HISTORY OF FLIGHTOn May 15, 2019, at 1151 Pacific daylight time, a Grumman Aircraft Corporation-Schweizer G-164C, N6787K, and a Gulfstream-Schweizer Aircraft Corporation Gulfstream AM G-164D, N6929K, were substantially damaged when they collided midair near Pleasant Grove, California. The two pilots were fatally injured. Both airplanes were operated as Title 14 Code of Federal Regulations Part 137 aerial application flights. The pilot of N6787K (the white airplane) departed on his first flight of the day to apply rice seed to a field about 3 nautical miles southeast of the departure point. The pilot of N6929K (the yellow airplane) departed from a different airstrip to spread rice seed on another field. The pilot of the white airplane was flying south toward the field he intended to treat, while the pilot of the yellow airplane had completed his application and was flying in a north/northwesterly direction while returning to the departure airstrip to land when the collision occurred. A witness near the accident site reported that he saw the yellow airplane fly south and spray the field behind his location. The airplane banked east, turned around, and flew north to spray another field. He looked away, and seconds later, heard the sound of an impact. When he looked back toward the airplane, he saw the white airplane in a “nosedive” toward a field and also saw the yellow airplane crash into a rice field. There was no Federal Aviation Administration (FAA) air traffic control radar data associated with either airplane. A SATLOC agricultural GPS unit recovered from the yellow airplane revealed its flight track on the day of the accident. (see Figure 1.) A similar unit recovered from the white airplane did not contain any recorded data from the day of the accident. Figure 1. Flight track information obtained from N6929K (yellow airplane) WRECKAGE AND IMPACT INFORMATIONThe wreckage and debris path was distributed over about ¼ mile of rural agricultural terrain. The yellow airplane impacted a flooded rice field, while the white airplane came to rest in a dry field south of the yellow airplane. Various debris associated with both airplanes was located between the two main wreckage sites. The white airplane came to rest on its left side adjacent to an unpaved road. The airplane was mostly intact; the engine remained attached to the airframe and the propeller remained attached to the engine. The left lower wing was located about 140 ft from the main wreckage. A yellow aileron and a portion of a yellow wing strut were located near the white airplane’s left lower wing. Flight control continuity was established with no anomalies noted. The yellow airplane was not accessible at the accident site due to its location in the flooded rice field. The majority of the airplane was within the confines of the main wreckage. The engine separated and forward of the main wreckage. The propeller assembly separated from the crankshaft.  MEDICAL AND PATHOLOGICAL INFORMATIONAn autopsy was performed on the pilot of the white airplane by the County of Sacramento Department of Coroner. The cause of death was multiple blunt force injuries. No significant natural disease was identified. Toxicology testing by the FAA Forensic Sciences Laboratory identified the non-sedating antacid medication ranitidine in the pilot’s urine. An autopsy of the pilot of the yellow airplane was performed by the County of Sacramento Department of Coroner. The cause of death was multiple injuries. The autopsy identified an enlarged heart (470 grams) and severe atherosclerosis with 70 to 90% narrowing in several coronary arteries and branches. Toxicological testing by the FAA Forensic Sciences Laboratory identified delta-9-tetrahyrdocannabinol (THC) at 1.7 ng/mL in the pilot’s femoral blood. 11-hydroxy-delta-9-THC, and carboxy-delta-9-THC (THC-COOH) were identified in urine at 9.9 ng/mL and 15.8 ng/mL, respectively. The non-impairing high blood pressure medications amlodipine and valsartan were detected in blood and urine. The cannabis plant (Cannabis sativa) contains chemicals called cannabinoids; tetrahydrocannabinol (THC) is the primary psychoactive cannabinoid compound. THC's mood-altering effects include euphoria and relaxation. In addition, cannabis causes alterations in motor behavior, perception, and cognition. Significant performance impairments are usually observed for at least 1-2 hours following cannabis use, and residual effects have been reported up to 24 hours. THC is rapidly metabolized, but the rate of metabolism is not linear and depends on the means of ingestion (smoking, oil, and edibles), potency of the product, frequency of use, and user characteristics. The primary metabolite, 11-hydroxy-delta-9-THC, is equally psychoactive, but is rapidly metabolized to the non-psychoactive metabolite THC-COOH. THC is fat soluble, so is stored in fatty tissues and can be released back into the blood long after consumption. So, while the psychoactive effects may last a few hours, THC can be detected in the blood for days or weeks. Commonly, peak THC concentrations of 100-200 ng/mL occur soon after smoking cannabis and are generally below 5 ng/mL after 3 hours. Very little THC is excreted in urine. Instead, THC-COOH can be found in urine days to weeks after the last use of the drug. Thus, both blood and urine test results do not necessarily reflect recent use and cannot be used to prove that the user was under the influence of the drug at the time of testing.