SEA94GA203

While returning to salt lake city from the southeast, the pilot reported seeing thunderstorms in the area. At about 1843 MDT, the helicopter changed course and proceeded over mountainous terrain with a mode c altitude readout of 9600 to 9800 feet MSL. The terrain elevation along the flight path changed from 7700 to about 10,000 feet. Shortly after passing over terrain near 10,000 feet, the pilot radioed a mayday and declared that the engine was losing power. The helicopter began a descent, roughly following a canyon drainage area. Subsequently, it crashed at an elevation of approximately 8850 feet after impacting trees. The longitudinal wreckage distribution was minimal, indicating slow forward velocity at tree impact. An exam of the engine revealed evidence of power production during impact; no preimpact part failure/malfunction was found. The density altitude was estimated to be about 12,000 feet, and the helicopter's gross weight at the time of the accident was estimated to be 1977 pounds. Performance evaluation showed that with these conditions, the helicopter would have been unable to maintain a hover out of ground effect. The rotor blades showed rotational energy impact signatures.

History of flight

On July 30, 1994, approximately 1850 hours mountain daylight time (MDT), a Hughes OH-6A helicopter, N12UT, registered to and operated by the Utah Department of Public Safety, and being flown by Doyle R. Thorne, a commercial pilot, was destroyed during a descent into a heavily wooded ridge line, approximately 26 nautical miles west of Duchesne, Utah. The pilot was fatally injured. The precise meteorological conditions in the vicinity of the accident site were unknown, and the helicopter was operating under flight following procedures with the Utah Highway Patrol at the time of the accident. The flight, which was returning from a cancelled search mission, was public use and was to have been operated in accordance with Utah State procedures. The flight originated from the Salt Lake City International Airport approximately 1725 hours.

According to recorded radio communications between N12UT and the Highway Patrol Dispatch facility (call sign "Control"), the search mission which the aircraft had been launched on was cancelled at 1843:10 (dispatch clock time) and the pilot acknowledged the cancellation.

At 1843:18 (dispatch clock time), the pilot was advised by Control of "thunderstorms south of the Salt Lake valley and they are eastbound" and the pilot responded "I'm looking at thunderstorms all around I'll keep up my [unintelligible]." Salt Lake Air Route Traffic Control Center (SLC ARTCC) radar showed the aircraft proceeding generally west-northwest at this time.

Approximately 1843 (SLC ARTCC clock time), the aircraft turned to a westerly heading and proceeded up the Pine Hollow drainage showing a mode C altitude readout of 9,600 to 9,800 feet MSL. The ground elevation along the flight path changed from approximately 7,700 feet MSL, climbing to a height of approximately 10,000 feet MSL (refer to CHART I).

At 1846:18 (SLC ARTCC clock time), the aircraft's radar target showed the aircraft at 40 degrees 02 minutes 38 seconds north and 110 degrees 58 minutes 08 seconds west at an altitude of 9,800 feet MSL. At 1847:29 (SLC ARTCC clock time) the aircraft's radar target showed the aircraft at 40 degrees 03 minutes 38 seconds north and 110 degrees 58 minutes 47 seconds west at an altitude of 9,600 feet MSL. This was the last recorded radar target, with the previous two targets showing the aircraft in a right-hand turn roughly following the drainage running northeast and beginning slightly east of Strawberry Peak (refer to ATTACHMENT RD-1 and CHART I). The average ground speed between these last two targets was calculated as approximately 56 knots whereas the average ground speed between the previous two targets was approximately 137 knots.

At 1850:01 (dispatch clock time), dispatch recorded a transmission from N12UT beginning with a keyed microphone followed at 1850:04 (dispatch clock time) by a call from the pilot "Control, Air twelve; Control, Air twelve." Dispatch responded at 1850:08 (dispatch clock time) and immediately thereafter (at 1850:09 dispatch clock time) the pilot radioed "Mayday, I have a problem...coming into the canyon...below ten thousand feet, I'm losing power...I'm going into the trees." No further transmissions were received from the aircraft.

Personnel information

The pilot possessed a commercial pilot's certificate with a reported 425 total flight hours in the OH-6A aircraft, all of which was pilot in command. He was also reported to have a total of 1,731 hours of rotorcraft flight time of which 1,128 hours were pilot in command. The pilot's flight records showed that he had received 5.0 and 6.5 hours of ground and flight time respectively in mountain flight operations on May 19, 1994.

Aircraft information

The aircraft's maximum gross takeoff weight was 2,550 pounds. The weight of the aircraft at the time of the accident was estimated to be 1,977 pounds based upon a fuel load of 202 pounds. Performance planning information provided by the manufacturer indicated that at a weight of 1,977 pounds, a temperature of 70 degrees Fahrenheit, and an altitude of 9,590 feet (MSL), the maximum weight of the aircraft to maintain hover out of ground effect (OGE) would be 1,950 pounds (refer to ATTACHMENT W-B). Increasing temperature or altitude (density altitude) would further degrade the aircraft's hovering capability out of ground effect.

Meteorological information

The 1848 local surface weather observation at Price, Utah, bearing 148 degrees magnetic and 29 nautical miles from the accident site, reported a surface temperature of 87 degrees F., with winds of 10 knots from 240 degrees (true) and a peak wind of 14 knots. The Price observation facility is 5921 feet MSL.

The 1854 local surface weather observation at Provo, Utah, bearing 268 degrees magnetic and 36 nautical miles from the accident site, reported a surface temperature of 71 degrees F., with winds of 17 knots from 130 degrees (true). The Provo observation facility is 4491 feet MSL (refer to CHART II).

The interpolated upper air winds from the Salt Lake City radiosonde for 1800 hours local were as follows:

ALTITUDE: TRUE DIR: SPEED: 9,000 ft 271 12 10,000 ft 267 14 11,000 ft 264 14 12,000 ft 263 14

Density altitude was estimated on a range of temperatures (from 64 degrees F. through 87 degrees F.) at 10,000 feet MSL. The lowest estimated density altitude (corresponding to 64 degrees) was 12,036 feet whereas the highest estimated density altitude (corresponding to 87 degrees) was 13,382 feet (refer to attached Weather Factual Report).

Communications

A digital recording of the communications between the Utah Highway Patrol Dispatch facility and N12UT was provided to the Safety Board and translated into audio format (refer to Factual Report of Investigation Seltronics Digital Recording).

Wreckage and impact information

The ground impact site was situated within a heavily wooded, moderately sloped mountainside. The latitude and longitude of the site was determined from a hand-held Trimble GPS and was 40 degrees 04.34 minutes north and 110 degrees 56.82 minutes west respectively. The elevation of the site was approximately 8,850 feet MSL (refer to CHART I). The crash site was slightly upslope along the east face of a drainage oriented approximately northeast/southwest. Although the upsloping terrain was heavily forrested, open areas free of trees existed along the base of the drainage (refer to photograph 01 and CHART III).

The main fuselage was observed lying on its upper right side, in between the trunks of several large conifer trees (refer to photograph 02). All four main-rotor blades were located at or adjacent to the main fuselage and were observed to have remained attached to the main rotor hub assembly, and the longitudinal axis of the fuselage was oriented along an approximate 040/220 degree magnetic bearing line (refer to photograph 03). The entire tail boom assembly and tail rotor were observed to be separated from the fuselage (refer to photograph 04).

The tail boom and tail-rotor assembly were located approximately 55 feet west of the fuselage (refer to photograph 05). The tail rotor was complete except for approximately 75% of one blade which was separated. The separated blade section was located lying on the ground near the base of topped trees and approximately 50 feet southwest of the fuselage (refer to photograph 06). The blade exhibited minimal leading edge damage. The opposing blade exhibited a 90 degree folding bend just outboard of the blade grip and minimal leading edge damage (refer to photograph 07).

The four main-rotor blades were subsequently detached from the main-rotor hub assembly. All four blades displayed extensive bending deformation but were continuous from hub attach point to their respective tips (refer to photographs 08 through 11).

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