On June 21, 2018, about 1200 mountain daylight time, a Piper PA-18-150 airplane, N1979P, was substantially damaged when it was involved in an accident near Yellow Pine, Idaho. The pilot was seriously injured. The airplane was being operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The accident airplane was a part of a flight of three, which had departed Cabin Creek US Forest Service Airport (I08), Big Creek Ranger Station, Idaho, for Dewey Moore Airstrip, Cascade, Idaho, which is a remote, mountainous, backcountry airstrip. One of the pilots in the flight of three reported that, as they approached the Dewey Moore Airstrip, the lead airplane pilot surveyed the area, and he reported that there was no other traffic. Subsequently, he and the lead pilot landed uneventfully, and he noted that “the conditions were excellent, with calm air, and the runway in good condition”; he had landed there the previous day. He then parked his airplane at the top of the airstrip facing directly down the runway, which allowed him to easily see the approach to the airstrip. As he was waiting for the accident airplane to come into view, he asked him, “…you’ve been in here before, right”? The accident pilot replied that he had not. He then reminded the accident pilot to fly “low over the creek” and added that, when the accident airplane came into view, it was higher than the lead airplane had been on its approach. When the accident airplane had almost reached the extended runway centerline, the accident pilot added power, after which, the airplane pitched up steeply. It then continued flying upstream of a nearby creek for about 1/2 mile with the nose up and “not climbing well, if at all.” The airplane then entered a left turn away from his position and directly over the creek. Subsequently, the left wing dipped, and the airplane entered a left spin. The pilot stated that he then saw the airplane make 1 1/4 left rotations before it descended out of his sight and impacted terrain. He added that the engine sounded like it was developing full power from the time of the initial go-around until impact. The wreckage came to rest on sloping terrain. The airframe and wings were substantially damaged. The pilot’s brother and one of the pilots who witnessed the accident reported that there were no preaccident mechanical malfunctions or failures with the airplane or engine that would have precluded normal operation. The pilot does not recall the events leading to the accident. Dewey Moore Airstrip is located at 4,494 ft mean sea level. The single gravel runway, 28/10, is a one-way in, one-way out airstrip: airplanes landed on runway 28 and departed from runway 10. The remarks section of the plan view of the airstrip recommended that it be used “only by mountain proficient pilots who have had a checkout specific to the airstrip.” In an Aircraft Owners and Pilots Association video made after the pilot had recovered from his injuries, the pilot stated that he was negligent in planning for the flight, which included not obtaining a briefing on the airstrip. The pilot also mentioned his lack of proficiency at the time of the accident.