On August 24, 1996, about 1450 eastern daylight time, a PZL-Bielsko SZD-59, N591SZ, a glider, was substantially damaged when it collided with trees on approach to runway 36, at the Franconia Airport, Franconia, New Hampshire. The pilot received minor injuries. Visual meteorological conditions prevailed for the local flight. No flight plan was filed for the personal flight conducted under 14 CFR Part 91. During a telephone interview, the pilot stated the following: He was conducting a local flight, south of the airport, and observed a squall line approaching from the west. He decided to return to the airport and conduct an approach and landing to runway 36. As he approached the pattern entry area, west of the runway, the forward visibility was zero with heavy rain showers. The altimeter indicated 1,800 feet above ground level. The pilot planned to perform a 360 degree turn in the pattern entry area to lose altitude. During the turn, the pilot observed that the actual altitude appeared lower than the indicated altitude, and flew over the runway to the east for a right downwind entry. The pilot observed a high sink rate and attempted to perform an abbreviated pattern, during which the visibility was 1/2 a mile with moderate rain showers; however, the glider impacted trees on the east side of the runway. In the NTSB form 6120.1/2, the pilot stated: ". . .As I crossed the airfield heading east, I was approximately at mid-field and estimated my altitude to be 500' AGL [above ground level]. I felt things looked good for an abbreviated pattern with a touchdown point @ [at] about mid-field. With a glide ratio of 40/1, from an altitude of 500', I could travel 20,000 ft along the ground. Cutting this 1/2 to allow for sink and provide a margin would provide a ground distance of slightly less than 2 miles. As I got to the east edge of the airfield I felt I was now too low for an abbreviated pattern and initiated a 180 degree turn back towards the field. During the turn it was obvious my sink rate was severe. As I rolled out to wings level I pushed some forward stick to insure adequate energy as I approached the rapidly rising tree line. Soon after wings level I had a tree strike on my right wing, and was able to maintain level flight. I believe there was a second strike on my right wing before a third impact threw me violently against the left side of the canopy and cockpit and I lost control of the aircraft." The pilot reported he had a total flight experience of 191 hours, all in gliders, of which, 18 hours were in this make and model. Examination of the wreckage by a Federal Aviation Administration Inspector revealed no evidence of mechanical malfunctions or failures with the glider. The weather at a nearby airport was reported as follows: Variable wind at 4 knots; Visibility 10 miles; Few clouds at 5,000 feet; Broken clouds at 6,500 feet; temperature 73 degrees F; Dewpoint 55 degrees F; Altimeter 30.04 inches Hg.