On July 22, 1998, approximately 1420 mountain daylight time, a Robinson R22 Beta, N8350N, was destroyed while maneuvering 1 mile east of Centennial Airport, Englewood, Colorado. The flight instructor and his private pilot-certificated student received minor injuries. The instructional flight was being operated by XL Helicopters Inc. under Title 14 CFR Part 91. Visual meteorological conditions prevailed at the time of the accident, and no flight plan had been filed. The flight originated approximately 1400. According to the instructor-pilot's accident report, he and his student were practicing 180 degree autorotations, preparing for the student's commercial and flight instructor checkrides. The maneuvers were begun between 610 and 660 feet above the ground. They completed several autorotations without incident. The instructor said that during the autorotation that culminated in the accident, both he and his student were operating the controls because he wanted to demonstrate another way of entering the (180 degree) turn. He said the helicopter "dipped slightly as our airspeed increased by maybe 5 knots, to 65-70 knots indicated. The main rotor rpm decreased slightly, so I made an aft cyclic input and lowered collective slightly, but it didn't seem to completely correct the attitude/airspeed, although I believe rpm increased. At this point, the rate of descent suddenly increased very, very rapidly. I decreased our bank angle with cyclic and. . .rolled throttle back on to re-engage the power and raised the collective. . .and aft cyclic. I remember flaring to get rid of as much of our rate of descent as possible. I vaguely remember pulling collective to cushion the landing." Citing a fast-moving thunderstorm that occurred 90 to 120 minutes after the accident, the instructor said he thought wind shear or a strong downdraft caused the accident. Centennial control tower reported the accident at 1420. In his accident report, the pilot indicated the accident occurred approximately 1515. The METAR observation made at 1553 was as follows: Wind 060 degrees at 9 kts; visibility 10 sm; ceiling 4,500 ft. broken, 8,000 ft. broken; 25,000 ft. broken; temperature 24 degrees C. (75.2 degrees F.); dew point 16 degrees C. (60.8 degrees F.); altimeter 30.22 in. Hg.; Remarks: rain began 1505, rain ended 1528; sea level pressure 1016.6 mb; mountains obscured southwest through northwest.