The private pilot, who had no mountain flying instruction, planned to fly a friend's newly-purchased experimental amateur-built airplane from Idaho to Georgia. After he took delivery of the airplane at one airport in Idaho, the pilot flew to another Idaho airport to meet his pilot-rated cousin. The next morning, the two departed in the airplane for an airstrip situated about 38 miles to the northeast, at an elevation about 5,800 ft above mean sea level (msl). The aeronautical chart of the region depicted mountainous terrain between the two airports, with peaks ranging from about 6,700 to 8,700 ft msl. While en route, the pilot entered a canyon and realized that the airplane was unable to outclimb the rising terrain. The pilot began a course reversal turn to escape the canyon, but during the turn, the airplane experienced an aerodynamic stall and impacted the ground. The airplane was destroyed, the pilot received serious injuries, and the passenger was fatally injured. The impact site was at an elevation of about 7,500 ft. The pilot's preparations for the flight were minimal, and he did not explicitly plan out the flight route or altitudes to ensure sufficient terrain clearance margins. The pilot did not reside in mountainous terrain, and had not taken any mountain-flying training courses. The pilot did not have or use any paper charts before or during the flight and did not program his intended flight route into his GPS device, which was equipped with a terrain database and terrain display and warning capability. Prior to the accident, the pilot had only accrued about 1 hour in the accident airplane make and model.After the accident, the pilot reported that he had over-estimated the airplane's climb performance. Aside from climb speeds, there was no climb performance information in the airplane Owner's Manual (OM). Because the pilot did not specify whether he used the airplane's best angle of climb airspeed for the climb, his conduct of the climb could not be evaluated, and the effect of the actual climb performance on the outcome could not be determined.Mountain flying training guidance advocates that when flying in canyons, pilots should select a flight path near the side, as opposed to the center, of the canyon in order to provide the maximum amount of terrain clearance in the event a course reversal turn becomes necessary. Because the pilot did not specify any details regarding the lateral position of the airplane before or during the course reversal turn, the Page 2 of 11 WPR17LA195pilot's execution of the turn could not be evaluated, and its effect on the outcome could not be determined.An airplane's minimum-radius turn requires use of the lowest airspeed and highest bank angle that still provide sufficient stall margin. Because the pilot did not specify the speed or bank angle that he used for his course reversal turn, the pilot's execution of the turn could not be evaluated, and its effect on the outcome could not be determined.The OM did not provide actual stall speed information, and the airplane was not equipped with a stall warning system. Because the weight, configuration, bank angle, and actual performance of the airplane were not known, the actual stall speed could not be determined. Although the pilot had deployed partial flaps at an undetermined time just prior to or during the turn in order to reduce his stall speed, that configuration change alone was insufficient to prevent the stall. Because the airplane dynamics and pilot responses were not known, the specific reason(s) for the stall were not able to be determined.The pilot's inadequate flight planning, preparation, and navigation, combined with his lack of familiarity with the airplane's performance capability, resulted in the airplane entering a canyon that the pilot was unable to exit by outclimbing the terrain. The pilot's decision to reverse course was either too late, and/or his execution of the course reversal turn was insufficient to enable successful escape from the canyon. During the course reversal turn, the airplane experienced an aerodynamic stall and impacted terrain.