On October 4, 1996, at 1240 mountain daylight time, a PDPS PZL-Bielsko SZD-59 glider, N597R, impacted the ground after entering uncontrolled flight while in the traffic pattern at Sterling Municipal Airport, Sterling, Colorado. The commercial pilot received fatal injuries and the aircraft sustained substantial damage. Visual meteorological conditions prevailed for this local area personal flight operating under Title 14 CFR Part 91 and no flight plan was filed. The flight departed Sterling, approximately 20 minutes prior to the accident. According to witnesses, the pilot had been practicing in the aerobatics area and was in the process of returning to the airport for landing. An eyewitness said that the tow plane landed on runway 33 and began back taxiing. The glider then came down the runway about 100 feet above ground level, wings level, and entered a left turn at bout 45 degrees angle of bank. The witness said the nose began to drop and the bank angle increased to about 60 degrees. The glider then dove into the ground beside the runway at approximately 60 degrees nose down attitude. The witness said the glider came to rest in an upright position facing to the south. The terrain around the runway was flat grass covered farm fields. An examination of the glider by an FAA airworthiness inspector provided no evidence of preimpact failure or malfunction. According to the flight manual for the aircraft, the minimum speed marking on the airspeed indicator is 51 knots (59 mph). This speed is 1.1 X Vs (velocity of stall), wings level, at the maximum gross weight approved for aerobatics. Accelerometer limits are +7 and -5. Stall/spin recovery requires a maximum of 623 feet loss of altitude for recovery.