Turbine-powered aircraft operating at cruise altitudes above FL180 in the vicinity of mountainous terrain may encounter moderate or greater turbulence associated with orographic winds. This type of turbulence may be characterized by relatively rapid onset and can lead to structural damage or airframe failure.

Structural damage is not the only danger associated with high-altitude turbulence encounters. It is possible to operate some turbine-powered aircraft at such weights and altitudes so that their cruise airspeed is only a few knots below the onset of Mach buffet and a like speed above stall buffet. In this situation (the so-called “coffin corner”), turbulent airspeed excursions of moderate or greater intensity (15 kt or more) can quickly lead to high-speed upset, Mach tuck, and loss of control.

Takeoff and landing concerns include experiencing turbulent air with inadequate stall margins, loss of directional control on or near the runway, rolling moments that surpass aircraft roll authority, and downdraft velocities that exceed the climb capability of the aircraft, particularly for airplanes with high wing and power-loading. Localized gusts in excess of 50 kt, with downdrafts greater than 1,500 fpm, are also possible.

Vortices spawned by the interaction of strong winds and high terrain can lead to severe turbulence and aircraft rolling moments that may exceed the pilot’s ability to maintain aircraft control.

Aircraft that engage in low-level flight operations over mountainous terrain in the presence of strong winds (20 kt or greater at ridge level) can expect to encounter moderate or greater turbulence, strong updrafts and downdrafts, and very strong rotor and shear zones. This is particularly true for General Aviation (GA) aircraft.

Mountain flying literature often cite 20 kt as the criterion for classifying a wind as “strong.” This criterion refers to the large-scale (or prevailing wind in the area as opposed to a local wind gust) wind speed at the crest of the ridge or level of the mountain peaks, upwind of the aircraft’s position. Such an ambient wind flow perpendicular to a ridge will lead to substantially stronger surface winds, with the likelihood of turbulence. Similar wind enhancements can be anticipated near the slopes of an isolated peak. In contrast, downdrafts over forested areas may be strong enough to force aircraft down into the trees, even when the aircraft is flown at the best rate-of-climb speed. This effect on the aircraft can be exacerbated by loss of aircraft performance because of the high-density altitude.