Overview
On September 26, 2011, at about 1230 Alaska daylight time, a Eurocopter AS350BA helicopter, registration N230CH, sustained substantial damage after landing at a remote mountain site approximately 22 miles northwest of Juneau, Alaska. The helicopter was operated by Coastal Helicopters Inc. under 14 CFR Part 135 as a visual flight rules on-demand charter flight. The commercial pilot and sole passenger were not injured. Visual meteorological conditions prevailed and company flight following procedures were in effect. The flight departed Juneau International Airport at about 1212.
Narrative
The pilot reported that the purpose of the flight was to pick up two US Forest Service scientists from a remote mountainside outcrop protruding from a steep slope. He had landed at the same site about two hours earlier to drop them off. Before departure, the pilot received a weather briefing indicating possible wind gusts in the 20-knot range, but reported that winds were "relatively calm" upon landing.
After landing, the pilot initiated the shutdown checklist: locking the collective down, applying cyclic friction, and retarding the engine throttle to ground idle for a 2-minute cool-down. About 1.5 minutes into the cool-down, both occupants noticed the helicopter began to buffet from strong gusty winds. The pilot observed small trees to his right and down slope being buffeted by a strong upslope wind. Shortly thereafter, the helicopter pivoted to the right. The pilot applied left tail rotor pedal but with no effect. With the collective locked down and engine at ground idle, the helicopter rocked backward, then lifted vertically about 3–5 feet in a slightly nose-high attitude. It then rolled left despite full right cyclic input, and the main rotor blades struck the ground. The helicopter came to rest on its left side, sustaining substantial damage to the fuselage, main rotor drive system, and tail boom. The pilot noted no preaccident mechanical issues.
Meteorological Information
The nearest weather reporting facility, Juneau International Airport (22 miles southeast), reported at 1053: wind 230 degrees true at 6 knots, visibility 10 statute miles, few clouds at 1,400 feet, temperature 48°F, dew point 44°F. Two witnesses working near the accident site reported increased wind gusts shortly after the helicopter landed. A strong low-pressure system was forecast to arrive over southeast Alaska that day with surface winds of 35–45 knots. About 20 miles north of the site, winds were recorded at 27 knots at 1200 and 29 knots at 1300.
Additional Information
According to the Airman's Information Manual, abrupt changes in wind direction and velocity are common in mountainous areas, with severe updrafts, downdrafts, and turbulence near abrupt terrain changes. Eurocopter lists the idle rotor speed for an AS350BA at approximately 260 RPM. Helicopter aerodynamics literature notes that increasing up-flow through a rotor system can reverse airflow and increase rotor RPM and thrust, leading to higher rotor speeds at lower pitch settings.
At the request of Coastal Helicopters, Eurocopter USA provided technical assistance. Eurocopter's aerodynamics department in France simulated the accident using site photographs, estimated gross weight, ground idle rotor speed, and terrain data. With a wind impact angle of -50 degrees (referenced to the rotor disk), the simulation predicted lift-off at a wind speed of 37 mph (32 knots). The simulation indicated that a greater negative wind angle (more upslope) reduces the wind speed required to lift the helicopter, and that the roll motion in the accident further exposed the rotor disk to increasing negative angles, amplifying lift.
A similar event occurred on December 17, 2008, involving a Kaman K-1200 helicopter (N267KA) near Santa Clarita, California, which was upset by a wind gust while at ground idle (NTSB # WPR09LA057). The United States Department of the Interior subsequently issued an interagency aviation accident prevention bulletin (No. IA APB 12-01) highlighting the hazards of orographic turbulence in steep, up-sloping terrain.