# NTSB CEN20LA353: Air Tractor AT602, N604MG, Inverness, MS, August 20, 2020 - Type: NTSB accident investigation - Classification: Accident · No injuries · Class 3 · Limited · Final report - Date: August 20, 2020, 09:00 local (14:00 UTC) - Place: Inverness, MS - Coordinates: 33.336944, -90.58139 (estimated) - Nearby events: https://openairdocs.org/map.md?lat=33.3369&lon=-90.5814&mi=10 - Phase of flight: [Approach](https://openairdocs.org/ntsb-phases/500) (OpenAirDocs' grouping) - Defining event: [Approach-IFR final approach: 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: Central Region - Parties: Hartzell, TSB Canada, Air Tractor, Federal Aviation Adminstration, Woodward Inc., PWC - NTSB case key (mKey): 101837 - NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=CEN20LA353 - NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/101837/pdf - NTSB docket: https://data.ntsb.gov/Docket?ProjectID=101837 - Page: https://openairdocs.org/ntsb/CEN20LA353 ## Probable cause The pilot’s decision to move the power control lever into beta during flight, which resulted in the power control lever and associated linkage becoming stuck in the beta position. ## Findings NTSB's coded findings; those marked are part of the probable cause. - [Action: incorrect action selection](https://openairdocs.org/ntsb-findings/02041010): Personnel issues › Action/decision · Pilot (probable cause); cases with it: [Air Tractor AT-602, 2 cases](https://openairdocs.org/ntsb-findings/02041010/cases.md?model=0390309), [All aircraft, 397 cases](https://openairdocs.org/ntsb-findings/02041010/cases.md) - [Use of equip/info: use of equip/system](https://openairdocs.org/ntsb-findings/02063015): Personnel issues › Task performance · Pilot (probable cause); cases with it: [Air Tractor AT-602, 3 cases](https://openairdocs.org/ntsb-findings/02063015/cases.md?model=0390309), [All aircraft, 1,391 cases](https://openairdocs.org/ntsb-findings/02063015/cases.md) - [Engine controls: power lever](https://openairdocs.org/ntsb-findings/01057603): Aircraft › Aircraft power plant · Incorrect use/operation (probable cause); cases with it: [Air Tractor AT-602, 1 case](https://openairdocs.org/ntsb-findings/01057603/cases.md?model=0390309), [All aircraft, 95 cases](https://openairdocs.org/ntsb-findings/01057603/cases.md) ## Sequence of events The events NTSB coded, in order, by phase of flight. The defining event best describes what happened. - [Approach: Off-field or emergency landing](https://openairdocs.org/ntsb-events/440); cases with it: [Air Tractor AT-602, 13 cases](https://openairdocs.org/ntsb-events/440/sequence.md?model=0390309), [All aircraft, 3,365 cases](https://openairdocs.org/ntsb-events/440/sequence.md) - [Approach-IFR final approach: Sys/Comp malf/fail (non-power)](https://openairdocs.org/ntsb-events/330) (defining event); cases with it: [Air Tractor AT-602, 2 cases](https://openairdocs.org/ntsb-events/330/sequence.md?model=0390309), [All aircraft, 994 cases](https://openairdocs.org/ntsb-events/330/sequence.md) ## Analysis The turbine-powered agricultural airplane was on short final to land at the private airstrip. In attempt to slow down the airplane, the pilot actuated the power control lever (PCL) into beta, which caused the PCL, cam box, and associated linkage to get stuck in place. The airplane required more engine power to make the runway so he attempted to advance the PCL, but it would not move. The pilot made a forced landing in a field short of the runway, the airplane came to rest upright, and sustained substantial damage to both wings. The engine data revealed that the PCL was moved to beta while the airplane was on short final. The beta signal was still on after engine shutdown, which indicates that the cam box was likely stuck in beta. A service letter issued by the airplane manufacturer states that the use of beta mode in flight is not authorized and can result in a dangerous situation, leading to a loss of control. In this event, when the PCL was actuated into beta in flight and the propeller servo pressure dropped to maintain control of the propeller, the servo pressure went into a negative range, and the propeller blade angle was quickly forced to the max reverse stop, which allowed the beta ring to move to the most forward position. Since the beta valve does not allow this amount of movement, the additional movement of the beta ring was transferred to the cam box and the cam box jammed in place. ## Factual narrative On August 20, 2020, about 0900 central daylight time, an Air Tractor AT602 airplane, N604MG, was substantially damaged when it was involved in an accident near Inverness, Mississippi. 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 when he was on short final, the airplane required more engine power to reach the runway. He attempted to advance the power control lever (PCL), but it would not move. He subsequently made a forced landing in a field and the airplane sustained substantial damage to both wings. An initial examination of the PCL revealed that it was stuck near the idle position and could not be moved. The pilot reported that during the accident flight, he positioned the PCL friction lock such that the lever would move smoothly and characterized its friction as a “medium range.” He said that during some previous flights, he had used beta in flight to quickly slow the airplane. He could not recall if he had put the PCL into beta range before it became stuck but said it was possible. The actual commanded position of the PCL provides an indication to the airplane’s MVP-50T engine monitor. The recorded data revealed that the PCL was moved into beta during the accident flight. Also, the beta signal was still on after engine shutdown. According to the Airplane Flying Handbook (FAA-H-8083-3B), the beta range of operation consists of power lever positions from flight idle to maximum reverse. Beginning at power lever positions just aft of flight idle, propeller blade pitch angles become progressively flatter with aft movement of the power lever until they go beyond maximum flat pitch and into negative pitch, resulting in reverse thrust. To move the PCL into beta, a pilot moves the thumb latch on the top of the PCL forward, which raises the idle stop and allows the PCL to move aft of the anti-reversing plate. A fence on the power quadrant keeps the idle stop in line with the anti-reversing lock plate to prevent the PCL from inadvertently moving into beta. When the PCL is actuated aft of the idle stop position and into the beta range it reduces the propeller blade angle below the flight idle position. Under some flight conditions blade angles below flight idle result in blade aerodynamic forces that cause the propeller servo pressure to decrease and the propeller to become uncontrolled by the propeller governor. In this condition the propeller blade angle would be aerodynamically forced – without pilot action – to the max reverse stop. When the propeller blade angle moves to the reverse stop, the beta ring moves to the most-forward position. If the propeller is not controlled by the governor/beta valve at this point, movement of the PCL out of beta range moves the beta valve to its max forward stop without corresponding aft movement of the propeller beta ring. With the beta ring and beta valve in those positions, no further movement of thereversing cable/reversing cam would be possible, and the cam box could get jammed. The cam box is the intermediate linkage between the cockpit levers, the propeller governor, and the fuel control unit. If the cam follower pin is in the reversing range of the cam when this occurs, it will prevent forward movement of the PCL. Air Tractor Service Letter 364 states in part: The use of beta mode in flight is not authorized on Air Tractor aircraft and can result in a dangerous situation, leading to aircraft loss of control…beta mode and reverse thrust settings are only authorized for ground operations. These settings may be used during taxi operations to reduce brake system wear and to reduce taxi speeds. Beta mode and reverse thrust may be used during the landing rollout, but only after all three wheels are on the ground. ## Aircraft - Registration: [N604MG](https://openairdocs.org/aircraft/N604MG), registered now, S/N 602-1306, built 2019 (OpenAirDocs) - Reported make and model: AIR TRACTOR AT602 - Registry model: [Air Tractor AT-602](https://openairdocs.org/model/air-tractor-at-602) (registry code 0390309) (OpenAirDocs) - Registry engine: [Pratt & Whitney Canada PT6A-60AG](https://openairdocs.org/engine/pratt-whitney-canada-pt6a-60ag) (registry code 52101, turbo-prop) (OpenAirDocs) - Category: Airplane - Serial number: 602-1306 - Year built: 2019 - Airframe hours: 375 - Engines: Pratt & Whitney Canada PT6A-60AG 1173 horsepower turbo prop - Damage: Substantial damage - Flight: Aerial application, Part 137 - Operator: Gary Flying Service - Registered owner: Gary Flying Service - Airworthiness certificate: Special, Restricted (Special) - Last inspection: Annual, December 19, 2019 - Departed: MS21, Inverness, MS - Destination: MS21, Inverness, MS - Airport: MS21, Lester Field, runway 18, 3,000 × 65 ft, grass/turf - Equipment: Stall warning system ## Flight crew As NTSB recorded them. NTSB leaves out names. - Pilot; age 38; injuries: none; flight instructor, commercial; airplane ratings: multi-engine land, single-engine land; instrument ratings: airplane; medical: class 2, without waivers/limitations; last flight review February 9, 2019; seat: single; restraint: 5-point; flight hours: 7,067 total, 445 in this make and model, 6,945 as pilot in command, 211 in the last 90 days, 63 in the last 30 days, 3 in the last 24 hours ## Weather - Conditions: Visual (VMC), Day - Wind: 40° at 6 knots - Visibility: 10 statute miles - Lowest ceiling: none - Temperature: 73 °F, dew point 70 °F - Altimeter: 29.93 inHg - Observed at: KGLH, 22 nm away ## 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 20200820016579A](https://openairdocs.org/incident/20200820016579A), Aug 20, 2020: Air Tractor Inc AT-602, Inverness, MS, Substantial damage. Inverness, MS (MS21): N604MG, AT602, made a forced landing in a field due to a throttle issue. The aircraft suffered damage to the wing and landing gear. POB:… ## Docket (11 files) The files NTSB released with the investigation on June 3, 2022, on NTSB's docket system: https://data.ntsb.gov/Docket?ProjectID=101837. 298 MB in all. - Pilot/operator aircraft accident report, NTSB form 6120.1 (item 1, PDF, 9 pages, 3.7 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=10175825&FileExtension=pdf&FileName=6120%20-%20Pilot_Redacted-Rel.pdf - Correspondence - pilot (item 2, PDF, 1 page, 121 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=12538264&FileExtension=pdf&FileName=ROC%20-%20Pilot%20(PCL%20to%20beta%20admission)-Rel.pdf - Investigation summary (item 3, PDF, 7 pages, 521 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=12569004&FileExtension=pdf&FileName=Investigation%20Summary-Rel.pdf - Computed tomography specialist's factual report (item 4, PDF, 46 pages, 40 photos, 284 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=11878181&FileExtension=pdf&FileName=CEN20LA353,%20AT602,%20FCU%20and%20prop%20governor,%20Inverness,%20MS%20-%20CT%20factual%20report%20-%20final-Rel.pdf - Air tractor service letter 364 (item 5, PDF, 2 pages, 52 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=12553778&FileExtension=pdf&FileName=SL364-Rel.pdf - Pratt & whitney Canada PT6A-60 control rigging (item 6, PDF, 8 pages, 274 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=12560073&FileExtension=pdf&FileName=PT6A-60%20engine%20control%20rigging-Rel.pdf - Photos (item 7, PDF, 5 pages, 3.5 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=12538202&FileExtension=pdf&FileName=Photos-Rel.pdf - Tabular engine data (item 8, CSV, 2.7 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=12538327&FileExtension=csv&FileName=Flt254_08-20-20f-Rel.csv - Airplane flying handbook excerpt - beta range (item 9, PDF, 1 page, 522 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=12568955&FileExtension=pdf&FileName=Airplane%20Flying%20Handbook%20(FAA-H-8083-3B)%20Beta%20range-Rel.pdf - Statement of party representatives to NTSB investigation (item 10, PDF, 3 pages, 2.1 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=12537497&FileExtension=pdf&FileName=Party%20forms%20-%20all_Redacted-Rel.pdf - Release of aircraft wreckage, NTSB form 6120.15 (item 11, PDF, 1 page, 1.2 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=12559994&FileExtension=pdf&FileName=Wreckage%20release%20form_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.