Bullfrog, UT, USA
N37929
HUGHES OH 6
During a hover about 50 ft above ground level over steeply sloped terrain, the helicopter suddenly made an uncommanded left yaw, and the rotor rpm started to drop. The pilot lowered the collective and performed an autorotation to a rock ledge. After touching down on the ledge, the helicopter tipped over and tumbled downslope about 60 ft, and the tailboom was severed from the helicopter. The rotor blade damage observed during the postaccident wreckage examination was consistent with low-rotor energy at the time of ground impact. Postaccident examination of the engine, rotor drive system, and fuel control unit revealed no anomalies that would have precluded normal operation. The engine's damaged external components were replaced, and a test run was conducted. The engine started and produced full power. The helicopter's operating manual stated that an uncommanded left yaw and a drop in rotor rpm could be indications of a loss of engine power. Given the uncommanded left yaw and drop in rotor rpm during the flight, it is likely that the engine lost power; however, the reason for the loss of engine power could not be determined.
HISTORY OF FLIGHTOn April 29, 2014, at 1028 mountain daylight time, a Hughes 369A (Army OH 6/A) helicopter, N37929, was in a 50-foot hover when it experienced an uncommanded yaw to the left. The pilot executed a hovering autorotation and impacted steeply sloped canyon terrain about 14.5 miles east of the Bullfrog Basin Airport, Glen Canyon National Recreation Area, Bullfrog, Utah. The commercial helicopter pilot and other crewmember were seriously injured. The helicopter was substantially damaged. The United States Department of Agriculture and Wildlife Services operated the helicopter as a public-use flight under the provisions of 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed for the flight, and a company flight plan was active. The flight originated at Hanksville Airport, Hanksville, Utah, at 0930. The pilot stated that the purpose of the flight was to locate a radio-tagged Bighorn sheep that had died. They were hovering about 50 feet above ground level (agl) over steeply sloped terrain when the helicopter suddenly yawed to the left in an uncommanded fashion, and rotor rpm started to drop. The pilot lowered the collective and autorotated to a rock ledge. After touching down on the ledge, the helicopter tipped over and tumbled downslope about 60 feet, severing off the tail boom in the process. The pilot and passenger egressed the helicopter, used a cell phone to request assistance, and activated his personal locator beacon (SPOT) device. A Utah Highway Patrol helicopter located the aircrew and wreckage, and coordinated a rescue. PERSONNEL INFORMATIONThe pilot, age 33, held a commercial pilot certificate with a rotorcraft-helicopter rating, and a second-class medical certificate issued on September 4, 2013, with no limitations or waivers. The pilot reported in the National Transportation Safety Board (NTSB) Pilot Accident Report that he had 3,263.6 hours of total rotorcraft flight time, and 1,000.6 hours in the accident helicopter make and model. His most recent flight review was conducted on May 17, 2012. AIRCRAFT INFORMATIONThe 4-place, single-engine, conventionally configured 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 was delivered to the U.S. Army on June 29, 1968, transferred to the Department of Homeland Security Border Patrol on July 11, 1991, and then transferred to the US Department of Agriculture on July 26, 2011. The maintenance records show that the helicopter had accumulated 19,761.8 hours total time prior to the mishap flight. The most recent 100-hour inspection was performed on March 21, 2014, at 19,761.1 flight hours. Total time on the engine at the time of the inspection was 4,172.2 hours. The most recent weight and balance was dated November 22, 2013, and showed an empty weight of 1,391.7 lbs. Maximum gross weight was 2,550 lbs. According to the Boeing/MD Helicopters technical representative, the helicopter maintenance appeared to be in compliance with published manufacturer's service letters, notices and bulletins, and applicable Federal Aviation Administration (FAA) airworthiness directives. The Boeing & MDHI wreckage examination technical notes state "The aircraft performance was reviewed within the scope of available documents and data representing the approximate power available conditions and aircraft configuration at the time of the mishap. Data indicates that there was sufficient power available for the conditions and configuration to perform the planned mission, and that the aircraft was capable of being operated within published operational parameters" The performance charts for a Hughes 369HS, a civilian variant comparable to an OH-6A, with a Rolls Royce 250-C20 engine, were used by the pilot to estimate the maximum out of ground effect (OGE) hover performance capability. The performance data showed that for a pressure altitude of 6,000 feet, and air temperature at 50 degrees Fahrenheit, the max out-of-ground-effect (OGE) hover weight was 2,550 lbs. The pilot reported that the helicopter's weight at the time of the accident was 2,095 lbs. According to the OH-6 Operators Manual (TM 55-1520-214-10), "The indication of an engine malfunction, either partial or complete power loss are: left yaw, drop in engine rpm, drop in rotor rpm, low rpm audio alarm, illumination of the engine-out warning light, an a change in engine noise." METEOROLOGICAL INFORMATIONPage Municipal Airport, Arizona, is 70 miles to the southwest, elevation 4,613 mean sea level (msl), reported the following weather at 1053 local time; clear sky, 55 degrees Fahrenheit and wind direction 060 at 14 knots. The pilot reported 5 knots of wind from the north at the time of the accident, and the Utah Highway Patrol helicopter pilot reported the wind was 30 knots at the time he arrived at the accident site. AIRPORT INFORMATIONThe 4-place, single-engine, conventionally configured 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 was delivered to the U.S. Army on June 29, 1968, transferred to the Department of Homeland Security Border Patrol on July 11, 1991, and then transferred to the US Department of Agriculture on July 26, 2011. The maintenance records show that the helicopter had accumulated 19,761.8 hours total time prior to the mishap flight. The most recent 100-hour inspection was performed on March 21, 2014, at 19,761.1 flight hours. Total time on the engine at the time of the inspection was 4,172.2 hours. The most recent weight and balance was dated November 22, 2013, and showed an empty weight of 1,391.7 lbs. Maximum gross weight was 2,550 lbs. According to the Boeing/MD Helicopters technical representative, the helicopter maintenance appeared to be in compliance with published manufacturer's service letters, notices and bulletins, and applicable Federal Aviation Administration (FAA) airworthiness directives. The Boeing & MDHI wreckage examination technical notes state "The aircraft performance was reviewed within the scope of available documents and data representing the approximate power available conditions and aircraft configuration at the time of the mishap. Data indicates that there was sufficient power available for the conditions and configuration to perform the planned mission, and that the aircraft was capable of being operated within published operational parameters" The performance charts for a Hughes 369HS, a civilian variant comparable to an OH-6A, with a Rolls Royce 250-C20 engine, were used by the pilot to estimate the maximum out of ground effect (OGE) hover performance capability. The performance data showed that for a pressure altitude of 6,000 feet, and air temperature at 50 degrees Fahrenheit, the max out-of-ground-effect (OGE) hover weight was 2,550 lbs. The pilot reported that the helicopter's weight at the time of the accident was 2,095 lbs. According to the OH-6 Operators Manual (TM 55-1520-214-10), "The indication of an engine malfunction, either partial or complete power loss are: left yaw, drop in engine rpm, drop in rotor rpm, low rpm audio alarm, illumination of the engine-out warning light, an a change in engine noise." WRECKAGE AND IMPACT INFORMATIONThe helicopter wreckage was located in a steep desert canyon, elevation 4,699 feet msl. The helicopter fuselage, minus the tail boom and skid tubes, was positioned on a steep rocky slope about 60 feet below a small ledge outcrop. Photos of the wreckage, on-scene, showed a dark stained wet area beneath the main fuselage, consistent with a liquid draining from the helicopter. Recovery personnel also stated that they observed approximately 30 gallons of fuel drain from the helicopter as they moved it. The wreckage was recovered, and transported to a storage facility in Phoenix, Arizona. On May 14, 2014, a full wreckage and engine examination was performed at Air Transport in Phoenix, Arizona, by representatives from the NTSB, Boeing, MD Helicopters, Rolls-Royce, and the United States Department of Agriculture Wildlife Services (USDA) under the direction of the NTSB investigator-in-charge (IIC). The airframe was lifted by the rotor head to a stabile position, and placed on stands. The cockpit windscreen was not present and the cockpit deck had buckled. The main landing gear skids were not attached to the airframe, and the struts had separated at the fuselage interface in a fashion consistent with overload. All main rotor blades were extensively damaged. Three of the four main rotor blades were detached from the rotor head outboard of the root fittings. The single retained blade had wrapped itself around the left side of the airframe and cockpit. Main rotor blade damage was consistent with low rotor energy at impact. The tail rotor blades and fork exhibited impact damaged. The drive fork bolt was fractured and the pitch change links bent and fractured. There was very little visible rotational damage noted to the blades or hub. All damage appeared to be overload as a result of the crash sequence. Control continuity was verified from the cockpit to the stationary swash plate on the main rotor, and to the tail rotor pitch change links. Power train continuity was verified from the engine to the transmission and to the tail rotor gear box. The tail boom had separated aft of the engine compartment. Inspection of the fuel system revealed that the aft section of the left fuel cell had been penetrated and was torn open. Due to airframe damage, an airframe fuel system vacuum check could not be completed. The self-sealing (frangible fitting) fuel supply line from the firewall to the engine fuel pump fractured at the fitting to the engine fuel pump. Pressurized air was applied to the fuel line on the pick-up side of the left fuel tank that passes through the firewall frangible fitting on to the engine fuel pump. Air and fuel vapor exited the broken fitting verifying that the valve was in an open position and that a clear passage existed from the tanks to the engine. Pressurized air was also used to verify that the firewall frangible fitting did not leak. The engine was removed and taken to Aeromaritime in Mesa, Arizona, for further examination and test run. The outer combustion case (OCC) and left air transfer tube sustained impact damage, and were replaced with undamaged spare components. The engine was placed in a test cell, and five engine starts were attempted. All five resulted in hot starts where the turbine outlet temperature (TOT) rose rapidly and would exceed 1,660 degrees Fahrenheit. Each start was stopped manually. The fuel control unit (FCU) was replaced with a slave unit. The engine then started successfully, and was operated through the entire engine acceptance test program. Under the supervision of the NTSB IIC, Honeywell conducted a functional test and disassembly of the fuel control, Model DP-N2, Part Number 2524644, Serial Number BR57615, on June 10 and 11, 2014, in South Bend, Indiana. Discounting impact damage to the power lever input shaft, the manufacturer's representative found no condition that would cause a sudden reduction in fuel flow. AIDS TO NAVIGATIONA Garmin GPSMAP 396 was being used by the pilot in the cockpit. The GPS was recovered during the wreckage examination and sent to the NTSB Vehicle Recorders Laboratory for data extraction and processing. The Garmin GPSMAP 396 is a GPS receiver with a 256-color TFT LCD display screen. The unit stores date, route-of-flight, and flight-time information for up to 50 flights. A detailed tracklog – including latitude, longitude, date, time, and GPS altitude information for an unspecified number of points – was stored within the unit whenever the receiver has lock on the GPS navigation signal. Position was updated within the tracklog as a function of time or distance moved, depending on how the unit had been configured. Once the current tracklog memory becomes full, new information either overwrites the oldest information or recording stops, depending on how the unit was configured. Upon arrival at the NTSB Vehicle Recorder Division, an exterior examination revealed the unit was in good condition. The unit was powered on, and the data was successfully downloaded using the manufacturer's recommended procedures. Visual track plots were created in Google Earth using data extracted from the Garmin GPSMAP 396. A graphical overlay was generated using Google Earth for the entire recorded event flight. The overlay showed the flight originating near Hanksville. Another overlay that highlights the last recorded data points along a ridge, about 10:25:10, the aircraft began a 180-degree turn left back towards a valley. The groundspeed began to decrease from 50 knots, to a minimum of 2 knots at 10:28:07, until the last recorded data point 10 seconds later.
A loss of engine power while the helicopter was hovering for reasons that could not be determined because postaccident examination of the engine and rotor drive system revealed no evidence of a mechanical malfunction or failure that would have precluded normal operation.
Source: NTSB Aviation Accident Database
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