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.
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'.
Illustration of mountain wave and associated turbulence
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.
|Type:||Educational Fact Sheet|
|Publication date:||12 October 2000|