NTSB Factual Report

On April 25, 2013, about 1016 mountain daylight time, a Bell 407, helicopter, N937GR, was substantially damaged when it landed on uneven snow covered terrain while maneuvering about 10 miles northeast of Bigfork, Montana. The airline transport pilot (ATP) and two passengers were not injured. The helicopter was registered to Two Bear Management LLC, and operated by the Flathead County Sheriff's Office as a public use flight under the provisions of 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed, and a company flight plan was filed and activated. The flight originated from Kalispell City Airport, Kalispell, Montana at about 0945.

The pilot reported that he flew directly to a telecommunications repeater site located near the summit of Mt. Aneas and after reaching the site he performed a reconnaissance orbit of the area. Mt. Aneas has a summit of 7,528 feet mean sea level (msl). The reconnaissance orbit was to determine the wind direction and speed in the area. While maneuvering about 60 knots, he stated that he lost directional control after crossing over a ridge line. He applied full left pedal to correct the right turn and lowered the nose and collective control. All attempts to regain directional control were unsuccessful. During the descent he made several power increases to avoid colliding with rocky terrain. As the helicopter impacted the uneven snow covered terrain the helicopter rolled onto its right side.

Examination of the helicopter revealed that the right side of the fuselage had crush damage and the tail boom was buckled aft of the horizontal stabilizer. The helicopter was relocated to a secure location and examined by a Federal Aviation Administration (FAA) inspector. The postaccident examination of the airframe and engine revealed no evidence of mechanical malfunctions or failures that would have precluded normal operation.

As reported by a Senior National Transportation Safety Board (NTSB) Meteorologist in a NTSB Weather Study, a plot of local observations and Remote Automated Weather Station (RAWS) data located about 15 miles east of the accident site revealed at 0921, winds were 29 miles per hour (MPH) with gusts of 41 MPH and a temperature of 44° F. At 1021, winds were 29 MPH gusting to 47 MPH and a temperature of 44° F. The meteorologist also reported that the temperature profile and windy environment in the area supported mountain wave formation with a potential wave near 7,000 feet msl with strong up/downdrafts.

According to FAA's Advisory Circular 90-95, loss of tail rotor effectiveness (LTE) "is a critical, low-speed aerodynamic flight characteristic that can result in an uncommanded rapid yaw rate which does not subside of its own accord and, if not corrected, can result in the loss of aircraft control. LTE is not related to a maintenance malfunction and may occur in varying degrees in all single main rotor helicopters in some of the conditions conducive to LTE to include a high power setting, low airspeed, and a tailwind or left crosswind."

NTSB Probable Cause

The pilot’s failure to maintain helicopter control while maneuvering over mountainous terrain in strong up/downdrafts and possible mountain wave activity, which resulted in a loss of tail rotor effectiveness.