On July 15, 2003, approximately 0740 mountain daylight time, a Piper PA-18A, N6792B, sustained substantial damage after impacting terrain while maneuvering 3 nautical miles north of Zurich, Montana. The airplane was registered to a private individual and operated by Prairie Aviation of Glasgow, Montana. The airline transport pilot and his sole passenger received serious injuries. Visual meteorological conditions prevailed for the 14 CFR Part 91 predator control flight, and a flight plan was not filed. The flight departed the Edgar G. Obie Airport, Chinook, Montana, at 0615. The pilot reported he was conducting low level operations in a "slow flight mode with flaps at 1/2." During a left turn from the west to the east, at approximately 30 to 40 feet above ground level (agl) he felt the initial indications of the airplane entering a stall. The pilot stated that after rolling wings level the aircraft entered a secondary stall, subsequently impacting the ground in an approximately 5 degree nose down, wings level attitude, coming to rest in an upright position. The pilot and passenger exited the airplane successfully, and no post-impact fire ensued. According the an FAA inspector, who traveled to the accident site, both landing gear had collapsed, both propeller blades were bent aft, and the left wing spar and aileron sustained substantial damage.