# NTSB CEN23LA227: Kolb Twinstar, N1025T, Starbuck, MN, June 2, 2023 - Type: NTSB accident investigation - Classification: Accident · Serious injuries · Class 3 · Limited · Final report - Date: June 2, 2023, 14:50 local (19:50 UTC) - Place: Starbuck, MN - Coordinates: 45.598893, -95.536149 (estimated) - Nearby events: https://openairdocs.org/map.md?lat=45.5989&lon=-95.5361&mi=10 - Phase of flight: [Landing](https://openairdocs.org/ntsb-phases/550) (OpenAirDocs' grouping) - Defining event: [Landing-flare/touchdown: Abnormal runway contact](https://openairdocs.org/ntsb-events/090) - Highest injury: Serious injuries - Injuries: 1 serious, 1 minor - Status: Completed - Report: Approved by Office Director - Investigation class: Class 3 - NTSB office: Central Region - Parties: Federal Safety Investigation Authority, FAA Minneapolis FSDO, Rotax Aircraft Engines - NTSB case key (mKey): 192316 - NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=CEN23LA227 - NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/192316/pdf - NTSB docket: https://data.ntsb.gov/Docket?ProjectID=192316 - Page: https://openairdocs.org/ntsb/CEN23LA227 ## Probable cause The pilot’s improper landing flare and hard landing, which resulted in the left axle fracturing from bending overstress. ## Findings NTSB's coded findings; those marked are part of the probable cause. - [Landing gear system: main gear strut/axle/truck](https://openairdocs.org/ntsb-findings/01023213): Aircraft › Aircraft systems · Failure (probable cause); cases with it: [Roger Thorstad/Brad Thorstad KOLB TWINSTAR MARK 3, 1 case](https://openairdocs.org/ntsb-findings/01023213/cases.md?model=05640FY), [All aircraft, 86 cases](https://openairdocs.org/ntsb-findings/01023213/cases.md) - [Landing gear system: main gear strut/axle/truck](https://openairdocs.org/ntsb-findings/01023213): Aircraft › Aircraft systems · Capability exceeded (probable cause); cases with it: [Roger Thorstad/Brad Thorstad KOLB TWINSTAR MARK 3, 1 case](https://openairdocs.org/ntsb-findings/01023213/cases.md?model=05640FY), [All aircraft, 86 cases](https://openairdocs.org/ntsb-findings/01023213/cases.md) - [Use of equip/info: aircraft control](https://openairdocs.org/ntsb-findings/02063040): Personnel issues › Task performance · Pilot (probable cause); cases with it: [Roger Thorstad/Brad Thorstad KOLB TWINSTAR MARK 3, 1 case](https://openairdocs.org/ntsb-findings/02063040/cases.md?model=05640FY), [All aircraft, 9,295 cases](https://openairdocs.org/ntsb-findings/02063040/cases.md) - [Performance/control parameters: landing flare](https://openairdocs.org/ntsb-findings/01062041): Aircraft › Aircraft oper/perf/capability · Not attained/maintained (probable cause); cases with it: [Roger Thorstad/Brad Thorstad KOLB TWINSTAR MARK 3, 1 case](https://openairdocs.org/ntsb-findings/01062041/cases.md?model=05640FY), [All aircraft, 1,285 cases](https://openairdocs.org/ntsb-findings/01062041/cases.md) - [Action: incorrect action performance](https://openairdocs.org/ntsb-findings/02041015): Personnel issues › Action/decision · Pilot (probable cause); cases with it: [Roger Thorstad/Brad Thorstad KOLB TWINSTAR MARK 3, 1 case](https://openairdocs.org/ntsb-findings/02041015/cases.md?model=05640FY), [All aircraft, 1,386 cases](https://openairdocs.org/ntsb-findings/02041015/cases.md) ## Sequence of events The events NTSB coded, in order, by phase of flight. The defining event best describes what happened. - [Landing-flare/touchdown: Abnormal runway contact](https://openairdocs.org/ntsb-events/090) (defining event); cases with it: [Roger Thorstad/Brad Thorstad KOLB TWINSTAR MARK 3, 1 case](https://openairdocs.org/ntsb-events/090/sequence.md?model=05640FY), [All aircraft, 1,506 cases](https://openairdocs.org/ntsb-events/090/sequence.md) - [Landing-flare/touchdown: Part(s) separation from AC](https://openairdocs.org/ntsb-events/338); cases with it: [Roger Thorstad/Brad Thorstad KOLB TWINSTAR MARK 3, 1 case](https://openairdocs.org/ntsb-events/338/sequence.md?model=05640FY), [All aircraft, 563 cases](https://openairdocs.org/ntsb-events/338/sequence.md) - [Landing-flare/touchdown: Loss of control on ground](https://openairdocs.org/ntsb-events/230); cases with it: [Roger Thorstad/Brad Thorstad KOLB TWINSTAR MARK 3, 1 case](https://openairdocs.org/ntsb-events/230/sequence.md?model=05640FY), [All aircraft, 4,585 cases](https://openairdocs.org/ntsb-events/230/sequence.md) - [Landing-flare/touchdown: Nose over/nose down](https://openairdocs.org/ntsb-events/096); cases with it: [Roger Thorstad/Brad Thorstad KOLB TWINSTAR MARK 3, 1 case](https://openairdocs.org/ntsb-events/096/sequence.md?model=05640FY), [All aircraft, 2,287 cases](https://openairdocs.org/ntsb-events/096/sequence.md) - [Post-impact: Evacuation](https://openairdocs.org/ntsb-events/460); cases with it: [Roger Thorstad/Brad Thorstad KOLB TWINSTAR MARK 3, 1 case](https://openairdocs.org/ntsb-events/460/sequence.md?model=05640FY), [All aircraft, 103 cases](https://openairdocs.org/ntsb-events/460/sequence.md) ## Analysis The flight instructor and the pilot, who was also the builder of the airplane, were practicing soft field landings with the tailwheel-equipped experimental airplane. They performed two three-point landings to the dry turf without incident. During touchdown on the third landing, with the pilot on the flight controls, the left wheel departed from the left strut. The airplane began sliding on the turf and nosed over, coming to rest inverted. The airplane sustained substantial damage to the fuselage, both wings, and the empennage. A postaccident metallurgical examination of the left main landing gear revealed that it failed when the left axle fractured from bending overstress consistent with upward forces on the wheel and outward portion of the axle. It is likely that the pilot executed an improper landing flare, which resulted in a hard landing that fractured the left axle in bending overstress, and a subsequent loss of control and nose over. ## Factual narrative On June 2, 2023, about 1450 central daylight time, a Kolb Twinstar Mark III airplane, N1025T, sustained substantial damage when it was involved in an accident near Starbuck, Minnesota. The flight instructor and the private pilot sustained serious injuries. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 instructional flight. The purpose of the flight was for the private pilot, who was also the airplane owner and builder, to work on soft field landings in the tailwheel-equipped experimental airplane. Before the flight, the pilot “topped off” the airplane with fuel and performed a preflight inspection with no anomalies noted. The flight instructor and the pilot also checked meteorological data before the training and observed nothing of concern. The airplane departed from the Chandler Field Airport (AXN), Alexandria, Minnesota, about 1409, for the Starbuck Municipal Airport (D32), Starbuck, Minnesota. The airplane entered the traffic pattern at D32 and the instructor and pilot performed two three-point landings to the dry turf on runway 15. During the touchdown on the third landing, with the pilot on the flight controls, the left wheel departed from the left strut. The airplane began sliding on the turf and nosed over, coming to rest inverted. The two occupants were able to egress from the airplane without further incident. The airplane sustained substantial damage to the fuselage, both wings, and the empennage. The airplane was equipped with an airframe parachute system manufactured by ASR-Pioneer that did not activate during the accident sequence. The pilot reported that the turf runway was in “great shape.” He added that the runway was examined after the accident for holes or ruts and nothing of concern was found. A postaccident metallurgical examination of the left main landing gear revealed that it failed when the left axle fractured from bending overstress consistent with upward forces on the wheel and outward portion of the axle. The chemical composition of the fractured axle was consistent with Type 4130 alloy steel. The hardness of the axle was consistent with typical tensile strengths for this alloy. There were no metallurgical deficiencies noted with the left main landing gear. The Federal Aviation Administration’s Airplane Flying Handbook (FAA-H-8083-3C) discusses a soft field landing with a tailwheel-equipped airplane and states: The tailwheel should touchdown simultaneously with or just before the main wheels and should then be held down by maintaining firm back-elevator pressure throughout the landing roll. This minimizes any tendency for the airplane to nose over and provides aerodynamic braking. The use of brakes on a soft field is not needed because the soft or rough surface itself provides sufficient reduction in the airplane’s forward speed. Often, it is found that upon landing on a very soft field, the pilot needs to increase power to keep the airplane moving and from becoming stuck in the softsurface. ## Aircraft - Registration: [N1025T](https://openairdocs.org/aircraft/N1025T), registered now, S/N M3X20-4-00116, built 2021 (OpenAirDocs) - Reported make and model: KOLB TWINSTAR (III) - Registry model: [Roger Thorstad/Brad Thorstad KOLB TWINSTAR MARK 3](https://openairdocs.org/model/roger-thorstad-brad-thorstad-kolb-twinstar-mark-3) (registry code 05640FY) (OpenAirDocs) - Registry engine: [Rotax 912UL](https://openairdocs.org/engine/rotax-912ul) (registry code 55592, reciprocating) (OpenAirDocs) - Category: Airplane, amateur-built - Serial number: M3X20-4-00116 - Year built: 2021 - Airframe hours: 40 - Engines: Rotax Aircraft Engines 912 ULS 100 horsepower reciprocating - Damage: Substantial damage - Flight: Instructional, Part 91 - Airworthiness certificate: Special, Experimental (Special) - Last inspection: Annual, May 15, 2023, 2 hours before - Departed: AXN, Alexandria, MN - Destination: Same as departure - Airport: D32, Starbuck Muni, runway 15/33, 2,512 × 198 ft, grass/turf - Equipment: ADS-B, Video recording device, Satellite tracking device, Onboard weather, Electronic primary flight display, Electronic multifunction display, Auto-pilot, Airframe parachute, Angle of attack indicator ## Flight crew As NTSB recorded them. NTSB leaves out names. - Flight instructor; age 35; injuries: minor; commercial, flight instructor; airplane ratings: multi-engine land, single-engine land; instrument ratings: airplane; medical: class 1, without waivers/limitations; last flight review February 5, 2022; seat: right; restraint: 4-point; flight hours: 1,581 total, 3 in this make and model, 1,534 as pilot in command, 245 in the last 90 days, 90 in the last 30 days, 7 in the last 24 hours - Pilot; age 57; injuries: serious; private; airplane ratings: single-engine land, single-engine sea; medical: class 3, without waivers/limitations; last flight review July 16, 2022; seat: left; restraint: 4-point; flight hours: 250 total, 8.9 in this make and model, 250 as pilot in command, 12 in the last 90 days, 6 in the last 30 days ## Weather - Conditions: Visual (VMC), Day - Wind: 190° at 8 knots, gusting 15 - Visibility: 10 statute miles - Lowest ceiling: none - Temperature: 86 °F, dew point 63 °F - Altimeter: 30.02 inHg - Observed at: KGHW, 9 nm away - METAR: KGHW 021955Z AUTO 19008G15KT 10SM CLR 30/17 A3002 RMK AO2 ## 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 20230602010169A](https://openairdocs.org/incident/20230602010169A), Jun 2, 2023: Roger Thorstad/Brad Thorstad KOLB TWINSTAR MARK 3, Starbuck, MN, Substantial damage. On June 2, 2023, a kolb twinstar mark iii airplane departed from the chandler field airport (axn), alexandria, Minnesota, at 1409 central daylight time, for… ## Docket (7 files) The files NTSB released with the investigation on July 10, 2024, on NTSB's docket system: https://data.ntsb.gov/Docket?ProjectID=192316. - Pilot/operator aircraft accident report, NTSB form 6120.1 (flight instructor) (item 1, PDF, 9 pages): https://data.ntsb.gov/Docket/Document/docBLOB?ID=16779423&FileExtension=pdf&FileName=NTSB%20Form%206120_1%20CFI_Redacted-Rel.pdf - Pilot/operator aircraft accident report, NTSB form 6120.1 (pilot/builder) (item 2, PDF, 7 pages): https://data.ntsb.gov/Docket/Document/docBLOB?ID=16780222&FileExtension=pdf&FileName=NTSB%20Form%206120__1%20Pilot_Builder_Redacted-Rel.pdf - Accident photographs (item 3, PDF, 3 pages): https://data.ntsb.gov/Docket/Document/docBLOB?ID=16778915&FileExtension=pdf&FileName=Accident%20Photographs-Rel.pdf - Pilot/builder - additional question (brake system) (item 4, PDF, 2 pages): https://data.ntsb.gov/Docket/Document/docBLOB?ID=16779647&FileExtension=pdf&FileName=Pilot%20-%20Additional%20Question_Redacted-Rel.pdf - Pilot's most recent flight review (item 5, PDF, 1 page): https://data.ntsb.gov/Docket/Document/docBLOB?ID=16786710&FileExtension=pdf&FileName=Pilot%27s%20Most%20Recent%20Flight%20Review_Redacted-Rel.pdf - Materials laboratory factual report (item 6, PDF, 15 pages, 17 photos): https://data.ntsb.gov/Docket/Document/docBLOB?ID=16449682&FileExtension=pdf&FileName=23-097%20-%20Materials%20Lab%20Factual%20Report%20(CEN23LA227)-Rel.pdf - Release of aircraft wreckage, NTSB form 6120.15 (item 7, PDF, 2 pages, 206 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=16779509&FileExtension=pdf&FileName=NTSB%20Wreckage%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.