NTSB Factual Report

On July 17, 2016, about 1409 Pacific daylight time, a Bell UH-1H helicopter, N790RJ, impacted mountainous terrain while maneuvering near a landing zone located at an elevation of about 7,800 feet in Mount Adams, Washington. Two pilots, four crewmembers and two passengers were not injured; however, the helicopter sustained substantial damage. The helicopter was registered to and operated by the King County Sherriff's Department and was supporting a Search and Rescue (SAR) mission at the time of the accident. Visual meteorological conditions prevailed at the time of the accident, and no flight plan had been filed for the public flight that originated from Yakima Air Terminal/McAllister Field (YKM), Yakima, Washington about 1330.

The pilot reported that after refueling at YKM, he departed to transport two SAR personnel onto Mt. Adams to retrieve an injured hiker. While en route, a sensor unit on the helicopter indicated 5 knots of wind. As they approached the hiker from the north, the pilot used too much left pedal and decided to abort the approach. He attempted a second approach from the south, but the helicopter yawed to the right, which the pilot attributed to a loss of tail rotor effectiveness (LTE). In an attempt to recover, the pilot reduced the collective and applied forward cyclic. The helicopter spun about 540 degrees, impacted the ground, and then departed the mountain. The pilot did not observe any indications of a malfunction with the rotor or the drive system on the helicopter. In his subsequent report, he reported that there were no mechanical malfunctions or anomalies that could have precluded normal operation. The helicopter then returned to YKM.

The FAA issued Advisory Circular (AC) 90-95, Unanticipated Right Yaw in Helicopters, in February 1995. The AC states that LTE is a critical, low- speed aerodynamic flight characteristic that could result in an uncommanded rapid yaw rate, which does not subside of its own accord and, if not corrected, LTE could result in the loss of aircraft control. It also stated, "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 stated: "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 stated: "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 stated: "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... (6) Stay vigilant to power and wind conditions."

NTSB Probable Cause

The pilot’s failure to maintain yaw control as he slowed the helicopter during a landing approach, which resulted in the loss of helicopter control due to the loss of tail rotor effectiveness.