Background
On April 28, 2017, about 1300 central daylight time, an Air Tractor AT-602 airplane, registration N604RH, impacted terrain while on final approach for runway 35 at Hollis Municipal Airport (O35), Hollis, Oklahoma. The pilot was fatally injured, and the airplane was substantially damaged. The aircraft was registered to the pilot and operated by Gray Ag Air under Title 14 Code of Federal Regulations Part 137 as an aerial application flight. Visual meteorological conditions prevailed; no flight plan was filed. The local flight had originated from O35 about 1200.
A colleague reported that the pilot was returning from his third or fourth application flight of the day. The pilot had treated a field about 4 miles northwest of the airport.
Accident Sequence
A witness working near the airport reported hearing the airplane and observing it approaching the airport. Nothing appeared unusual about the flight path, attitude, or appearance, but an unusual noise was heard, possibly associated with the engine or propeller pitch change. Shortly afterward, a loud explosion was heard.
Ground-based video footage from a surveillance camera about 1.65 miles southwest of the accident site showed the airplane as a black dot approaching runway 35 from the south. About 8 seconds after appearing, the airplane entered a rapid descent and went out of sight behind a tree line. A smoke plume became visible seconds later.
Analysis of the video estimated the airplane altitude at about 140 ft above ground level (agl) as it entered the field of view. Shortly before the rapid descent, altitude was estimated at 92 ft agl with a descent rate of about 6 ft/sec (360 ft/min). Groundspeed was estimated at 121 ± 18 knots. During the final descent, the rate of descent was about 80 ft/sec (4,800 ft/min). The airplane's stall speeds ranged from 61 to 71 knots with flaps extended, depending on gross weight.
Aircraft Information
The airplane flight manual "Glide and Approach (Before Landing)" procedure instructed pilots to adjust engine power to provide the required rate of descent, noting: "Do not move power lever below idle stop position. Check Beta light – OUT." It also stated that after landing, "reverse thrust may be selected as necessary." To prevent inadvertent reverse thrust, the pilot must press the reverse trigger and move the lever aft to enter beta mode.
On November 29, 2017, Air Tractor issued a service letter regarding the use of propeller beta mode in flight, informing operators that beta use in flight is prohibited and may result in loss of control due to disrupted airflow over the empennage.
Wreckage and Impact Information
The airplane impacted a wheat field about 101 yards south of the runway 35 threshold, with the fuselage coming to rest about 26 yards north of the initial impact point.
A postaccident examination by the NTSB investigator-in-charge, with assistance from airframe, engine, and propeller manufacturers, revealed no anomalies consistent with a preimpact failure or malfunction. Propeller blade angles were in the normal operating range at impact. The propeller governor was damaged and could not be functionally tested. The wing flap actuator extension was consistent with flaps being down.
Medical and Pathological Information
The Oklahoma Board of Medical Investigations performed an autopsy attributing the pilot's death to multiple blunt force injuries. Toxicology testing by the FAA Bioaeronautical Sciences Research Laboratory identified cetirizine (levels too low to quantify), oxycodone (0.116 ug/ml), and oxymetazoline in cavity blood. In urine, cetirizine, oxycodone, oxymetazoline, acetaminophen, oxazepam, and oxymorphone were detected.
Cetirizine is an antihistamine used for allergies; oxycodone is an opioid for pain; oxymetazoline is a vasoconstrictor in nasal sprays; acetaminophen is an analgesic; oxazepam is a benzodiazepine for anxiety; oxymorphone is a metabolite of oxycodone. Cetirizine, oxycodone, and oxazepam are known to be sedating.