On July 31, 2002, approximately 1445 mountain daylight time, a Blanik L-13, N70741, impacted a ditch after landing short of the runway at Gallatin Field Airport, Belgrade, Montana. The aircraft was owned by Big Sky Soaring Inc., and operated by Sunbird Aviation. The private pilot and his passenger received minor injuries, and the glider sustained substantial damage. The 14 CFR Part 91 personal pleasure flight departed Gallatin Field Airport, Belgrade, Montana, approximately one hour prior to the accident, and was being operated in visual meteorological conditions. No flight plan had been filed. According to the pilot, he released from the tow plane just south of Saddle Peak, and soared Bridger Ridge until about 1430. At that time he called Bozeman Tower to report leaving the ridge and to request landing on the grass/sod runway located just to the right of the paved runway (see attached satellite image). The tower reported the current winds as 300 degrees at 20 knots, and instructed the pilot to report 30 seconds prior to entering the downwind for sod runway 30. When he was 30 seconds from a downwind entry, the pilot advised the tower of this position, and after being cleared to land on the sod runway, entered a left downwind. Just about the time the pilot was ready to initiate his base turn, the tower controller asked him if he saw the traffic on a one mile final to the paved runway. According to the pilot, he then let go of the spoiler handle (the spoilers were partially deployed), in order to reach the microphone that was stored in a pocket on the right side of the glider. He advised the tower that he did not have the traffic in sight, put the microphone back in the pocket, and then reached back for the spoiler handle. At that time he discovered that the spoilers had deployed to the full open position. The pilot then immediately retracted the spoilers and made a turn to final. After turning final, the pilot realized that he probably would not be able to make it to the threshold of the sod runway, but because of the aircraft on final for the paved runway, he made the decision not to request a change to that runway (the threshold of which was approximately 2,500 feet closer to his position at the time). As the pilot continued on toward the threshold of the sod runway, he attempted to maneuver toward its right side in order to avoid some stakes that were between him and the runway threshold, and in order to land well clear of the paved runway. Ultimately the aircraft touched down approximately 200 feet short of the sod runway threshold and 50 feet to the right of its right edge. Although the initial touchdown in the tall weeds was uneventful, almost immediately after starting the landing roll, the aircraft encountered a ditch that the pilot had not been aware of. Upon impacting the far wall of the ditch, the aircraft sustained substantial damage generated by the force of the collision.