Mountain wave and associated turbulence
In Australia, mountain waves are commonly experienced over and to
the lee of mountain ranges in the south-east of the continent. They
often appear in the strong westerly wind flows on the east coast in
late winter and early spring.

Mountain waves are a different phenomena to the mechanical
turbulence found in the lee of mountain ranges, and can exist as a
smooth undulating airflow or may contain clear air turbulence in
the form of breaking waves and 'rotors'. Mountain waves are defined
as 'severe' when the associated downdrafts exceed 600 ft/min and/or
severe turbulence is observed or forecast.

'Breaking waves' and 'rotors' associated with mountain waves are
among the more hazardous phenomenon that pilots can experience.
Understanding the dynamics of the wind is important in improving
aviation safety.

Windflow over obstacle

Mountain wave turbulence breaking wave

Glider pilots learn to use these mountain waves to their
advantage; typically to gain altitude. However, some aircraft have
come to grief in those conditions. Encounters have been described
as similar to hitting a wall. In 1966, clear air turbulence
associated with 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.

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 perturbation develops into a series of standing waves
downstream from the barrier, and may extend for hundreds of
kilometres over clear areas of land and open water.

Mountain waves are likely to form when the following atmospheric
conditions are present:
•    the wind flow at around ridge height is nearly
perpendicular to the ridge line and at least 25 kts
•    the wind speed increases with height
•    there is a stable layer at around ridge
height.

If the wave amplitude is large enough, then the waves become
unstable and break, similar to the breaking waves seen in the surf.
Within these 'breaking waves', the atmospheric flow becomes
turbulent.

The crests of the waves may be identified by the formation of
lenticular clouds (lens-shaped), if the air is sufficiently moist.
Mountain waves may extend into the stratosphere and become more
pronounced as height increases. Some pilots have reported mountain
waves at 60,000 feet. The vertical airflow component of a standing
wave may exceed 8,000 ft/min.

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 on the up-flow side of a rotor and dissipate on the down-flow
side if the air is sufficiently moist.

Many dangers lie in the effects of mountain waves and associated
turbulence 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 the climb rate of an aircraft.
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, "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 mountain barrier into wind also reduces the
groundspeed of an aircraft and has the effect of keeping the
aircraft in the area of downdraft for longer, while an aircraft
flying downwind on the upwind side of a mountain range is likely to
initially encounter updrafts as it approaches rising ground. Rotors
and turbulence may also affect low level flying operations near
hills or 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 a
factor in the accident.

Research into 'braking 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 usually forecast reasonably well by the Bureau of
Meteorology, many local factors may effect the formation of
'breaking waves' and 'rotors'. When planning a flight a pilot
should 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 disturbances on water or wheat fields and the
formation of clouds, provided there is sufficient moisture for
cloud to form.

Prudent flight planning may include allowing for the possibility
of significant variations in the aircrafts altitude if updrafts and
downdraughts are encountered. A margin of at least the height of
the hill or mountain from the surface should be allowed, and
consideration given to the need to adopt a manoeuvring airspeed
appropriate to the circumstances. 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 containing 'breaking waves' and
'rotors'.

Further Reading
Bureau of Meteorology. (2007). Manual of Aviation Meteorology.
Second Edition, pp 59, 60, 68. Airservices Australia.

Bureau of Air Safety Investigation Journal. (1991, September).
Downslope winds are dangerous. BASI Journal, 9, pp 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.

McCann, Donald W. (2006). Diagnosing and forecasting aircraft
turbulence with steepening mountain waves. National Weather Digest,
pp 77-92.

New Zealand Civil Aviation Authority (2006), Good Aviation
Practice, Mountain Flying booklet.

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.

 


Revised: 29 October 2009.

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