Air Tractor INC AT 602 accident near Hollis, Oklahoma, April 28, 2017

The NTSB's narrative final · quoted from the NTSB record

The pilot was returning from an application flight at the time of the accident. A witness observed the airplane approaching the airport. He noted that an unusual noise came from the airplane but was unsure of the exact nature. Shortly afterward, he heard a loud explosion. Ground-based video footage depicted the airplane entering a rapid descent while on final approach until it went behind an intervening tree line. A plume of smoke rose from that location few seconds later. A postaccident examination of the airframe, engine and propeller did not reveal any anomalies consistent with a preimpact failure or malfunction. However, an examination of some components was limited due to the extent of the impact damage. Toxicology testing revealed that the pilot had used three sedating drugs: cetirizine, oxycodone, and oxazepam. However, only oxycodone was still present at a level that could have been impairing. Because patients using opioids may develop significant tolerance to the sedating side effects, there is no accepted relationship between blood levels and impairment from these medications. Therefore, the extent of impairment, if any, could not be determined. The reason for the abrupt descent and impact with terrain while on final approach could not be determined because the postaccident examination did not reveal any preimpact anomalies.

The complete narrative as the NTSB published it. The NTSB's docket holds the report as a PDF and any photographs, statements and other documents from the investigation.