Monday, June 21, 2021

Flight Control System Malfunction / Failure: Piper PA-25-235, N6917Z; accident occurred June 20, 2021 at Twisp Municipal Airport (2S0), Okanogan County, Washington

Aviation Accident Final Report - National Transportation Safety Board 

The National Transportation Safety Board did not travel to the scene of this accident.

Additional Participating Entity:
Federal Aviation Administration / Flight Standards District Office; Spokane, Washington

Investigation Docket - National Transportation Safety Board:

Evergreen Soaring Inc 

Location: Twisp, Washington
Accident Number: WPR21LA238
Date and Time: June 20, 2021, 12:10 Local 
Registration: N6917Z
Aircraft: Piper PA-25-235 
Aircraft Damage: Substantial
Defining Event: Flight control sys malf/fail 
Injuries: 1 None
Flight Conducted Under: Part 91: General aviation - Glider tow


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.

Probable Cause and Findings

The National Transportation Safety Board determines the probable cause(s) of this accident to be:
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.


Aircraft Control column section - Malfunction
Aircraft Directional control - Not attained/maintained
Aircraft Fasteners - Not specified

Factual Information

History of Flight

Landing Flight control sys malf/fail (Defining event)

Pilot Information

Certificate: Airline transport 
Age: 79, Male
Airplane Rating(s): Single-engine land; Multi-engine land
Seat Occupied: Single
Other Aircraft Rating(s): None
Restraint Used: Unknown
Instrument Rating(s): Airplane
Second Pilot Present: No
Instructor Rating(s): None
Toxicology Performed:
Medical Certification: None With waivers/limitations 
Last FAA Medical Exam: March 24, 2020
Occupational Pilot: Yes 
Last Flight Review or Equivalent: May 19, 2021
Flight Time: (Estimated) 15077 hours (Total, all aircraft), 516.9 hours (Total, this make and model), 34.5 hours (Last 90 days, all aircraft), 27 hours (Last 30 days, all aircraft), 2.3 hours (Last 24 hours,all aircraft)

Aircraft and Owner/Operator Information

Aircraft Make: Piper
Registration: N6917Z
Model/Series: PA-25-235
Aircraft Category: Airplane
Year of Manufacture: 1964 
Amateur Built:
Airworthiness Certificate: Normal 
Serial Number: 25-2660
Landing Gear Type: Tailwheel
Seats: 1
Date/Type of Last Inspection: March 11, 2021 Annual
Certified Max Gross Wt.: 2900 lbs
Time Since Last Inspection: 
Engines: 1 Reciprocating
Airframe Total Time: 7278.8 Hrs as of last inspection
Engine Manufacturer: Lycoming
ELT: Not installed
Engine Model/Series: O-540-B2C5
Registered Owner: 
Rated Power: 235 Horsepower
Operating Certificate(s) Held: None

Meteorological Information and Flight Plan

Conditions at Accident Site: Visual (VMC)
Condition of Light: Day
Observation Facility, Elevation: KOMK,1300 ft msl
Distance from Accident Site: 24 Nautical Miles
Observation Time: 11:53 Local 
Direction from Accident Site: 73°
Lowest Cloud Condition: Clear 
Visibility: 10 miles
Lowest Ceiling: None 
Visibility (RVR):
Wind Speed/Gusts: 10 knots / 18 knots
Turbulence Type Forecast/Actual:  /
Wind Direction: 20°
Turbulence Severity Forecast/Actual:  /
Altimeter Setting: 30.08 inches Hg
Temperature/Dew Point: 27°C / 8°C
Precipitation and Obscuration: No Obscuration; No Precipitation
Departure Point: Twisp, WA
Type of Flight Plan Filed: None
Destination: Twisp, WA 
Type of Clearance: None
Departure Time: Type of Airspace: Class E

Airport Information

Airport: TWISP MUNI 2S0 
Runway Surface Type: Grass/turf
Airport Elevation: 1602 ft msl 
Runway Surface Condition: Dry
Runway Used: 10/28 
IFR Approach: None
Runway Length/Width: 2701 ft / 60 ft 
VFR Approach/Landing: Forced landing

Wreckage and Impact Information

Crew Injuries: 1 None
Aircraft Damage: Substantial
Passenger Injuries:
Aircraft Fire: None
Ground Injuries: 
Aircraft Explosion: None
Total Injuries: 1 None
Latitude, Longitude: 48.350585,-120.09395 (est)

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