History of Flight
On April 29, 2014, at 1028 mountain daylight time, a Hughes 369A (Army OH-6A) helicopter, registration N37929, was in a 50-foot hover over steeply sloped terrain when it experienced an uncommanded yaw to the left. The pilot executed a hovering autorotation and the helicopter impacted the canyon terrain. The location was about 14.5 miles east of the Bullfrog Basin Airport, Glen Canyon National Recreation Area, Bullfrog, Utah. The commercial helicopter pilot and another crewmember were seriously injured. The helicopter was substantially damaged. The flight was operated by the United States Department of Agriculture and Wildlife Services as a public-use flight under 14 CFR Part 91. Visual meteorological conditions prevailed, and a company flight plan was active. The flight originated at Hanksville Airport, Hanksville, Utah, at 0930. The purpose was to locate a radio-tagged Bighorn sheep that had died. The pilot reported that while hovering at about 50 feet above ground level, the helicopter suddenly yawed left without command and rotor rpm dropped. The pilot lowered the collective and autorotated to a rock ledge. After touchdown, the helicopter tipped over and tumbled downslope about 60 feet, severing the tail boom. The pilot and passenger egressed, used a cell phone to request assistance, and activated a personal locator beacon (SPOT). A Utah Highway Patrol helicopter located them and coordinated rescue.
Personnel Information
The pilot, age 33, held a commercial pilot certificate with a rotorcraft-helicopter rating and a second-class medical certificate issued September 4, 2013, with no limitations. He reported 3,263.6 hours total rotorcraft flight time and 1,000.6 hours in the accident make and model. His most recent flight review was May 17, 2012.
Aircraft Information
The four-place, single-engine helicopter with skids, Model 369A, serial number 680518 (Army serial number 67-16133), was manufactured in 1967. It was powered by a Rolls-Royce 250-C20B, 420-shp engine, serial number CAF-837003. The helicopter had accumulated 19,761.8 hours total time. The most recent 100-hour inspection was on March 21, 2014, at 19,761.1 hours. The engine had 4,172.2 hours at that inspection. Weight and balance showed empty weight 1,391.7 lbs and maximum gross weight 2,550 lbs. Maintenance appeared compliant with manufacturer's service letters and FAA airworthiness directives. The Boeing/MDHI wreckage examination notes stated that there was sufficient power available for the conditions and configuration, and the aircraft could be operated within published parameters.
Meteorological Information
Page Municipal Airport, Arizona (70 miles southwest, elevation 4,613 ft msl), reported at 1053 local: clear sky, 55°F, wind 060° at 14 knots. The pilot reported 5 knots of north wind at the accident time. The Utah Highway Patrol helicopter pilot reported 30 knots of wind upon arrival at the accident site.
Wreckage and Impact Information
The wreckage was in a steep desert canyon at elevation 4,699 ft msl. The fuselage, minus tail boom and skid tubes, was on a steep slope about 60 feet below a small ledge. A dark wet area beneath the fuselage suggested fluid drainage. Recovery personnel observed about 30 gallons of fuel drain during movement. On May 14, 2014, a full wreckage and engine examination was performed in Phoenix, Arizona. The airframe was lifted using the rotor head. The cockpit windscreen was missing, deck buckled, main landing gear skids detached, struts separated overload. All main rotor blades extensively damaged; three detached from rotor head outboard of root fittings, one blade wrapped around left side of airframe and cockpit. Damage consistent with low rotor energy at impact. Tail rotor blades and fork impact damaged; drive fork bolt fractured, pitch change links bent and fractured. Little visible rotational damage; all damage appeared overload from crash. Control continuity verified from cockpit to stationary swash plate and tail rotor pitch change links. Power train continuity verified from engine to transmission and tail rotor gearbox. Tail boom separated aft of engine compartment. Fuel system inspection revealed aft section of left fuel cell penetrated and torn open. Self-sealing fuel supply line from firewall to engine fuel pump fractured at fitting to pump. Pressurized air applied verified clear passage from tanks to engine and firewall fitting did not leak.
Tests and Research
The engine was removed and taken for examination and test run. Outer combustion case and left air transfer tube had impact damage and were replaced. In a test cell, five engine starts were attempted; all resulted in hot starts (TOT rapidly exceeded 1,660°F) and were stopped manually. After replacing the fuel control unit (FCU) with a slave unit, the engine started and operated through the entire acceptance test program. Honeywell conducted a functional test and disassembly of the FCU (Model DP-N2, P/N 2524644, S/N BR57615) on June 10-11, 2014. Discounting impact damage to the power lever input shaft, no condition was found that would cause a sudden reduction in fuel flow. A Garmin GPSMAP 396 was recovered and data extracted. The GPS showed the flight originating near Hanksville. The last recorded data points showed the aircraft began a 180-degree left turn back toward a valley at about 10:25:10, groundspeed decreased from 50 knots to 2 knots at 10:28:07, until the last point 10 seconds later.