On May 8, 2005, at 1700 mountain standard time, a Schempp-Hirth Discus 2b glider, N15MM, was damaged during an encounter with rough terrain during a forced landing 10 miles north of Pleasant Valley Airport, Peoria, Arizona. The owner operated the glider under the provisions of 14 CFR Part 91. The glider rated private pilot was not injured, and the glider was substantially damaged. Visual meteorological conditions prevailed, and no flight plan had been filed. The flight originated at the Pleasant Valley Airport. The pilot stated to the National Transportation Safety Board investigator that he was on a circular cross-country flight designed to depart and then return to the Pleasant Valley Airport. Along the route there were numerous navigation "turn points." The pilot said that about 250 miles into the flight, and at the second to last turn point, Black Canyon City, he determined that he did not have enough altitude to make it to the last turn point before heading back to Pleasant Valley. His flight computer indicated that he had enough altitude to make it to Pleasant Valley Airport with a 1,000-foot altitude margin. While on the return leg he encountered some "downward subsiding air" of about 7-8 knots (700-800 feet per minute downdraft). At this point he determined that he would not make it to Pleasant Valley, and decided to divert to an auxiliary dirt landing strip (N33:57.13, W112:18.35) to make a precautionary landing. The pilot did not make it all the way to the auxiliary strip and landed about 1.5 miles south, in rocky desert terrain. The pilot stated that the glider had no mechanical failures or malfunctions during the flight.