On October 12, 1997, at 1412 eastern daylight time, a Schweizer SGS-233 glider, N5762S, operated by a commercial rated glider pilot, collided with the terrain when the glider landed short of Runway 18 at the Ionia County Airport, Ionia, Michigan. The personal 14 CFR Part 91 local flight was operating in visual meteorological conditions without a flight plan. The pilot and passenger both received minor injuries. The glider was substantially damaged. The flight originated from Ionia, Michigan, at 1334 edt. The pilot reported that he entered the traffic pattern at 850' above ground level and made a normal downwind until he perceived that the glider had developed an unusual sink rate. He reported, "Cut downwind short an estimated 200 yards. Executed right turn to base chose not to fly base, but take the shortest, diagonal track to nearest point of runway." The pilot continued to report that the sink rate apparently increased and he was unable to clear trees in his path at which time he chose to land the glider in a clearing to the side of the trees. The glider contacted several small trees prior to contacting a large tree with the left wing. The glider came to rest approximately 1,000' short of the approach end of the runway. Two witnesses who saw the glider in the traffic pattern reported that it appeared that the pilot had extended his downwind leg too far. Inspectors from the Grand Rapids Michigan FAA Flight Standards District Office inspected the airplane. They reported that the flight controls were intact and properly rigged with the exception of the spoilers which were jammed due to impact damage.