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 soft surface.