ANC17LA046

The pilot was transporting bulk fuel to a remote lodge. During the landing to the dirt and gravel runway, the tail of the airplane came down, and the pilot heard a loud bang. He reported the empennage "went clear" to the ground and that he had no rudder authority. The pilot attempted to control the airplane, but the airplane departed the runway and impacted a ditch, which resulted in substantial damage to the left wing, fuselage, and empennage. The tailwheel spring assembly and a fractured attachment bolt were recovered from the airplane. An examination revealed that the deformation of the bolt and appearance of the fracture surface were consistent with bending overstress. The spring and a portion of the bolt exhibited corrosion and pitting; signs of fatigue were not noted. The airplane flight manual states to lower the tail smoothly during the landing roll. Because the deformation of the bolt and the fracture surface were the result of overstress, it is likely that the pilot allowed an excessive landing load on the tailwheel when it touched down, which resulted in a loss of control and a subsequent runway excursion and impact with terrain.

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. 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. The airplane did not carry, nor was required to carry, a crashworthy flight data recorder. 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. Thepilot 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). The pilot reported no weather conditions that affected the safe operation of the airplane. 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. 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. 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).