On July 21, 1995, at 0500 hours Pacific daylight time, an Aero Commander S2R, N5609X, collided with the terrain near Huron, California. The pilot completed the initial aerial application pass on a visual flight rules aerial application flight under 14 CFR 137. The airplane, registered to and operated by Willett Flying Service, Inc., Huron, California, sustained substantial damage. The certificated commercial pilot, the sole occupant, sustained minor injuries. Visual meteorological conditions prevailed. The flight originated from a private airstrip at Huron at 0457 hours. The operator reported that the field the pilot was spraying is about 1 mile east of the airstrip. The pilot said in his written accident report that the ground flaggers were not in the proper position when he made the initial pass. He pulled up and reached down to get his map to verify the flaggers proper position and turned on the overhead light. He trimmed the airplane to a "slightly nose up" position. When he was reaching to turn the light out, the airplane struck the ground. The pilot sent a letter to the Federal Aviation Administration (FAA), Fresno [California] Flight Standards District Office. He said that the accident was ". . . caused by engine failure while I was checking my map to confirm the flaggers were positioned incorrectly while I had my overhead light on. . . ." The pilot said in the letter that after the accident he took a pain pill before going to the hospital. He said that on July 20, 1995, between 1200 and 1300 hours he took 1/2 of the 5-milligram (mg) Valium tablet. He took the Valium so that he could get some sleep before reporting to work at 2100 hours. The pilot told the inspector that he began his aerial application activities at 2300 hours. These activities continued until the accident. A ground flagger reported that he was flagging the airplane from the north center position of the field. The airplane made the first pass from the south to the north on the east side of the field. After completing the pass, the airplane pulled up and made a left turn. The airplane struck the ground as it continued the turn. The flagger said that he did not hear any unusual noises from the airplane before it struck the ground. The pilot's son took the pilot to Hanford Community Medical Center, Hanford, California, for treatment. The doctors treated the pilot for a cracked sternum and a sprained foot, and suggested that he be hospitalized for additional tests. The pilot elected not to be hospitalized and left the hospital against medical advice. A Hanford Community Medical Center laboratory technician did toxicological examinations on the pilot on July 21, 1995, at 0839 hours. The National Transportation Safety Board subpoenaed the pilot's medical treatment and toxicological records from Hanford Community Medical Center. The toxicological examinations were positive for benzodiazapine (Valium) and opiates (codeine). The concentration of these drugs was not quantified. The laboratory technician told Safety Board investigators that the threshold cutoff for the benzodiazapine and opiates was 300 nanograms per milliliter (ng/ml). The FAA, Fresno Flight Standards District Office sent the Safety Board a record of the pilot's pharmaceutical purchases beginning on January 2, 1990. Examination of the records showed that the pilot continually purchased many drugs, including multiple purchases of Valium and Tylenol with codeine, until May 26, 1995.