# NTSB CEN21LA287: Grumman G164, N62375, Sedgwick, AR, June 20, 2021 - Type: NTSB accident investigation - Classification: Accident · No injuries · Class 3 · Limited · Final report - Date: June 20, 2021, 07:00 local (12:00 UTC) - Place: Sedgwick, AR · Rice field - Coordinates: 35.986718, -90.895871 (estimated) - Nearby events: https://openairdocs.org/map.md?lat=35.9867&lon=-90.8959&mi=10 - Phase of flight: [Enroute](https://openairdocs.org/ntsb-phases/400) (OpenAirDocs' grouping) - Defining event: [Enroute: Loss of engine power (total)](https://openairdocs.org/ntsb-events/341) - Highest injury: No injuries - Injuries: 1 uninjured - Status: Completed - Report: Approved by Office Director - Investigation class: Class 3 - NTSB office: Central Region - Parties: Honeywell, Federal Aviation Administration - Little Rock FSDO - NTSB case key (mKey): 103314 - NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=CEN21LA287 - NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/103314/pdf - NTSB docket: https://data.ntsb.gov/Docket?ProjectID=103314 - Page: https://openairdocs.org/ntsb/CEN21LA287 ## Probable cause The total loss of engine power due to the failure and disengagement of the torque sensor assembly front spur gear from its splined shouldered shaft due to a loose self-locking nut. ## Findings NTSB's coded findings; those marked are part of the probable cause. - [Engine (turbine/turboprop): accessory drives](https://openairdocs.org/ntsb-findings/01057260): Aircraft › Aircraft power plant · Malfunction (probable cause); cases with it: [Ag-Cat G-164B, 1 case](https://openairdocs.org/ntsb-findings/01057260/cases.md?model=1260000), [All aircraft, 12 cases](https://openairdocs.org/ntsb-findings/01057260/cases.md) ## Sequence of events The events NTSB coded, in order, by phase of flight. The defining event best describes what happened. - [Enroute: Loss of engine power (total)](https://openairdocs.org/ntsb-events/341) (defining event); cases with it: [Ag-Cat G-164B, 1 case](https://openairdocs.org/ntsb-events/341/sequence.md?model=1260000), [All aircraft, 3,820 cases](https://openairdocs.org/ntsb-events/341/sequence.md) ## Analysis The pilot was conducting an agricultural flight when the airplane had a sudden total loss of engine power. The pilot stated that the sound of the engine went from normal to silent, with no indication of an engine issue before the loss of engine power. The airplane impacted a levee and nosed over in a flooded rice field during the forced landing. The airplane’s empennage, aft fuselage, and engine mounts were substantially damaged during the forced landing. A postaccident engine examination and disassembly revealed the torque sensor assembly front spur gear was disengaged from the splined shouldered shaft on which it was installed. Additionally, the self-locking nut that secured the front spur gear to the shaft had backed off which allowed the front spur gear to disengage from the shaft. The examination and testing were unable to determine if the front spur gear self-locking nut was improperly torqued at installation or if it became loose during operation due a loss of retention. However, had the self-locking nut not been seated properly at installation, the disengagement of the front spur gear would likely have occurred well before the accident (2,541.5 hours since the overhauled torque sensor was installed on the engine). Additionally, no anomalies were found with the self-locking nut to suggest why it would have lost retention during operation. The engine-driven fuel pump and fuel control unit (FCU) are driven through a series of gears in the accessory gearbox, including the front spur gear, and any failure in the gear train that drives the fuel pump and FCU would prevent the fuel pump from delivering fuel to the FCU to sustain engine operation, resulting in an uncommanded engine shutdown. ## Factual narrative On June 20, 2021, about 0700 central daylight time, a Grumman G-164B airplane, N62375, was substantially damaged when it was involved in an accident near Sedgwick, Arkansas. The pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 137 aerial-application flight. The pilot stated that he had completed 5 to 6 aerial-application passes over the intended rice field when the airplane had a total loss of engine power while in level flight. The pilot stated that the sound of the engine went from normal to silent, with no indication of an issue before the loss of engine power. The airplane impacted a levee and nosed over in a flooded rice field during the forced landing. The airplane’s empennage, aft fuselage, and engine mounts were substantially damaged during the forced landing. A postaccident engine examination and disassembly revealed the torque sensor assembly front spur gear was loose and disengaged from the splined shouldered shaft on which it was installed. Furthermore, the self-locking nut that secured the front spur gear to the shaft had backed off which allowed the front spur gear to disengage from the shaft. A disassembly of the torque sensor assembly revealed that the internal spline teeth of the front spur gear and the corresponding external forward spline teeth of the splined shouldered shaft were heavily damaged, smeared over, and exhibited loss of teeth material. Metallurgical examination of the splined shouldered shaft found stepped wear and deformation consistent with meshing with the fretted internal spline teeth of the front spur gear. Wear and deformation were also observed on the last (rearward) threads of the splined shouldered shaft consistent with contact with the worn threads of the self-locking nut. The internal spline teeth of the front spur gear exhibited corresponding wear and deformation consistent with meshing with the fretted external forward spline teeth of the splined shouldered shaft. Dimensional inspection of the splined shouldered shaft away from the damaged region was consistent with the manufacturing drawing. The average hardness of the splined shouldered shaft and front spur gear were within their respective manufacturing drawing ranges. The self-locking nut that secured the front spur gear to the splined shouldered shaft has slightly ovalized threads so that the nut must elastically deform to mate with the threads of the splined shouldered shaft, causing increased surface friction that resists loosening when fully and properly torqued. Metallurgical examination of the self-locking nut exhibited wear marks on one side of each wrenching flat consistent with contact from the installation/torque tool, wear marks on the aft face of the nut consistent with contact with the mating washer, and wear of the internal threads consistent with engagement with the splined shouldered shaft. The average hardness of the self-locking nut met the minimum hardness design requirement.However, the examination and testing were unable to determine if the self-locking nut had been improperly torqued at installation or if it became loose during operation due a loss of retention. According to overhaul documentation, a new self-locking nut was used when the torque sensor assembly, part number 3101726-3, serial number P-4184C, series 2, was last overhauled on October 3, 2013. The engine logbook indicated that the overhauled torque sensor assembly was installed on the engine on February 11, 2015. At the time of the accident, the torque sensor assembly had accumulated 2,541.5 hours since its last overhaul. The engine-driven fuel pump and fuel control unit (FCU) are driven through a series of gears in the accessory gearbox, including the front spur gear, and any failure in the gear train that drives the fuel pump and FCU would prevent the fuel pump from delivering fuel to the FCU to sustain engine operation, resulting in an uncommanded engine shutdown. ## Aircraft - Registration: [N62375](https://openairdocs.org/aircraft/N62375), registered now, S/N 806B, built 1988 (OpenAirDocs) - Reported make and model: GRUMMAN G164 (B) - Registry model: [Ag-Cat G-164B](https://openairdocs.org/model/ag-cat-g-164b) (registry code 1260000) (OpenAirDocs) - Registry engine: [AiResearch TPE331 series](https://openairdocs.org/engine/airesearch-tpe331-series) (registry code 01517, turbo-prop) (OpenAirDocs) - Category: Airplane - Serial number: 806B - Year built: 1988 - Airframe hours: 14472 - Engines: Honeywell TPE331-1-151G 665 horsepower turbo prop - Damage: Substantial damage - Flight: Aerial application, Part 137 - Operator: Scott Flying Service - Registered owner: Scott Flying Service - Airworthiness certificate: Special, Restricted (Special) - Last inspection: Annual, March 1, 2021, 91 hours before - Departed: PVT, Fontaine, AR - Equipment: Stall warning system ## Flight crew As NTSB recorded them. NTSB leaves out names. - Pilot; age 64; injuries: none; commercial; airplane ratings: single-engine land, multi-engine land; medical: class 2, with waivers/limitations; last flight review February 25, 2021; seat: single; restraint: 4-point; flight hours: 16,020 total, 13,430 in this make and model, 15,998 as pilot in command, 100 in the last 90 days, 100 in the last 30 days, 7 in the last 24 hours ## Weather - Conditions: Visual (VMC), Day - Wind: 200° at 10 knots - Visibility: 10 statute miles - Lowest ceiling: none - Temperature: 73 °F, dew point 72 °F - Altimeter: 29.82 inHg - Observed at: ARG, 8 nm away - METAR: METAR KARG 201156Z AUTO 20010KT 10SM CLR 23/22 A2982 RMK AO2 SLP098 T02330222 10250 20228 56002= ## 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 20210620008609A](https://openairdocs.org/incident/20210620008609A), Jun 20, 2021: Ag-Cat Corporation G-164B, Sedgwick, AR, Substantial damage. On 06/20/2021 @ 0700 aircraft N62375, 1988 Ag-Cat, model G-164B, serial number 806B [privacy data omitted] was involved in an accident. The aircraft was on the… ## Docket (11 files) The files NTSB released with the investigation on August 16, 2023, on NTSB's docket system: https://data.ntsb.gov/Docket?ProjectID=103314. 65.8 MB in all. - Pilot/operator aircraft accident report, NTSB form 6120.1 (item 1, PDF, 9 pages, 4.5 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=15902473&FileExtension=pdf&FileName=01%20-%20Pilot%20Report_Redacted-Rel.pdf - Record of conversation (randy howard scott) (item 2, PDF, 2 pages, 95 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=15902608&FileExtension=pdf&FileName=02%20-%20Record%20of%20Conversation%20(Randy%20Howard%20Scott)-Rel.pdf - Powerplants group chairman's factual report (item 3, PDF, 28 pages, 4.8 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=15897345&FileExtension=pdf&FileName=TPE%20331%20Factual%20Group%20Chairmans%20Report%20CEN21LA287-Rel.pdf - Powerplants attachment 1 - Honeywell metallurgical report, ERS-asys-peng-MA-0000865-B (item 4, PDF, 53 pages, 17.7 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=14541898&FileExtension=pdf&FileName=ERS-ASYS-PENG-MA-0000865-B-Rel.pdf - Memorandum for the record (maintenance logbook calculations) (item 5, PDF, 4 pages, 99 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=15902091&FileExtension=pdf&FileName=05%20-%20Memorandum%20for%20the%20Record%20(Maintenance%20Logbook%20Calculations)-Rel.pdf - Airframe logbook (excerpts) (item 6, PDF, 7 pages, 9.2 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=15902287&FileExtension=pdf&FileName=06%20-%20Airframe%20Logbook%20(Excerpts)_Redacted-Rel.pdf - Engine logbook (excerpts) (item 7, PDF, 20 pages, 25.3 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=15919873&FileExtension=pdf&FileName=07%20-%20Engine%20Logbook%20(Excerpts)_Redacted-Rel.pdf - Torque sensor overhaul documentation (form 8130-3 and list of parts replaced) (item 8, PDF, 2 pages, 1.3 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=15902754&FileExtension=pdf&FileName=08%20-%20Torque%20Sensor%20Overhaul%20Documentation%20(Form%208130-3%20and%20List%20of%20Parts%20Replaced)_Redacted-Rel.pdf - Weather summary (item 9, PDF, 2 pages, 107 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=15902350&FileExtension=pdf&FileName=09%20-%20Weather%20Summary-Rel.pdf - Photos (item 10, PDF, 3 pages, 3 photos, 2.3 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=15902483&FileExtension=pdf&FileName=10%20-%20Photos-Rel.pdf - Statement of party representatives to NTSB investigation (item 11, PDF, 1 page, 380 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=15932871&FileExtension=pdf&FileName=11%20-%20NTSB%20Party%20Form%20(CEN21LA287)_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.