On May 18, 2006, approximately 1210 central daylight time, a Grumman G-164A, agricultural bi-plane, N6629, and an Ayres S2R low-wing agricultural airplane, N4121G, collided in-flight while maneuvering near DeWitt, Arkansas. Both airplanes sustained substantial damage. The commercial pilot and sole occupant of the G-164A was fatally injured. The commercial pilot and sole occupant of the S2R, received serious injuries. The G-164A was registered to Aufderheide Flying Service, Inc., of DeWitt, Arkansas, and operated by an individual. The S2R was registered to and operated by Wilco Flying Service, LLC., of DeWitt, Arkansas. Visual meteorological conditions prevailed and flight plans were not filed for either flight. Both airplanes were operating under 14 Code of Federal Regulations Part 137 for aerial application flight. The Grumman G-164A Ag-Cat is a turbine powered bi-plane and has conventional landing gear, a single-seat and was initially fitted with a piston (reciprocating) engine. A popular conversion for the Ag-Cat is the replacement of the reciprocating engine with a turbine engine. The Ayres S2R Thrush is a single seat, low wing airplane with conventional landing gear. The accident airplane was powered with a Pratt & Whitney PT6A series turbine engine. There were no reported eyewitnesses to the accident; however the owner of the G-164A stated that his pilot was returning to DeWitt Municipal Airport, 5M1, (located approximately 3 miles southeast of the city) from an aerial application flight. The owner also reported that the field the pilot was working was located approximately 2 to 3 miles directly south of the accident site. The general location of the field and the airport would put the pilot of the G-164A bi-plane returning to the airport on a northerly heading. The owner also added that it's typical to fly between 200-300 feet above the ground when en-route to, and from the airport. The pilot of the S2R told the NTSB Investigator-in-Charge (IIC) that he departed a private airstrip, located about a mile west of DeWitt and was en route to his field, which was located approximately three-quarters of a mile south of the accident site. The general location of the private airstrip and the field being worked would put the pilot of the S2R on a southeasterly heading. The pilot also reported that he didn't recall seeing anything other than "maybe a flash" just before "fighting the controls" during the uncontrolled descent to the ground. At 1155, the automated weather observing system at Stuttgart Municipal airport (SGT), near Stuttgart, Arkansas, located approximately 26 miles northwest of the accident site, reported wind from 320 degrees at 11 knots, 10 statute miles visibility, a clear sky, temperature 79 degrees Fahrenheit, dew point 55 degrees Fahrenheit, and an altimeter setting of 29.87 inches of Mercury. A review of Federal Aviation Regulation (FAR) (Section 91.113(b): Right-of-way rules) states in part: When weather conditions permit, vigilance shall be maintained by each person operating an aircraft so as to see and avoid other aircraft. Additionally, FAR 91.113(d) states: When aircraft of the same category are converging at approximately the same altitude (except head-on, or nearly so), the aircraft to the other's right has the right-of-way. In selecting a cruising altitude the general requirements of FAR 91.159 apply; that on a magnetic course of 0 through 179 degrees, choose an altitude of, odd thousand altitude, plus 500 feet; on a magnetic course of 180 through 359 degrees, choose an even thousand, plus 500 feet. The FAR also states that the requirement for selecting an altitude based on aircraft course is only applicable at attitudes above 3,000 feet above the surface. The GPS logging unit was not installed in the G-164A at the time of the accident. The GPS memory card from the S2R was removed and sent to the manufacturer for further download. However, the GPS manufacturer reported that the unit was not recording information at the time of the accident, and no data was available with regards to the accident. The Federal Aviation Administration’s Civil Aerospace Medical Institute conducted toxicology tests on the remains of the Grumman G-164A pilot. Tetrahydrocannabinol (THC/marijuana) was detected in the pilot’s heart and lung tissue. Tetrahydrocannabinol carboxylic acid (THC-COOH, a marijuana metabolite) was detected in lung, heart, and urine samples. The testing indicated that the pilot had recently used marijuana, which resulted in his impairment. The Arkansas State Crime Laboratory, Medical Examiner Division, performed an autopsy on the G-164A pilot on May 19, 2006. The autopsy lists the cause of death as blunt force injuries sustained during an aircraft crash. The FAA Toxicology Accident Research Laboratory, near Oklahoma City, Oklahoma, conducted toxicological testing on the G-164A pilot. Tetrahydrocannabinol (Marihuana) was detected in the lung and heart. Tetrahydrocannabinol Carboxyic acid (Marihuana) was also detected in the liver, lung, heart, and urine.