On June 19, 2010, at 1103 mountain daylight time, an experimental Oliver RV-9A, N559B, impacted the terrain about one-quarter mile south of May Airport, May, Idaho. The pilot, who was the sole occupant, received fatal injuries, and the airplane, which was owned and operated by the pilot, sustained substantial damage. The 14 Code of Federal Regulations Part 91 personal flight, which departed Challis, Idaho, about 15 minutes prior to the accident, was being operated in visual meteorological conditions. No flight plan had been filed. The pilot of the RV-9A and the pilot of another airplane were returning from a back country aerial camping trip. Both pilots had landed at McCall, Idaho, on June 18, and then flown into Johnson Creek Airstrip, where they spent the night. On the morning of June 19, they flew from Johnson Creek to Challis, Idaho, where they were on the ground for about 50 minutes. Both pilots then took off from Challis, and climbed over the hills between Challis and May. The other pilot landed first, and then interacted with the pilot of the RV-9A via VHF radio concerning the roughness of the May Airport dirt/turf landing surface. According to the pilot who was already on the ground, as he talked to the RV-9A pilot via the VHF radio, he and another person who was with him watched the pilot maneuver over the airfield as he visually evaluated the roughness of the surface prior to deciding whether to land there or not. Eventually the pilot rolled wings level about 200 feet above ground level (AGL) and proceeded along the south side of the airfield for a short period of time, whereupon the airplane suddenly rolled to the left and made a one-half turn spin into the ground. According to the witnesses, it appeared that the airplane's forward speed may have slowed prior to the sudden left roll, but they reported that there had been no audible change in engine power, nor any sounds that would indicate an engine anomaly. According to the data a National Transportation Safety Board technician downloaded from a Garmin 696 global position system (GPS) unit recovered from the wreckage, the pilot entered a shallow left turn when he was about one and one-quarter mile south of the town of May. As he reached a point about one-quarter mile east of May, he tightened up his left turn and circled the town in a turn with a diameter of about one-third mile. As he again reached the point where he had first tightened up his turn, he decreased his bank angle so that his continuing left turn was shallower, which resulted in a second left turn with a diameter of about three-quarters of a mile. By doing so, his second left turn took him over the western half of the May Airport, then off the west end of the airport, and back to the southern side of the airport, where he rolled wings-level and paralleled the runway on an easterly heading (see Terminal Flight Track – GPS). Once wings-level on the south side of the runway, he was in a position where he could continue to view the runway by looking out the left side of his canopy. About 20 seconds after he rolled out of the turn to parallel the runway, the airplane descended into the terrain. A further examination of the GPS data showed that from the time he initially began to enter the first left turn, until he finally rolled wings level along the south side of the airstrip approximately 240 seconds (4 minutes) had elapsed. The data also showed that after the pilot rolled out along the south side of the airport that he maintained an airspeed between 81 and 83 miles per hour (30 plus mph over level flight stall speed of 48 mph). A post-accident examination of the airplane's flight control system and primary structure performed by a Federal Aviation Administration (FAA) Airworthiness Inspector did not reveal any evidence of a malfunction or anomaly that would have contributed to the rapid roll and descent into the terrain. The investigation further revealed that the forensic toxicological examination performed by the Federal Aviation Administration's Civil Aerospace Medical Institute (CAMI) on specimens taken from the pilot found 0.173 (ug/ml, ug/g) of Diphenhydramine in his blood (heart), and an undesignated level of Diphenhydramine in his urine. Diphenhydramine is an over-the-counter antihistamine that can commonly cause sedation, tiredness, sleepiness, dizziness, and disturbed coordination. This level of Diphenhydramine in the pilot's blood would be expected to result in impairment. It is possible that such impairment resulted in or contributed to confusion or spatial disorientation.