Chowchilla, CA, USA
N502WQ
Air Tractor AT-502
Near the end of an aerial application flight, the pilot maneuvered the airplane 1/3-mile northwest of the designated field and began reversing course. A flagger in the field reported observing the airplane's main light turn on, which was unusual because the airplane was neither over a designated spray field nor an airstrip. Thereafter, the airplane rolled completely inverted and impacted the ground. About 6 weeks earlier, the pilot had experienced pain related to the presence of a kidney stone. He reported pain in his abdominal area, in his right flank, and above his hip. The pilot was taken to the hospital and was diagnosed as having 2 kidney stones; one in the ureter and one nearer the kidney. Days later, the pilot passed 2 fragments of the ureter stone; however, the one nearer the kidney was likely still there. When the pilot reported for work on the morning of the accident day, company personnel observed him rubbing his sides above his hip, indicative of the remaining stone inducing pain and discomfort. The pilot reported that he felt somewhat uncomfortable and he felt "some pain in his sides." The operator reported that the activation switch for the airplane's lights is located on the flight control stick. The operator simulated holding onto the control stick and doubling over. When these actions were performed, the airplane was capable of quickly rolling inverted. The pilot likely experienced a significant incapacitating event due to pain from the kidney stone, which resulted in his losing control of the airplane.
HISTORY OF FLIGHT On August 5, 2006, about 0408 Pacific daylight time, an Air Tractor, Inc., AT-502, N502WQ, crashed into an open field while maneuvering about 7 nautical miles south-southwest of Chowchilla, California. The agricultural application flight accident occurred during dark nighttime and visual meteorological conditions. The airplane was owned and operated by Thiel Air Care, Inc., Chowchilla, California, and it was substantially damaged upon impacting the ground in an inverted attitude. The commercial pilot was killed. The flight was performed under the provisions of Title 14 Code of Federal Regulations Part 137, and it originated from Chowchilla about 0340. At the time of the accident, the operator's ground employee (flagger) was standing near the field being sprayed and was watching the accident airplane. The operator reported to the National Transportation Safety Board investigator that a few seconds prior to the accident, the flagger heard the pilot on the company radio. The pilot had inquired about the local wind condition. The operator reported that this type of inquiry was a common practice for the pilot to make nearing the end of the designated spray operation over the field. The pilot did not indicate that he was experiencing any problem. The flagger also indicated that the airplane's "main work lights" became visible (were turned on) just before the crash. This was an unusual sighting and event because normally the lights are not activated when the airplane is in that location or when the airplane reverses course. The accident occurred near the end of the aerial application flight. PERSONNEL INFORMATION The pilot, age 64, held a commercial pilot certificate for airplane single engine and multiengine land, and an instrument airplane rating. The pilot's second-class airman medical certificate was issued April 25, 2006, with limitations to wear corrective lenses. According to the Federal Aviation Administration (FAA), as of April 2006, the instrument rated commercial pilot had an estimated 30,026 hours of total flight time. The operator reported that the pilot had flown at night approximately 15,000 hours. The pilot's total flight time in the accident make and model of airplane was about 1,500 hours. During the preceding 90 days, the pilot had flown the accident model of airplane about 350 hours. He had flown the airplane about 6 hours during the 24-hour period immediately preceding the accident. His last flight review was accomplished in May 2005. The operator reported that he has known and worked with the pilot for about 30 years. The operator indicated that the pilot was well known in the agricultural flying community and was highly respected for being a skilled and conscientious pilot who characteristically flew in a conservative manner. AIRCRAFT INFORMATION The operator maintained his turboprop airplane on an annual inspection basis. The airframe's total time was about 7,000 hours. The Pratt & Whitney's PT6-34AG engine had been operated about 4,500 hours since its last overhaul. The operator reported that, at the time of the accident, the airplane's application load would have been almost exhausted; the hopper was nearly empty. The airplane would have been near its empty weight except for approximately 100 gallons of fuel. The pilot had initiated the accident flight with a total of about 150 gallons of fuel. METEOROLOGICAL INFORMATION According to the operator, about the time of the accident, the sky was clear, the surface wind was from 300 degrees at 6 knots, and the visibility was about 20 miles. No moon was visible. FLIGHT RECORDERS The airplane was not required to be equipped with either a cockpit voice or flight data recorder. To assist in documenting the location where the aerial application flight was performed, the operator had installed in the airplane a flight tracking global positioning system (GPS) recording device. The recording device was a Satloc, Model M3 CPU, receiver, part number 806-1001-10A, and it bore serial number 7755-0008. The device was recovered from the accident airplane. Except for the wiring between the device and the airframe, no external damage was evident to the device's housing. Internally, the device contained a SanDisk compact flash (CF) recording media card that was found with no visually apparent damage. The CF card was dated July 3, 2003, and it bore the following numbers: SDCFB-32-101-01, and 002333J. The CF card was removed from the Satloc box, and its files were downloaded. The Safety Board investigator's examination of the data revealed the presence of multiple files bearing successive date and timestamps beginning on June 26, 2006. In total, the CF card contained about 20.1 megabytes (MB) of files, and about 10.2 MB of free disk space remained on the card. According to the operator, the last file that recorded data on the CF card included a portion of the accident flight. The recording commenced on August 5, 2006 at 0347:42.57, and it ended at 0407:46.99. The GPS coordinates for the airplane's last recorded location were 37.036021 degrees north latitude by 120.355724 degrees west longitude. Also, data for this final location/timestamp (as integrated between it and the preceding time stamp about 1 second earlier) indicated that the airplane's ground speed was 114.68 miles per hour, and its altitude was 128.41 feet mean sea level. The distance between the airplane's last recorded GPS position and the accident site is about 1/3 mile. WRECKAGE AND IMPACT INFORMATION The main wreckage was found oriented in a westerly direction at the following approximate GPS coordinates: 37 degrees 02.084 minutes north latitude by 120 degrees 21.265 west longitude. The accident site elevation was about 170 feet mean sea level. An FAA aviation safety inspector responded to the accident site and examined the wreckage. The FAA inspector reported observing evidence of propeller blade deformation that he opined was consistent with the engine producing power at impact. Based upon the observed ground scar, the airplane appeared to have impacted the soft open wheat field while completely inverted and while in a shallow rate of descent. At the initial point of impact (IPI), a swath was observed consistent in appearance with the airplane's vertical stabilizer. Also, an imprint of both wings was noted in the soil. The main wreckage slid to a stop about 200 feet beyond the IPI. There was no fire. According to the FAA inspector, the IPI was located about 1/3-mile northwest of the field being sprayed. The orientation of the ground scar was consistent with the pilot being approximately midway through his course reversal turn back toward the field. The FAA inspector additionally reported that all of the airplane's spray apparatus was accounted for at the accident site. The flight control surfaces and their respective control systems were examined. The elevator and aileron push-pull control rods, and the rudder cables, were found attached. Except for impact-related damage, no evidence of any preexisting mechanical malfunction or flight control system anomaly was found. MEDICAL AND PATHOLOGICAL INFORMATION The pilot's wife indicated to the Safety Board investigator that in June 2006, she had observed her husband with his hand over his chest, and he reported experiencing pain in his abdominal area, in his right flank, and above his hip. The pilot received a physical examination at the Community Medical Center, San Joaquin Valley, California, and a CT scan performed by the Department of Medical Imaging on June 22, 2006. Pertinent findings were as follows: "The left kidney is a normal size and contour. No hydronephrosis on the left. No left sided stones are noted. The left ureter is not dilated. On the right side there is prominent right pararenal edema. There is a cyst seen in the right kidney medially. There is moderate dilatation of the right collecting system. There is a small stone within the right mid pole renal calyx. In addition, there is a 3.5 mm calculus in the right proximal ureter. The distal ureters are normal in appearance." The following impression was indicated in the medical records: (1) 3.5 mm proximal right ureteral calculus causing partial obstruction. (2) Right mid pole renal calyceal stone also is noted. In addition there is an upper pole right renal cyst. A note was written on the medical record indicating that a 1 mm stone was passed on June 25, and a 3 mm stone was passed on June 28. Additionally, the stones were taken to another physician for evaluation. The note indicates that the small stone in the kidney "should not be a problem. Have appt. in 3 weeks." The pilot's wife reported to the Safety Board investigator that her husband was rested when he reported for work on August 4. She was unaware of the pilot being in any physical discomfort, and he appeared healthy. The operator reported that the pilot was in good spirits when he arrived at work to begin his 12-hour-long nighttime shift. The operator also reported that when the pilot's work shift began on the evening of August 4, it "was standard," and "nothing out of the ordinary" occurred. Subsequently, the operator became aware that 2 of his employees (a crewmember and shop manager) had observed the pilot arrive at work. The employees reported in written statements and during follow up interviews with the Safety Board investigator that the pilot's behavior was, in one respect, unusual. They both opined that while the pilot appeared healthy and alert, he was rubbing his sides above his hip. They had not previously observed the pilot manifest that type of behavior. The pilot indicated to them that he felt somewhat uncomfortable and he felt "some pain in his sides." On August 11, 2006, an autopsy was performed on the pilot by the Madera Sheriff - Coroner, Madera, California, 93638. The autopsy findings indicated the cause of death was "multiple severe blunt force injuries." Forensic toxicology was performed on specimens from the pilot by the FAA Bioaeronautical Sciences Research Laboratory, Oklahoma City, Oklahoma. The toxicology report stated no ethanol was detected. Naproxen was detected in urine. TESTS AND RESEARCH The operator stated to the Safety Board investigator during an August 30 and early September telephone conversations that his review of the Satloc data indicated the pilot had performed a normal spray pattern over the designated field. The airplane impacted the ground in what appeared to be a course reversal turn back toward the field. The accident site was within an estimated 1,500 feet northwest of the northwest corner of the field. For undetermined reasons, the Satloc stopped recording the airplane's position a few seconds prior to the impact. The operator additionally reported that he was familiar with the handling characteristics of the accident airplane. He considered the accident pilot to have been highly skilled. The operator reported that: (1) there was no field to be sprayed in the area where the accident occurred; (2) there was no reason related to spraying for the pilot to turn on the airplane's lights in the vicinity of the accident site; and (3) the activation switch for the lights is located on the control stick, and if the pilot had squeezed the stick in the vicinity of the light switch, the lights would turn on. The operator, using an exemplar airplane, simulated doubling over in the cockpit. When performing this motion, he moved the control stick in a manner consistent with what he opined the accident pilot might have done. If the pilot had abruptly moved the control stick to the left, the airplane was fully capable of quickly rolling inverted.
The pilot's loss of control while maneuvering resulting from an incapacitating physiological event.
Source: NTSB Aviation Accident Database
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