On February 28, 2013, about 1510 Alaska standard time, a ski-equipped, experimental/homebuilt Isaak Super Cub airplane, N130Z, sustained substantial damage when it collided with terrain following an in-flight loss of control, about 2 miles northwest of Sterling, Alaska. The airplane was being operated by the pilot as a visual flight rules (VFR) personal flight, under 14 Code of Federal Regulations, Part 91, when the accident occurred. The certificated private pilot and passenger sustained serious injuries. Visual meteorological conditions prevailed, and no flight plan was filed. During a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge on February 29, a witness who was familiar with the accident airplane stated that, typically when the pilot retuned from a flight, he would make a low pass to let friends or family members know he was back. During the accident flight, the witness stated that the engine seemed to be running smoothly, although the engine was operating at low power setting. He said that the airplane was flying very slowly, approximately 300 feet above ground level, and then it “appeared to drop out of the sky.” The airplane entered a nose down, near vertical spiraling descent to the right, collided with a tree, and came to rest in a snow bank adjacent to a house. The witness added that during the vertical descent, the engine rpm increased significantly, which continued until impact. On February 29, 2013, a Federal Aviation Administration (FAA) airworthiness inspector from the Anchorage Flight Standards District Office, traveled to the accident site, and examined the airplane. The inspector reported that his examination revealed no preaccident mechanical anomalies that would have precluded normal operation. Due to injuries sustained in the accident, the passenger was unable to be interviewed until after his recovery. During a telephone conversation with NTSB, IIC on April 19 the passenger seated in the rear seat stated that the purpose of the flight was to go look at a moose kill, and spot for coyotes in the area. The flight departed a remote frozen snow-covered lake near his house. Upon their return they spotted some friends, and made a low pass at approximately 200 feet above the ground. They had come around for a second pass, flaps were set at one notch, and the airplane was running smoothly, when the right wing dipped, and the airplane rolled over impacting the ground. The airplane sustained substantial damage to the wings and fuselage. Due to injuries sustained in the accident, the pilot was unable to be interviewed until after his recovery. During a telephone conversation with the NTSB IIC on April 19, 2013, the pilot reported that he was unable to remember anything about the accident. The pilot did not submit an NTSB Pilot/Operator Aircraft Accident Report (NTSB Form 6120.1).