On June 15, 2018, about 1538 Alaska daylight time, a tailwheel-equipped Cessna 180K airplane, N2957K, sustained substantial damage when it collided with hilly, sand covered-terrain after takeoff from a remote off airport site, about 26 miles southeast of Ambler, Alaska. The airplane was registered to Airframe Innovations Incorporated and operated by Northstar Aero LLC as a 14 Code of Federal Regulations (CFR) Part 135 visual flight rules flight when the accident occurred. Of the three occupants, the airline transport pilot sustained no injuries, one passenger sustained minor injuries, and the remaining passenger sustained serious injuries. Visual meteorological conditions prevailed, and company flight following procedures were in place. The flight departed the Fairbanks International Airport (PAFA), Fairbanks, Alaska, about 1024, and it was returning at the time of the accident. In the pilot's written statement provided to the National Transportation Safety Board, he reported, in part, that after landing on a hilly sand dune, the wind at the top of the sand dune increased to 10 to 20 knots over a two-hour time period. After considering and reviewing several downhill departure routes, the pilot said he marked off the most favorable departure route that was into the wind. The pilot reported that, after loading his two passengers into the airplane, and in preparation for takeoff, he ensured both passengers were wearing their BAS shoulder harness system, then he began the downhill takeoff run. He described the takeoff run as a typical sand dunes departure, with some rocking and bouncing, but the airplane accelerated rapidly downhill, and into the wind. He said that, as the airplane became airborne near the bottom of the dune, he made a slight right turn towards lower terrain, and the airspeed initially increased. He said that, as the airspeed increased to near Vx (Best angle of climb airspeed), the headwind diminished, and the airplane began losing altitude and the stall warning horn sounded. To avoid a collision with a sharp wall of hard packed sand ahead, the pilot said that he continued the slight right turn towards the down sloping terrain, but the airplane continued to descend towards a 60° sloped sand dune. The airplane subsequently collided with the sand dune, sustaining substantial damage to the forward fuselage and serious injuries to the passenger seated in the right front seat. The pilot stated to the National Transportation Safety Board investigator-in-charge that he credited the lack of more serious injuries to a recently installed BAS, Inc., four-point shoulder harness and lap belt restraint system. The pilot reported no mechanical malfunctions or anomalies that would have precluded normal operation.