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Mountain wave turbulence

Aviators need to be always aware of the wind and to seek to understand its potential effects and read the environment to appreciate and anticipate its effects on aircraft.

Wind effects around mountains and large features are the result of an interaction between the features, solar heating or cooling, mechanical turbulence caused by obstacles such as trees, and the ambient wind. The effects can be felt as anabatic and katabatic winds (resulting from solar heating and cooling), mountain waves, and rotors or eddies. Mountain waves and rotors are among the more hazardous phenomena aircraft can experience and understanding the dynamics of the wind is important to improving aviation safety.

Encounters with mountain waves can be sudden and catastrophic. Although glider pilots learn to use these mountain waves to their advantage, other aircraft have come to grief. Encounters have been described as similar to hitting a wall. In 1966, a mountain wave ripped apart a BOAC Boeing 707 while it flew near Mt Fuji in Japan. In 1968, a Fairchild F-27B lost parts of its wings and empennage and in 1992, a Douglas DC-8 lost an engine and wingtip in mountain wave encounters. In Australia, mountain waves are commonly experienced over and to the lee of mountain ranges in the southeast of the continent. They also often appear in the strong westerly wind flows Australia's east coast experiences in late winter and early spring.

Mountain waves are the result of flowing air being forced to rise up the windward side of a mountain barrier, then as a result of certain atmospheric conditions, sinking down the leeward side. This `bounce' forms a series of standing waves downstream from the barrier and may extend for hundreds of kilometres; being felt over clear areas of land and open water. Formation of the mountain waves relies on several conditions. The atmosphere is usually stable and an inversion may exist. The wind needs to be blowing almost constantly within 30 degrees of perpendicular to the barrier at a minimum speed of about 20 to 25 knots at the ridgeline. Wind speed needs to also increase uniformly with height and remain in the same direction. Wave `crests' can be upwind or downwind from the range and their amplitude seems to vary with the vertical stability of the flow. The crests of the waves may, (depending on the air having sufficient moisture content), be identified by the formation of lens-shaped or lenticular clouds. Mountain waves may extend into the stratosphere and become more pronounced as height increases with U2 pilots reportedly experiencing mountain waves at 60,000 feet. The vertical airflow component of a standing wave may exceed 8,000 feet per minute.

Rotors, or eddies can also be found embedded in mountain waves. Formation of rotors can also occur as a result of down slope winds. Their formation usually occurs where wind speeds change in a wave or where friction slows the wind near to the ground. Often these rotors will be experienced as gusts or windshear. Clouds may also form within a rotor.

Many dangers lie in the effects of mountain waves and rotors on aircraft performance and control. In addition to generating turbulence that has demonstrated sufficient ferocity to significantly damage aircraft or lead to loss of aircraft control, the more prevailing danger to aircraft in the lower levels in Australia seems to be the effect on an aircraft's climb rate. General aviation aircraft rarely have performance capability sufficient to enable the pilot to overcome the effects of a severe downdraft generated by a mountain wave, or the turbulence or windshear generated by a rotor. In 1996, three people were fatally injured when a Cessna 206 encountered lee (mountain) waves. The investigation report concluded that, "It is probable that the maximum climb performance of the aircraft was not capable of overcoming the strong downdrafts in the area at the time."

Crossing a barrier into wind also means that an aircraft's groundspeed would be reduced, remaining in an area of downdraft for longer. Flying downwind would likely put the aircraft in updraft as it approached rising ground. Rotors and turbulence may also affect low level flying operations near hills or even trees. In 1999, a Kawasaki KH-4 hit the surface of a lake during spraying operations at 30 feet. The lack of sufficient height to overcome the effects of wind eddies and turbulence was implicated as a factor involved in the accident.

Research into mountain waves and rotors or eddies continues but there is no doubt that pilots need to be aware of the phenomenon and take appropriate precautions. Although mountain wave activity is normally forecast, many local factors may effect the formation of rotors and eddies. When planning a flight, the pilot needs to take note of the winds and the terrain to assess the likelihood of waves and rotors. There may be telltale signs in flight, including the formation of clouds (provided there is sufficient humidity to provide for cloud formation) and disturbances on water or wheat fields. Some considerations include allowing for the possibility of significant variations in the aircraft's altitude if up and downdraughts are encountered. A margin of at least the height of the hill or mountain from the surface should be allowed. Ultimately, it may be preferable for pilots to consider diverting or not flying, rather than risk flying near or over mountainous terrain in strong wind conditions conducive to mountain waves and rotors.

Further Reading:

Bureau of Meteorology. (1988). Manual of meteorology part 2: Aviation meteorology. Canberra, ACT: Australian Government Publishing Service.

Bureau of Meteorology. (1991, September). Downslope winds are dangerous. BASI Journal, 9, 38-39.

Jorgensen, K. (undated). Mountain flying: A guide to helicopter flying in mountainous and high altitude areas. Westcourt, QLD: Cranford Publications.

Lester, P. F. (1993). Turbulence: A new perspective for pilots. Englewood, CO: Jeppesen Sanderson.

Welch, John, F. (Ed.). (1995). Van Sickles modern airmanship (7th Ed). New York, NY: McGraw-Hill.

Woods, R. H., & Sweginnis, R. W. (1995). Aircraft accident investigation. Casper, WY: Endeavor Books.

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