Events
Near the end of an aerial application flight, the pilot maneuvered the airplane about one-third 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.
Medical History
Approximately six weeks before the accident, 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. He was taken to the hospital and diagnosed with two kidney stones: one in the ureter and one nearer the kidney. Days later, the pilot passed two fragments of the ureter stone; however, the stone nearer the kidney was likely still present.
Pre-Accident Observations
On the morning of the accident day, company personnel observed the pilot rubbing his sides above his hip, indicative of the remaining stone inducing pain and discomfort. The pilot reported that he felt somewhat uncomfortable and felt "some pain in his sides."
Operator Simulation
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.
Probable Cause
The pilot likely experienced a significant incapacitating event due to pain from the kidney stone, which resulted in his losing control of the airplane.