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.