On August 29, 1998, at 1315 hours Pacific daylight time, a Boeing Stearman A75N1, N56856, struck wires and impacted terrain after encountering downdrafts near Big Bear City, California. The aircraft, operated under the provisions of 14 CFR Part 91 by Twin-D Enterprises Inc., Hemet, California, was destroyed in the impact sequence and postimpact fire. The commercial pilot was not injured, and the passenger received minor injuries. Visual meteorological conditions existed for the flight and no flight plan was filed. The flight originated at the Big Bear airport at 1308 as a for-hire sightseeing operation. The pilot stated that this was a flight of two aircraft. He reported that the winds were from 080 degrees at 8 knots and the crosswind leg was into rising terrain. No discrepancies were noted with the takeoff or upwind portion of the flight. The pilot reported that after turning on to the crosswind leg, the aircraft encountered a downdraft. He noted that the airspeed decayed from the initial climbout airspeed of 70 mph to 55 mph. He reported that he lowered the nose of the aircraft slightly to target 60 mph for recovery and declared an emergency with the temporary tower. After declaring the emergency he turned toward lower terrain. The pilot stated that the aircraft continued to lose altitude, with full power being applied. The estimated lost altitude was approximately 500 feet agl and the aircraft was approximately 100 feet agl when the sink rate stopped. The aircraft impacted telephone wires, then the ground, and caught on fire. The pilot stated that rain showers were present approximately 10 miles south of the airport and may have caused windshear or downdrafts. A San Bernardino Sheriff's Department officer interviewed the pilot, who stated that the aircraft was on a southerly heading when "he caught a downdraft from the storm." Witnesses to the accident stated that the aircraft was on a southerly heading and the engine was sputtering. The aircraft hit a telephone pole and came down inverted. After colliding with the ground the aircraft started to burn. Another witness observed that rain showers existed approximately 5 miles southeast of the airport. When the aircraft turned from the crosswind leg to the downwind leg, the aircraft was not maintaining altitude. The individual stated that "there were no other indications of wind shear other than the rain shower." He further reported that visibility in all directions was in excess of 10 miles, except where the rain shower was. The witness stated that prior to impact the aircraft struck power and guide lines on the south side of the road, and before coming to rest, the aircraft traveled approximately 30 feet. While assisting with the victims, the witness observed that the elevator, aileron, and rudder cable connections had all melted in the cockpit. The rudder and trim tab cables were connected at the rudder. Weather reported at the accident site was 5,000 scattered, 7,000 broken, and 9,500 overcast. Visibility was 10 miles; winds were reported from 100 degrees at 5 knots; the temperature was 77 degrees Fahrenheit; and the dew point was 51 degrees Fahrenheit. A Safety Board meteorologist obtained weather data, to include radar and satellite images. A convective build-up was present near the airport, with a weak line of instability. The pilot of the other aircraft stated that both aircraft lost approximately 700 feet in the downdraft. He stated that the accident aircraft turned left and he turned right. He stated that the accident aircraft was not in a stalled condition, it was producing power until it hit power lines. A Federal Aviation Administration (FAA) inspector stated that there was convective activity in the area and both aircraft were caught in a downdraft. The accident flight was the sixth flight of the day with no problems noted with the previous flights. He stated that the weather was deteriorating on the accident flight.