On May 16, 1998, at 1415 mountain daylight time, an AS+ Ltd. AC-4 glider, N158R, was destroyed while landing at Cedar Valley Airport, near Cedar Fort, Utah. The private pilot and sole occupant received serious injuries. The aircraft was being operated by a private individual under Title 14 CFR Part 91. Visual meteorological conditions prevailed for this local area personal flight which originated approximately 5 minutes before the accident. No flight plan was filed. According to available information, the wind was calm. Witnesses reported that while on down wind, the aircraft assumed a nose low attitude and impacted the ground. According to FAA records, the aircraft was first registered approximately a month before the accident and the pilot reported he had accumulated 5 hours of flight experience in the accident aircraft make and model. Examination of the aircraft by an FAA inspector provided no evidence of preimpact failure or malfunction. The pilot provided minimal information on NTSB Form 6120.1/2 (Pilot/Operator Report Form) and did not return calls from either the Safety Board IIC or FAA inspector assigned.