# NTSB WPR21LA238: Piper PA-25-235, N6917Z, Twisp, WA, June 20, 2021 - Type: NTSB accident investigation - Classification: Accident · No injuries · Class 4 · Limited · Final report - Date: June 20, 2021, 12:10 local (19:10 UTC) - Place: Twisp, WA - Coordinates: 48.350585, -120.09395 (estimated) - Nearby events: https://openairdocs.org/map.md?lat=48.3506&lon=-120.0940&mi=10 - Phase of flight: [Landing](https://openairdocs.org/ntsb-phases/550) (OpenAirDocs' grouping) - Defining event: [Landing: Flight control sys malf/fail](https://openairdocs.org/ntsb-events/333) - Highest injury: No injuries - Injuries: 1 uninjured - Status: Completed - Report: Approved by Office Director - Investigation class: Class 4 - Basic report with no Prelim publish - NTSB office: Western Pacific Region - Parties: FAA - NTSB case key (mKey): 103319 - NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=WPR21LA238 - NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/103319/pdf - NTSB docket: https://data.ntsb.gov/Docket?ProjectID=103319 - Page: https://openairdocs.org/ntsb/WPR21LA238 ## Probable cause An in-flight separation of the control stick from the torque tube assembly, which resulted in a loss of directional control during the landing roll and subsequent on-ground collision. ## Findings NTSB's coded findings; those marked are part of the probable cause. - [Flight control system: control column section](https://openairdocs.org/ntsb-findings/01022701): Aircraft › Aircraft systems · Malfunction (probable cause); cases with it: [Piper PA-25-235, 1 case](https://openairdocs.org/ntsb-findings/01022701/cases.md?model=7102504), [All aircraft, 21 cases](https://openairdocs.org/ntsb-findings/01022701/cases.md) - [Performance/control parameters: directional control](https://openairdocs.org/ntsb-findings/01062020): Aircraft › Aircraft oper/perf/capability · Not attained/maintained (probable cause); cases with it: [Piper PA-25-235, 3 cases](https://openairdocs.org/ntsb-findings/01062020/cases.md?model=7102504), [All aircraft, 3,958 cases](https://openairdocs.org/ntsb-findings/01062020/cases.md) - [Misc hardware: fasteners](https://openairdocs.org/ntsb-findings/01071430): Aircraft › Fluids/misc hardware · Not specified (probable cause); cases with it: [Piper PA-25-235, 1 case](https://openairdocs.org/ntsb-findings/01071430/cases.md?model=7102504), [All aircraft, 58 cases](https://openairdocs.org/ntsb-findings/01071430/cases.md) ## Sequence of events The events NTSB coded, in order, by phase of flight. The defining event best describes what happened. - [Landing: Flight control sys malf/fail](https://openairdocs.org/ntsb-events/333) (defining event); cases with it: [Piper PA-25-235, 1 case](https://openairdocs.org/ntsb-events/333/sequence.md?model=7102504), [All aircraft, 410 cases](https://openairdocs.org/ntsb-events/333/sequence.md) ## Analysis The pilot in the tailwheel-equipped airplane reported that, during the sixth flight of a glider tow operation, the control stick felt as though it had “some play” with regard to the ailerons. The control stick moved about four inches laterally before a corresponding movement of the ailerons could be observed. He landed the airplane, performed a flight control check and noticed a “little slack” between the control stick inputs and subsequent aileron movement. He determined that the control stick felt secure and took off on his seventh tow flight. During the climb out, the response to his control stick inputs diminished. The airplane encountered turbulence and the control stick separated from the torque tube assembly. He maintained control of the airplane by using trim, rudder control and throttle variations. The airplane subsequently landed on the grass surface to the right of the runway. During a firm landing, the airplane veered right, and the right wing collided with a berm. The airplane sustained substantial damage to the leading-edge of the right wing. A subsequent examination of the airplane’s flight controls, revealed that the bolt which secured the control stick to the torque tube assembly had backed out. The castellated nut and cotter pin which secured the bolt were not located. ## Aircraft - Registration: [N6917Z](https://openairdocs.org/deregistered/N6917Z-2021-12-03), registration cancelled 2021-12-03, S/N 25-2660, built 1964 (OpenAirDocs) - Reported make and model: PIPER PA-25-235 - Registry model: [Piper PA-25-235](https://openairdocs.org/model/piper-pa-25-235) (registry code 7102504) (OpenAirDocs) - Registry engine: [Lycoming 0-540 series](https://openairdocs.org/engine/lycoming-0-540-series) (registry code 41532, reciprocating) (OpenAirDocs) - Category: Airplane - Serial number: 25-2660 - Year built: 1964 - Airframe hours: 7278.8 - Engines: Lycoming O-540-B2C5 235 horsepower reciprocating - Damage: Substantial damage - Flight: Glider tow, Part 91 - Operator: Evergreen Soaring Inc - Registered owner: Evergreen Soaring Inc - Airworthiness certificate: Standard, Normal - Last inspection: Annual, March 11, 2021 - Destination: Same as departure - Airport: 2S0, Twisp Muni, runway 10/28, 2,701 × 60 ft, grass/turf - Equipment: None ## Flight crew As NTSB recorded them. NTSB leaves out names. - Pilot; age 79; injuries: minor; airline transport; airplane ratings: single-engine land, multi-engine land; instrument ratings: airplane; medical: with waivers/limitations; last flight review May 19, 2021; seat: single; flight hours: 15,077 total, 516.9 in this make and model, 34.5 in the last 90 days, 27 in the last 30 days, 2.3 in the last 24 hours ## Weather - Conditions: Visual (VMC), Day - Wind: 20° at 10 knots, gusting 18 - Visibility: 10 statute miles - Lowest ceiling: none - Temperature: 81 °F, dew point 46 °F - Altimeter: 30.08 inHg - Observed at: KOMK, 24 nm away - METAR: METAR KOMK 201853Z AUTO 02010G18KT 10SM CLR 27/08 A3008 RMK AO2 SLP177 T02720083= ## 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 20210620007919A](https://openairdocs.org/incident/20210620007919A), Jun 20, 2021: Piper PA-25-235, Twisp, WA, Substantial damage. Glider tow plane while towing glider lost aileron and elevator control function when hit turbulence and stick detached from torgue tube pivot point at 4000'… ## Docket (3 files) The files NTSB released with the investigation on November 12, 2021, on NTSB's docket system: https://data.ntsb.gov/Docket?ProjectID=103319. 4.3 MB in all. - Pilot/operator aircraft accident report, NTSB form 6120.1 (item 1, PDF, 9 pages, 3.8 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=12486839&FileExtension=pdf&FileName=6120_1_Redacted-Rel.pdf - Photographs (item 2, PDF, 2 pages, 392 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=12926349&FileExtension=pdf&FileName=Photographs-Rel.pdf - Record of conversation a&p/ia (item 3, PDF, 1 page, 156 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=13129361&FileExtension=pdf&FileName=Record%20of%20Conversation%20A%26P_IA-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.