WPR14GA281

A loss of tail rotor effectiveness, which resulted in an uncontrolled rotation and descent into terrain. Contributing to the accident was a sudden shift in wind conditions and the helicopter's high power demand condition due to its load weight and operating altitude.

The pilot was positioning the helicopter with an external load into a mountain supply drop zone located along a barren ridge at an elevation of about 8,600 feet mean sea level (msl). The helicopter had completed numerous drops that day; however, the accident drop was the first since the drop zone had been moved about 100 yards downslope, on the leeward side of the ridge. The helicopter was using a 100-foot long line with a 972-pound external load. According to the pilot, as the helicopter approached the drop zone and the load was about 10 ft above the ground, the helicopter started swaying left and right, suddenly dropped in altitude, and began rotating counterclockwise. The pilot attempted to stop the uncommanded left yaw with right pedal input; he also increased collective to stop the sudden loss of altitude. The pilot set the load down and attempted to release it; however, the long line did not completely release from the hook assembly, most likely due to the helicopter's rotation. The helicopter continued to rotate to the left and descend, despite the pilot's continued application of right pedal and collective. It landed hard and rolled over onto its right side, resulting in substantial damage. The engine could be heard winding down, and a small fire erupted out of the engine compartment. Ground personnel assisted the pilot's egress, and the fire extinguished itself. No preaccident malfunctions or anomalies were identified with the helicopter, and the engine had been operating at maximum power during the accident sequence. Wind observations taken by the ground crew on scene showed surface wind speeds between 5 and 15 knots from the southwest to west at the time of the accident. The wind just above the surface, however, was likely gusting between 5 and 25 knots as a result of an unstable atmospheric layer between the surface and 25,000 feet msl. As the wind flowed down the leeward side of the ridge, the air followed the contour of the terrain and became increasingly turbulent, resulting in downward pressure and reverse wind flow (eddy). The irregular wind conditions, high operating altitude, and the high-power, low airspeed state of the helicopter, were conducive to the development of a loss of tail rotor effectiveness, which subsequently resulted in a loss of directional control.

On August 28, 2014, under the oversight of the NTSB IIC, the EDR was removed from the sealed box, and the engine data downloaded. The EDR contained the continuous recording of the last 50 hours of engine parameters recorded at 1 hertz. The data revealed that the engine power check (EPC) performed the previous flight was with in normal parameters. The power-on time indicated that the accident flight was 91 minutes long. At 16 seconds from the end of recorded data, a steady rise of the collective over a 7 second period was observed, accompanied by the right antitorque pedal moving beyond 70 percent, N1 (gas generator turbine) accelerated to 102.5 percent, fuel flow increased to 222 liters per hour (LPh), and the N2 (main rotor rpm) begins to droop. At 11 seconds before the end of recorded data the main rotor rpm was at its lowest value, 378 rpm. One second before the end of recorded data a fault is detected that is consistent with the main rotor striking the ground. Wind Conditions in Mountainous Terrain Chapter 11 of the FAA Pilot's Handbook of Aeronautical Knowledge describes the effects of obstructions on wind. "While the wind flows smoothly up the windward side of the mountain and the upward currents help to carry an aircraft over the peak of the mountain, the wind on the leeward side does not act in a similar manner. As the air flows down the leeward side of the mountain, the air follows the contour of the terrain and is increasingly turbulent. This tends to push an aircraft into the side of a mountain. The stronger the wind, the greater the downward pressure and turbulence become. Due to the effect terrain has on the wind in valleys or canyons, downdrafts can be severe." Loss of Tail Rotor Effectiveness (LTE) Chapter 11 of the FAA Rotorcraft Flying Handbook discusses the loss of tail rotor effectiveness phenomena. "The required tail rotor thrust is modified by the effects of the wind. The wind can cause an uncommanded yaw by changing tail rotor effective thrust. Certain relative wind directions are more likely to cause tail rotor thrust variations than others. Flight and wind tunnel tests have identified three relative wind azimuth regions that can either singularly, or in combination, create an LTE conducive environment. These regions can overlap, and thrust variations may be more pronounced. Also, flight testing has determined that the tail rotor does not actually stall during the period. When operating in these areas at less than 30 knots, pilot workload increases dramatically." "At higher altitudes, where the air is thinner, tail rotor thrust and efficiency is reduced. When operating at high altitudes and high gross weights, especially while hovering, the tail rotor thrust may not be sufficient to maintain directional control and LTE can occur. In this case, the hovering ceiling is limited by tail rotor thrust and not necessarily power available. In these conditions gross weights need to be reduced and/or operations need to be limited to lower density altitudes." "This alteration of tail rotor thrust can be affected by numerous external factors. The main factors contributing to LTE are: 1. Airflow and downdraft generated by the main rotor blades interfering with the airflow entering the tail rotor assembly. 2. Main rotor blade vortices developed at the main blade tips entering the tail rotor. 3. Turbulence and other natural phenomena affecting the airflow surrounding the tail rotor. 4. A high power setting, hence large main rotor pitch angle, induces considerable main rotor blade downwash and hence more turbulence than when the helicopter is in a low power condition. 5. A slow forward airspeed, typically at speeds where translational lift and translational thrust are in the process of change and airflow around the tail rotor will vary in direction and speed. 6. The airflow relative to the helicopter; a. Worst case – relative wind within ± 15° of the 2 o'clock position (for clockwise turning main rotor), generating vortices that can blow directly into the tail rotor. This is dictated by the characteristics of helicopters aerodynamics of tailboom position, tail rotor size and position relative to the main rotor and vertical stabilizer, size and shape. b. Weathercock stability – tailwinds from 120° to 240° c. Tail rotor vortex ring state (030° to 150° for clockwise rotating main rotors with the tail rotor on the right side of the helicopter). Winds within this region result in the development of vortex ring state of the tail rotor." Hourly weather surface observations were recorded by the firefighting crew located at the cargo drop zone. Weather observation at 1400 PDT; dry bulb 84 degrees F, wet bulb 53 degrees F, relative humidity (RH) 12%, wind from the south/southwest at 10 mph, 15-20 mph gusts. Weather observation at 1500 PDT; dry bulb 84 degrees F, wet bulb 51 degrees F, RH less than 10%, wind from the south at 10 mph gusts to 20 mph. Weather observation at 1600 PDT; dry bulb 79 degrees F, wet bulb 53 degrees F, RH 12%, wind from the southwest 10-25 mph. Weather observation at 1700 PDT; dry bulb 80 degrees F, wet bulb 54 degrees F, RH 13%, wind from the south-southwest 3-5 mph with 10 mph gusts. The closest official weather surface station was at Fallon, NV (KNFL), about 54 miles west of the accident site. Fallon weather observation at 1756 PDT; wind from 310 degrees at 7 knots, 10 miles visibility, few clouds at 6,000 feet agl, temperature 37 degrees Celsius, dew point temperature -8 degrees Celsius, altimeter 29.91 inHg. Remarks, sea level pressure 1010.2 hPa, temperature 36.7 degrees Celsius, dew point temperature -7.8 degrees Celsius. Fallon weather observation at 1656 PDT; wind from 300 degrees at 9 knots with gusts to 14 knots, 10 miles visibility, few clouds at 6,000 feet agl, temperature 37 degrees Celsius, dew point temperature -9 degrees Celsius, altimeter 29.92 inHg. Remarks, sea level pressure 1010.7 hPa, temperature 36.7 degrees Celsius, dew point temperature -8.9 degrees Celsius, 6-hourly maximum temperature of 37.2 degrees Celsius, 6-hourly minimum temperature of 31.1 degrees Celsius, 3-hourly pressure decrease of 1.7 hPa. An upper air sounding was taken from a weather computer model for the accident site at 1700 PDT. The upper air sounding showed an unstable or conditionally unstable environment from the surface through 25,000 feet msl. These conditions in the vertical would allow for gusty winds at the accident flight level just above the ridge line and around any mountainous terrain. The wind was mainly from the west to southwest perpendicular or nearly perpendicular to the terrain, creating conditions conductive for mountain wave formation and mountain wave turbulence. The 1700 PDT weather computer weather model generated sounding, indicated that mountain wave conditions of +/- 1500 feet per minute downdrafts and updrafts, that would have been likely near the ridgeline or leeward of the ridgeline at 10,000 feet msl. WRECKAGE & IMPACT INFORMATION The main wreckage was positioned on a barren hill top and examined by personnel from the Bureau of Land Management (BLM) and US Department of Interior Office of Aviation Services. The helicopter was positioned on its right side. The right main landing skid had bent and separated from the right side at the upper cross tube. The right horizontal stabilizer was bent in an "L" shape against the ground. The tail boom had separated about 12 inches forward of the tail rotor gear box. Both tail rotor blades had separated from the tail rotor hub, and both pitch links remained attached to the hub. The tail rotor blades were located about 300 feet from the helicopter, and both blades exhibited damage consistent with ground impact. The 3 main rotor blades remained attached to the rotor hub and all displayed ground impact damage. The tail boom was creased where it attaches to the airframe. The area of the airframe below the