On August 18, 2018, about 1150 mountain daylight time, a Cessna 182Q airplane, N84CP, was substantially damaged when it was involved in an accident near Salmon, Idaho. The airline transport pilot and pilot-rated passenger were seriously injured; the pilot succumbed to his injuries 3 months after the accident. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot-rated passenger reported that the accident pilot fueled the airplane then conducted a formation flight of two airplanes from McCall Municipal Airport (MYL), McCall, Idaho, to Flying B Ranch Airstrip (12ID), Salmon, Idaho., with the accident airplane in the lead. The trail pilot reported that, during the briefing for the return flight to MYL, he and the accident pilot decided to take off to the north parallel to the west side of a river, and shortly after, to turn 180° back toward MYL. The pilot-rated passenger stated that the flight departed about 1115 with the accident pilot still in the lead airplane, that the takeoff and initial climb seemed normal, and that the airplane had power and climbed as expected. The trail pilot reported that he watched the takeoff and confirmed that everything seemed normal. Shortly after takeoff, the trail pilot asked the accident pilot, “where are you going?” and he replied, “We are going down.” The pilot-rated passenger stated that he was unaware of any issues with the airplane before this transmission. (He added that, after the accident, the pilot told him that he thought that the loss of lift was due to the airplane encountering “a downdraft.”) After the transmission, the passenger looked at the cockpit panel and saw the manifold pressure gauge, which read 20 inches of manifold pressure, and that the airplane was descending. The accident pilot then told him that he was going to “put the [air]plane in the trees” then turned left. Subsequently, the airplane impacted a stand of trees “slow and level” as the stall warning horn was “chirping”. After the left wing impacted a tree, the airplane rotated 180° counterclockwise. The tail then impacted another tree as the airplane continued to rotate then break in half. The airplane continued rotating, and its right side impacted a third three and then came to rest. Subsequently, the pilot-rated passenger noticed fire emanating from the engine cowling, and he and the pilot egressed the airplane. Shortly after, the airplane exploded and was subsequently consumed by fire. The trail pilot added that, when he was about 1/2 mile north of 12ID, he saw the airplane turn about 90° to the left on a westerly heading and that he thought that the accident pilot was then going to make the 180° turn “to head upstream as previously briefed.” However, he then saw the accident airplane head up a creek, which he said confused him because “you cannot out climb the rising terrain to the west, and it is a tight little canyon to turn in.” The trail pilot added that, in the backcountry they primarily speak about flying upstream and downstream, so he was confused about what the accident pilot meant when he said he was “going down.” He said that he thought, “does he think he’s going downstream through an impassable canyon?” He added that he asked the accident pilot, “downstream?” and that the accident pilot responded, “’crashing’ or something similar.” He received no further communication from the accident pilot. Examination of the area that bordered the departure route revealed that it was very confined and had a turn radius of about 800 ft. Terrain rising higher than 6,500 ft mean sea level (msl) existed on both the east and west sides of the north/south-oriented river. The airplane came to rest lodged in a tree between about 10 and 12 ft above ground level in an approximate 15°-nose-down attitude. The airframe and engine were destroyed by the postaccident fire. Postaccident examination of the available airframe and engine components did not reveal evidence of any preaccident mechanical malfunctions or failures that would have precluded normal operation. At 1155, the Automated Surface Observing System at Challis Airport (LLJ), Challis, Idaho, located about 35 nautical miles southeast of the accident site, elevation 5,072 ft msl, reported wind calm, temperature 22°C, dewpoint 8°C, and an altimeter setting of 30.28 inches of mercury. Based on the atmospheric conditions that existed at the time of the accident, the density altitude was calculated to be 6,197 ft.