Tuesday, June 09, 2020

Hard Landing: Rockwell International Thrush S-2R-800, N32LF; accident occurred August 16, 2017 at Stephan Lake Lodge Airport (AK61), Talkeetna, Alaska

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

Additional Participating Entities:

Federal Aviation Administration / Flight Standards District Office; Anchorage, Alaska
Thrush Aircraft Inc; Albany, Georgia

Aviation Accident Factual Report - National Transportation Safety Board: https://app.ntsb.gov/pdf

Investigation Docket - National Transportation Safety Board: https://dms.ntsb.gov/pubdms


Location: Talkeetna, AK
Accident Number:ANC17LA046 
Date & Time: 08/16/2017, 1800 AKD
Registration: N32LF
Aircraft: THRUSH S2R
Aircraft Damage: Substantial
Defining Event: Hard landing
Injuries: 1 None
Flight Conducted Under: Part 91: General Aviation - Business 

On August 16, 2017, about 1800 Alaska daylight time, a turbine-powered Thrush (formerly Rockwell International) S-2R-800 aerial application airplane, N32LF, lost control and impacted terrain while landing at the Stephan Lake Lodge Airport (AK61), Talkeetna, Alaska. The commercial pilot who was the sole occupant, sustained no injury. The airplane sustained substantial damage. The airplane was registered to, and operated by, Glenn Air, Inc., Palmer, Alaska, as a visual flight rules flight under the provisions of 14 Code of Federal Regulations (CFR) Part 91. Day visual meteorological conditions prevailed at the time of the accident, and company flight following procedures were in effect. The flight originated from the Willow Airport (UUO), Willow, Alaska about 1730.

The pilot, who is also the president of the company, reported that the purpose of the flight was to transport bulk fuel to AK61. AK61, serving the remote Stephan Lake Lodge, is located on the west side of Stephan Lake and is about 46 miles northeast of Talkeetna in the Talkeetna Mountains. Upon taking off from UUO, the pilot reported he heard a "wack" noise emit from the rear of the airplane which he attributed to a rock impacting a flap.

After an uneventful flight, the pilot proceeded to land into the wind to runway 18 at AK61. Upon touchdown on the dry dirt and gravel runway, the tail of the airplane came down, and the pilot reported he heard a "loud bang." He reported the empennage "went clear" to the ground and he had "zero" rudder authority. The pilot maintained a straight path for about 600 ft, before the airplane veered right and departed the runway. The airplane impacted a ditch parallel to the runway. The pilot shutdown the airplane and was able to egress without further incident. A postaccident inspection by the pilot revealed that the attachment bolt for the tail wheel spring assembly was fractured.

Photograph 1 - View of the airplane.
 (courtesy of the pilot).

Pilot Information

Certificate: Commercial
Age: 52, Male
Airplane Rating(s): Multi-engine Land; Single-engine Land; Single-engine Sea
Seat Occupied: Single
Other Aircraft Rating(s):None 
Restraint Used:
Instrument Rating(s): Airplane
Second Pilot Present: No
Instructor Rating(s):None 
Toxicology Performed: No
Medical Certification: Class 2 Without Waivers/Limitations
Last FAA Medical Exam: 05/08/2017
Occupational Pilot: Yes
Last Flight Review or Equivalent: 05/13/2016
Flight Time: (Estimated) 18000 hours (Total, all aircraft)

Aircraft and Owner/Operator Information

Aircraft Make:THRUSH 
Model/Series:S2R 800 
Aircraft Category:Airplane 
Year of Manufacture:1977 
Amateur Built:No 
Airworthiness Certificate: Restricted
Serial Number:5089R 
Landing Gear Type:Tailwheel 
Seats: 1
Date/Type of Last Inspection: 08/08/2017, Annual
Certified Max Gross Wt.: 10300 lbs
Time Since Last Inspection:
Engines: 1 Turbo Prop
Airframe Total Time: 10527.9 Hours as of last inspection
Engine Manufacturer: Honeywell Aerospace
ELT: C126 installed, activated, did not aid in locating accident
Engine Model/Series: TPE331-6-252M
Registered Owner: Glenn Air, Inc.
Rated Power:800 hp 
Operator: Glenn Air, Inc.
Operating Certificate(s) Held: Agricultural Aircraft (137)
Operator Does Business As: Glenn Air, Inc.
Operator Designator Code: 

The tail wheel-equipped airplane was configured for transporting bulk fuel at the time of the accident. The pilot reported that the airplane was below the maximum gross weight at the time of the accident with no abnormalities noted with the airplane's center of gravity values.

An examination of the airplane's maintenance records revealed that the airplane had an annual inspection of the airframe on August 8, 2017. During the annual inspection, the tail wheel was serviced, and it was confirmed that the tail wheel lock was rigged properly. No evidence of uncorrected mechanical discrepancies was found in the airplane's maintenance records. 

Meteorological Information and Flight Plan

Conditions at Accident Site: Visual Conditions
Condition of Light: Day
Observation Facility, Elevation: PATK, 350 ft msl
Distance from Accident Site: 40 Nautical Miles
Observation Time: 0153 UTC
Direction from Accident Site: 236°
Lowest Cloud Condition:
Visibility:  10 Miles
Lowest Ceiling: Broken / 4500 ft agl
Visibility (RVR):
Wind Speed/Gusts:  3 knots /
Turbulence Type Forecast/Actual: None / None
Wind Direction: 120°
Turbulence Severity Forecast/Actual:N/A / N/A 
Altimeter Setting:29.52 inches Hg 
Temperature/Dew Point: 11°C / 8°C
Precipitation and Obscuration: Light - Rain
Departure Point: WILLOW, AK (UUO)
Type of Flight Plan Filed: Company VFR
Destination: Talkeetna, AK (AK61)
Type of Clearance: None
Departure Time: 1730 AKD
Type of Airspace: Class G

The pilot reported no weather conditions that affected the safe operation of the airplane. 

Airport Information

Runway Surface Type: Dirt; Gravel
Airport Elevation: 1900 ft
Runway Surface Condition:Dry 
Runway Used:18 
IFR Approach:None 
Runway Length/Width: 3000 ft / 50 ft
VFR Approach/Landing: Full Stop 

Wreckage and Impact Information

Crew Injuries: 1 None
Aircraft Damage: Substantial
Passenger Injuries: N/A
Aircraft Fire: None
Ground Injuries:N/A 
Aircraft Explosion: None
Total Injuries: 1 None
Latitude, Longitude: 62.696667, -148.918056 (est) 

A review of postaccident photographs provided by the pilot showed that the airplane sustained substantial damage to the left wing, the fuselage, and the empennage.

Photograph 2 – View of the airplane in the ditch. 
(courtesy of the pilot). 

Flight Recorders

The airplane did not carry, nor was required to carry, a crashworthy flight data recorder.

Tests And Research

Metallurgical Examination

The tail wheel spring assembly and a portion of the attachment bolt with a nut were recovered and transported to the National Transportation Safety Board (NTSB) Materials Laboratory in Washington, District of Columbia. An examination of the components revealed that the tail wheel spring assembly had a portion of a fractured bolt shank protruding from the attachment bolt hole on one side. The spring and protruding portion of the bolt exhibited corrosion and pitting. The fracture surface of the recovered portion of the bolt with the nut still attached, matched the portion of the bolt protruding from the spring. The deformation of the bolt and appearance of the fracture surface were consistent with bending overstress. Refer to the NTSB Materials Laboratory Factual Report in the public docket.

Photograph 3 – View of the tail wheel spring assembly and fractured bolt.
 (courtesy of the NTSB).

Photograph 4 – View of the tail wheel spring assembly airframe attachment point.
 (courtesy of the NTSB).

Types of Overstress

Aircraft Accident Investigation (2006) by Richard H. Wood and Robert W. Sweginnis discusses the types of structural failures in aircraft accident investigations. This book states that overstress occurs when the part sustains more stress than it was designed to withstand. Overstress can occur in three ways and this book states in part:

1. Pilot induced overstress. Such as aerobatics, over reaction to turbulence or controlled flight departure, improper recovery techniques or excessive landing loads, or any other operations outside of the aircraft structural envelope.

2. Weather induced overstress. Such as excessive gust loading (turbulence) or wind shear.

3. Wake turbulence induced overstress. Such as downwash or wing tip vortices.

Organizational And Management Information

Glenn Air, Inc., in addition to transporting bulk fuel, provides aerial application services of fertilizer, herbicide, live fish stocking, pesticide, seed, and timber seeding as a 14 CFR Part 137 agricultural aircraft operations certificate holder throughout Alaska.

Additional Information

Thrush S-2R Landing Procedures

The Federal Aviation Administration-approved Thrush Model S-2R Airplane Flight Manual contains the normal procedures and subsequent instructions for the pilot to execute in the airplane. The manual discusses the procedures for landing and states in part:

1. Airspeed on Final – 130% power off stall speed.

2. Wing Flaps – Fully extended (30°).

3. Touchdown – Main wheels.

4. Landing Roll – Lower tail smoothly, close throttle.

5. Brakes – Minimum required.

6. Wing Flaps – Retract after clearing runway.

7. Carburetor Heat – Off.

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