WPR13FA343

The pilot's failure to maintain helicopter control while operating in conditions conducive to a loss of tail rotor effectiveness.

The helicopter was flying northeast following a line of utility poles for the aerial survey flight. A surviving passenger, who occupied the left rear seat, reported that the helicopter began to rotate in a clockwise direction just before impact. A second surviving passenger, who occupied the left front seat, stated that the helicopter was flying straight and level before it began to spin. He added that, before impact, he heard the low rotor rpm warning horn. The helicopter impacted heavily forested terrain in a steep nose-down, right-bank attitude. At the time of the accident, the helicopter was about 200 lbs below its maximum gross weight. Wind was calculated to be between 2 and 16 knots from the southwest with maximum gusts of about 20 knots near the accident site, which would have resulted in a tailwind condition. Examination of the helicopter did not reveal any anomalies that would have precluded normal operation.   Video footage recorded by a passenger showed the helicopter traveling about 39 knots on a northeasterly heading and at an altitude of about 200 ft above ground level. The groundspeed then began to decay to about 30 knots over a period of about 30 seconds. The helicopter then yawed right, and the groundspeed dropped to 22.6 knots. The helicopter then appeared to develop an uncontrollable right spin, and the video ended with the helicopter crashing into the forest below. It is likely that the combination of the helicopter's high gross weight, the reduction in airspeed, and the tailwind condition led to a loss of tail rotor effectiveness, which resulted in the right yaw from which the pilot did not recover control.

Examination of Fuel Tanks On August 29, 2013, under the supervision of an NTSB accident investigator, both the main and auxiliary fuel tanks, along with the instrument cluster, underwent functional testing at the facilities of Robinson Helicopter Company, Torrance, California. The results of the examination revealed the following: Main Fuel Tank The main fuel tank, which held a total of 30.5 US gallons, was visually examined. The aluminum skins were dented and/or creased, and the mounting holes were torn away at the edges. Portions of the mounting brackets remained attached to the tank. The tank was temporarily fitted to an exemplar airframe, ensuring proper angles. The instrument cluster was wired to the fuel quantity sending unit, and a warning light was wired to the Low Fuel Warning (LFW) sending unit. With power applied, the Main Fuel Tank Operating Indicator (MFI) read EMPTY, and the LFW light illuminated. Subsequent to 30 gallons of water poured into the tank, the MFI read FULL. When 9.5 gallons was drained, the MFI continued to read FULL. A light tap on the tank resulted in the MFI dropping to just below the 3/4 mark. When the MFI was observed at the 1/2 mark, 14.34 gallons of fuel had been drained, 14.55 gallons remained. When the MFI was at the 1/4 mark, 21.28 gallons had been drained, leaving 7.61 gallons remaining. After draining 24.46 gallons, the LFW light illuminated; 4.43 gallons of fuel remained. When the MFI was at the EMPTY mark and the flow of water stopped, 28.89 gallons had been drained, leaving about 1.11 gallons of unusable liquid in the tank. Auxiliary Fuel Tank A visual inspection of the tank, which had a capacity of 17.2 US gallons, revealed that the aluminum skins were dented, which reduced the capacity of the tank, and the mounting holes were torn away at the edges. The tank was temporarily fitted to an exemplar airframe, which insured proper angles. The instrument cluster was wired to a power source and the sending unit. When power was applied, the Auxiliary Fuel Tank Operating Indicator (AFI) read empty. Approximately 17 gallons of water was poured into the tank; the AFI needle read FULL. When the AFI was at the 1/2 mark, 8.80 gallons had been drained, with 8.2 gallons remaining. When the AFI was at the 1/4 mark, 12.91 gallons had been drained, with 4.09 gallons remaining. When the AFI was observed at the EMPTY mark and the flow of water halted, 17 gallons had been drained. Both fuel quantity sending units, the Low Fuel Sending unit, and both indicators were observed to have functioned within factory specifications. The FAA Rotorcraft Flying Handbook, publication FAA-H-8030-21, Unanticipated Yaw/Loss of Tail Rotor Effectiveness (LTS), states in part that unanticipated yaw is the occurrence of an uncommanded yaw rate that does not subside of its own accord and, which, if not corrected, can result in the loss of helicopter control. This uncommanded yaw rate is referred to as a loss of tail rotor effectiveness (LTE) and occurs to the right in helicopters with counter-rotating main rotor and to the left in helicopters with a clockwise main rotor rotation. LTE is not related to an equipment or maintenance malfunction and may occur in all single-rotor helicopters at airspeeds less than 30 knots. It is the result of the tail rotor not providing adequate thrust to maintain directional control. 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. FAA Advisory Circular (AC) 90-95, Unanticipated Right Yaw in Helicopters, dated February 26, 1995 states that the loss of tail rotor effectiveness (LTE) is a critical, low-speed aerodynamic flight characteristic which could result in an uncommanded rapid yaw rate which does not subside of its own accord and, if not corrected, could result in the loss of aircraft control. It also states, "LTE is not related to a maintenance malfunction and may occur in varying degrees in all single main rotor helicopters at airspeeds less than 30 knots." Paragraph 6 of the AC covered conditions under which LTE may occur. It states: "Any maneuver which requires the pilot to operate in a high-power, low-airspeed environment with a left crosswind or tailwind creates an environment where unanticipated right yaw may occur." Paragraph 8 of the AC states: "OTHER FACTORS...Low Indicated Airspeed. At airspeeds below translational lift, the tail rotor is required to produce nearly 100 percent of the directional control. If the required amount of tail rotor thrust is not available for any reason, the aircraft will yaw to the right." Paragraph 9 of the AC states: "When maneuvering between hover and 30 knots: (1) Avoid tailwinds. If loss of translational lift occurs, it will result in an increased high power demand and an additional anti-torque requirement. (2) Avoid out of ground effect (OGE) hover and high power demand situations, such as low-speed downwind turns. (3) Be especially aware of wind direction and velocity when hovering in winds of about 8-12 knots (especially OGE). There are no strong indicators to the pilot of a reduction of translation lift. (4) Be aware that if considerable amount of left pedal is being maintained a sufficient amount of left pedal may not be available to counteract an unanticipated right yaw. (5) Be alert to changing aircraft flight and wind conditions which may be experienced when flying along ridge lines and around buildings. (6) Stay vigilant to power and wind conditions." Robinson Helicopters Safety Notice SN-42, UNANTICIPATED YAW, issued May, 2013, states that a pilot's failure to apply proper pedal inputs in response to strong or gusty winds during hover or low-speed flight may result in an unanticipated yaw. Some pilots mistakenly attribute this yaw to loss of tail rotor effectiveness (LTE), implying that the tail rotor stalled or was unable to provide adequate thrust. Tail rotors on Robinson helicopters are designed to have more authority than many other helicopters and are unlikely to experience LTE. To avoid unanticipated yaw, pilots should be aware of conditions (a left crosswind, for example) that may require large or rapid pedal inputs. Practicing slow, steady-rate hovering pedal turns will help maintain proficiency in controlling yaw. Hover training with a qualified instructor in varying wind conditions may also be helpful. An autopsy of the pilot was performed at the Montana Division of Forensic Science, Missoula, Montana, on July 29, 2013. The cause of death was listed as "blunt force injuries." Toxicological testing on the pilot was performed by the FAA Civil Aeromedical Institute's (CAMI) Forensic Toxicology and Accident Research Center at Oklahoma City, Oklahoma. The toxicological tests were negative for alcohol and drugs. An NTSB Meteorological Specialist reported that a review of the available weather in the area of where the accident occurred, included the following: The National Weather Surface (NWS) Surface Analysis Chart for 1200 MDT depicted that a low pressure center was located at the central portion of Montana's border with Canada. A stationary front extended south-southeastward from the low pressure center into north-central Colorado. Another low pressure center was identified along the eastern portion of the Washington/Oregon border. Many station models in the accident region depicted clear skies, with winds across the region generally 10 knots or less, with direction variable. Temperatures near the accident site were from the mid-70 degrees F to the mid-80 degrees F, with dew points ranging from about 30 degrees F to 60 degrees F. A composite radar imagery mosaic at 1300 MDT of the accident region from the National Severe Storms Laboratory's National Mosaic and Q2 System did not identify any areas of reflectivity near the accident site. An Automated Surface Observing System station (ASOS) named KMLP, was