On May 22, 2018, about 1400 mountain daylight time, an experimental amateur-built Sportavia-Putzer RF5B Sperber motor-glider, N66GL, collided with a roadside barrier following an encounter with downdrafts near Lookout Pass, Montana. The airline transport pilot received minor injuries. The glider sustained substantial damage to the left wing, firewall, and empennage. The glider was registered to and operated by the pilot under the provisions of Title 14 Code of Federal Regulations Part 91 as a personal cross-country flight. Visual meteorological conditions prevailed about the time of the accident and no flight plan was filed. The flight originated from Missoula, Montana at 1300 and was destined for Kellogg, Idaho.

The pilot had recently purchased the glider in New York and was returning with it to Seattle, Washington. In preparation for the long cross-county flights, he modified the glider to include an extra fuel tank in the back seat adding eight gallons or 54 pounds. This additional fuel combined for a total fuel weight of 108 lbs. The pilot stated by the time of the accident, the extra tank was empty, and even when it was full, the glider was well within center of gravity (CG) limits.

The pilot had been following the interstate 90 (I-90) freeway so that he had an emergency landing area if needed. On the accident leg of the flight he was flying towards Lookout Pass, the lowest point of rising mountainous terrain, at 4,692 ft msl. While climbing through 6,100 ft to his desired altitude of 8,500 ft mean sea level (msl), the engine oil temperature began to rapidly rise. He corrected this by leveling the glider off in order to cool the engine oil. At this point, he decided to continue towards the pass and to the destination airport, as it was closer than the departure airport. When the engine oil cooled to an acceptable level he continued the climb. After reaching an altitude of about 6,500 ft, the glider encountered a downdraft. The pilot applied maximum engine power and the engine oil started to heat beyond "red line", the oil pressure started to drop, and the glider continued to descend. The pilot determined he could not safely turn around and decided to make a precautionary landing on the west-bound lanes of the I-90 freeway. While descending, he observed a vehicle obstructing the landing area. He made a turn to the left to land on the eastbound traffic lanes. While in the turn, the left wing struck a freeway barrier and the glider yawed to the left. The engine then struck the barrier, and the glider came to rest upright against the barrier. A small fire ensued and was quickly put out by witnesses.

A video of the accident was discovered online and shows the motor-glider descending towards the west-bound lanes of the freeway then turning left moments before impacting a barrier. The elevation of the accident site was 4,685 ft. The pilot reported no mechanical failures or malfunctions of the airframe or engine and stated, "the engine just got hot and didn't have the power to overcome the down draft."

At 1400 mountain daylight time, the Mullan Pass automated surface observation system, located about 2 miles from the accident site, at an elevation of 6,089 ft, reported the wind from 130° at 7 knots. No turbulence was forecast.