On September 1, 1997, at 1100 central daylight time, a Piper PA-28-140, N6566W, sustained substantial damage on impact with the terrain while maneuvering at low altitude, near Monona, Iowa. The pilot said he was circling his property checking crops when the airplane struck the ground. The pilot and one passenger reported no injuries. The personal 14 CFR Part 91 flight was operating in visual meteorological conditions. No flight plan was on file. The local flight departed about 1030. The pilot stated that he was conducting low level flight to inspect his crops. He said he attempted to gain altitude to avoid contact with trees; however, the airplane encountered a downdraft and he was unable to arrest the descent prior to impact with the terrain. Subsequent to the accident an examination of the airplane and engine failed to reveal any preexisting anomalies.