On April 20, 2019, at 1930 central daylight time, a Robinson Helicopter Company R66, N766WC, impacted an unmarked powerline and terrain during an aerial application of a field near St Jacob, Illinois. The airline transport pilot received serious injuries and the helicopter sustained substantial damage. The helicopter was registered to and operated by Skyline Helicopters LLC under Title 14 Code of Federal Regulations Part 137 as an aerial application flight that was not operating on a flight plan. Day visual meteorological conditions prevailed at the time of the accident. The local flight originated from St Jacob, Illinois at 1915. The pilot stated the wheat fields, which were not conjoined, were positioned on a north/south orientation; he approached the fields from the south to apply pesticide. When he arrived at the fields, he performed a reconnaissance by flying near their perimeters at about 100-150 feet above ground level to compare their features to those on customer-provided maps of the fields. During the reconnaissance, he identified the northern field to have powerlines/poles that traversed east/west and branched in the middle of the field toward the northwest. He set an east/west A-B line (imaginary reference line set for a field that a tractor/spray guidance system would follow) for the north field and used a back and forth spray pattern that progressed northward. Upon reaching the northern edge of the north field, he set a north/south A-B line for the southern section of the north field and the entire south field, both of which were sprayed simultaneously. The pilot said that his selection of the north/south A-B line was less than ideal for spraying, but it provided better visibility of the powerlines because it avoided direct flight into the sun. He completed the spraying of the north/south A-B lines without incident. He then lined up to perform an eastbound trim pass along the southern-edge of the north field, which had a tree line. The sun was behind him, and he retracted his helmet visor. During the trim pass, he noticed a powerline in front of him, but he was "too close to avoid [it]." He felt the powerline contact the helicopter mast and "shortly" thereafter, the helicopter began to "shake violently" and then impacted the ground. The pilot stated that he had not previously identified the powerline struck by the helicopter, even while flying the north/south spray passes. He said the powerline was oriented north/south and spanned an "abnormally long distance." The powerline spanned from a pole located at the southern edge of the field to a northern pole that was part of the east/west powerline structure. The pilot stated there was no mechanical malfunction/failure of the helicopter. Post-accident examination of the accident site revealed the powerline struck by the helicopter was a copper wire approximately 1/4-inch in diameter and green in color. The powerline pole at the southern edge of the north field was within the tree line, and the field vegetation was green.