On July 15, 1998, about 1800 central daylight time, a Texas Helicopters OH-13H/M-47A, N1001F, registered to Gulf Helicopters, Inc., and operating as a 14 CFR Part 137 aerial application flight, crashed near the town of Citronelle, Alabama. Visual meteorological conditions prevailed and no flight plan was filed. The helicopter received substantial damage and the commercial-rated pilot was not injured. The flight originated from a truck-mounted helipad about five minutes before the accident. The pilot stated that after flying a short upwind leg to gain altitude from the helipad launch, and executing a 180 degree turn en route to his spray tract, in straight and level flight at 100 feet agl, at about 50 mph, he started sinking. Despite application of full throttle and dumping of his load of 100 gallons of herbicide, the sinking condition continued until collision with the tree tops situated near the top of a ridgeline. The pilot further stated he is convinced the accident was not a result of engine power degradation. He stated that prevailing winds at his location at the time of the accident were 2 to 5 mph from a westerly direction, or a tailwind.