On April 6, 1999, approximately 1730 Pacific daylight time, a main rotor blade of an Enstrom F-28C helicopter, N51770, impacted the aircraft's tail boom during an emergency landing near Pendleton, Oregon. The aircraft, which was owned and operated by General Aircraft Services, sustained substantial damage, but the commercial pilot reported that he was not injured. No flight plan had been filed for the VFR 14 CFR Part 137 aerial application flight, and there was no report of an ELT activation. According to the pilot, he had just taken off from the refueling site and was beginning to initiate a climb when the engine lost power. He attempted to autorotate to a landing, but since the aircraft had not attained sufficient airspeed or altitude to perform a successful autorotation, it hit hard and the main rotor blades flexed downward and impacted the tail boom. It was later determined that the fuel in the portable refueling barrel was contaminated with water. While the pilot kept the aircraft running during the refueling operation (hot refueling), water-contaminated fuel was transferred from the barrel to the helicopter's fuel system and remained undetected. It could not be determined how water was introduced into the barrel itself.