pacific Region - Parties: Kamatics Corporation, Federal Aviation Administration - NTSB case key (mKey): 105378 - NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=WPR22LA236 - NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/105378/pdf - NTSB docket: https://data.ntsb.gov/Docket?ProjectID=105378 - Page: https://openairdocs.org/ntsb/WPR22LA236 ## Probable cause 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 power to the transmission and a subsequent hard landing. ## Findings NTSB's coded findings; those marked are part of the probable cause. - [Flight control system](https://openairdocs.org/ntsb-findings/01022700): Aircraft › Aircraft systems · Incorrect service/maintenance (probable cause); cases with it: [Bell 206B, 1 case](https://openairdocs.org/ntsb-findings/01022700/cases.md?model=1181511), [All aircraft, 65 cases](https://openairdocs.org/ntsb-findings/01022700/cases.md) - [Flight control system](https://openairdocs.org/ntsb-findings/01022700): Aircraft › Aircraft systems · Failure (probable cause); cases with it: [Bell 206B, 1 case](https://openairdocs.org/ntsb-findings/01022700/cases.md?model=1181511), [All aircraft, 65 cases](https://openairdocs.org/ntsb-findings/01022700/cases.md) ## Sequence of events The events NTSB coded, in order, by phase of flight. The defining event best describes what happened. - [Maneuvering-low-alt flying: Sys/Comp malf/fail (non-power)](https://openairdocs.org/ntsb-events/330) (defining event); cases with it: [Bell 206B, 6 cases](https://openairdocs.org/ntsb-events/330/sequence.md?model=1181511), [All aircraft, 994 cases](https://openairdocs.org/ntsb-events/330/sequence.md) ## Analysis The pilot reported that he was on an agricultural spray run. He had just performed a 180° turn and, about six feet above ground level (agl), 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 postaccident examination revealed that the KAflex driveshaft was separated and there were several sheared bolts and pieces of the KAflex located underneath the engine. Subsequent metallurgical examination determined that the failure of the KAflex coupling resulted from fatigue observed in two flex plate segments and an adjacent attachment bolt, indicating these components were the initial points of failure. The fatigue cracking initiated at areas adjacent to wear and material loss. With the fracture of flex plates and bolts, the interconnect and end fitting could rotate, resulting in the assembly failure. One flex plate and bolt exhibited more fatigue, suggesting longer crack propagation and earlier fracture, while thumbnail cracks on a second flex plate indicated incipient propagation when other components failed in overstress. The bolt and flex plate with fatigue features showed severe wear and loss of cross-sectional area, potentially concentrating stress at these locations. Although the investigation could not determine which fractured first, the joint had become loose, likely due to wear over time or inadequate assembly/maintenance. Most of the fractured bolts did not have discernable fracture features, likely lost to post fracture wear and battering. Visual inspections during maintenance might reveal a loose bolt, but fatigue cracks in these components would be more difficult to detect, leading to rapid propagation in rotary aircraft parts. ## Factual narrative HISTORY OF FLIGHT 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. TESTS AND RESEARCH 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. Theseinitiation 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. Top of Form ADDITIONAL INFORMATION 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. ## Aircraft - Registration: [N915B](https://openairdocs.org/deregistered/N915B-2023-07-07), registration cancelled 2023-07-07, S/N 720, built 1971 (OpenAirDocs) - Reported make and model: BELL 206B - Registry model: [Bell 206B](https://openairdocs.org/model/bell-206b) (registry code 1181511) (OpenAirDocs) - Registry engine: [Allison 250-C20 series](https://openairdocs.org/engine/allison-250-c20-series) (registry code 03020, turbo-prop) (OpenAirDocs) - Category: Helicopter - Serial number: 720 - Year built: 1972 - Engines: Rolls Royce 250-C20B 420 hp turbo shaft - Damage: Substantial damage - Flight: Aerial application, Part 137 - Operator: Bettencourt Flying Service Inc - Registered owner: Bettencourt Flying Service Inc - Airworthiness certificate: Special, Restricted (Special) - Departed: Atwater, CA - Destination: Same as departure - Airport: NA - Equipment: Handheld GPS ## Flight crew As NTSB recorded them. NTSB leaves out names. - Pilot; age 59; injuries: none; commercial; airplane ratings: single-engine sea, single-engine land, multi-engine land; medical: class 2, with waivers/limitations; last flight review January 21, 2021; seat: rear; restraint: 3-point; flight hours: 25,150 total, 11,254 in this make and model, 25,079 as pilot in command, 76 in the last 90 days, 65 in the last 30 days, 3 in the last 24 hours ## Weather - Conditions: Visual (VMC), Day - Wind: 330° at 5 knots - Visibility: 10 statute miles - Lowest ceiling: none - Temperature: 88 °F, dew point 54 °F - Altimeter: 29.84 inHg - Observed at: KMCE, 4 nm away - METAR: KMCE 241653Z AUTO 33005KT 10SM CLR 31/12 A2984 RMK AO2 SLP103 T03060117 ## FAA reports on this event (1) The FAA's own accident and incident reports, matched to the aircraft by N-number and date (OpenAirDocs). - [Accident 20220624008519A](https://openairdocs.org/incident/20220624008519A), Jun 24, 2022: Bell 206B, Merced, CA, Substantial damage. On June 24, 2022, at approximately 0927 PDT, N915B, a Bell 206B, helicopter experienced a hard landing while dispensing pesticides. The aircraft was in process… ## Docket (5 files) The files NTSB released with the investigation on April 25, 2024, on NTSB's docket system: https://data.ntsb.gov/Docket?ProjectID=105378. 19.3 MB in all. - Materials laboratory factual report (item 1, PDF, 29 pages, 34 photos, 8.5 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=14584901&FileExtension=pdf&FileName=22-074%20-%20Materials%20Lab%20Factual%20Report%20(WPR22LA236)-Rel.pdf - Pilot/operator aircraft accident report (item 2, PDF, 8 pages, 1.6 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=16991458&FileExtension=pdf&FileName=Z%20-%20Attachment%2001%20-%206120%20Pilot_Redacted-Rel.pdf - Records of conversation (item 3, PDF, 1 page, 133 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=16445867&FileExtension=pdf&FileName=Z%20-%20Attachment%2003%20-%20Record%20of%20Conversation%2022LA236-Rel.pdf - Kamatics service instructions (item 4, PDF, 39 pages, 8.9 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=16445212&FileExtension=pdf&FileName=SIN%202348%20Rev.%20K-%20Service%20Instruction%20Manual%20-%20206B%20Driveshaft-Rel.pdf - Statement of party representatives to NTSB investigation (item 5, PDF, 1 page, 153 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=16445427&FileExtension=pdf&FileName=Party%20Form%20WPR22LA236%20-%20Signed%20KAM%202022.07.22_Redacted-Rel.pdf From the NTSB's public API. Aircraft are matched to registry aircraft by N-number and event date, checked against the reported serial number, make and model; the registry's history has gaps, so older matches can be wrong. Compare the reported make and model. --- OpenAirDocs.org is an independent research project of [Contrail Labs](https://contraillabs.com/). It is not affiliated with the FAA or the NTSB. Source documents are U.S. government works in the public domain.