On July 19, 2017, about 0921 eastern daylight time, a Grumman G164 airplane, N691Y, was substantially damaged when it struck power lines and impacted a field while spraying a field near Marydel, Delaware. The commercial pilot was seriously injured. The airplane was registered to and operated by Air Enterprises, Inc., as a Title 14 Code of Federal Regulations Part 137 aerial application flight. Visual meteorological conditions prevailed at the accident site and no flight plan was filed for the flight that departed Johnson's Airport (DE09), Magnolia, Delaware, about 0910.According to the operator, the pilot had completed two spray runs and had returned to DE09 to reload. He then departed for his third spray run when the airplane struck a set of four power lines and impacted terrain after completing the first pass of the field. In an interview, the pilot told a Federal Aviation Administration (FAA) inspector that he did not remember anything about the accident. Postaccident examination revealed the airplane impacted a field adjacent to the field he was spraying and sustained substantial damage to both wings and the fuselage. There was no postimpact fire. Data downloaded from the onboard AgNav P151 GPS revealed the airplane approached the field on a westerly heading, passed low over the field as the sprayer was turned on then off, before it began a climb near the power lines. The airplane continued to travel west until data stopped at 0921 when the airplane was on the opposite side of the power lines. The pilot held a commercial pilot certificate with ratings for airplane single-engine land, and instrument airplane. His last FAA second-class medical certificate was issued on March 7, 2017. The pilot reported a total of 913 hours, with 63 hours in the same make/model as the accident airplane. Weather reported at Dover Air Force Base (DOV), Dover, Delaware, about 10 miles southeast of the accident site, at 0858, was wind from 200° at 5 knots, visibility 10 miles and clear skies. The temperature was 27° C and the dew point was also 27° C.