Aviation
Lee waves provide a possibility for gliders to gain altitude or fly long distances when soaring. World record wave flight performances for speed, distance or altitude have been made in the lee of the Sierra Nevada, Alps, Patagonic Andes, and Southern Alps mountain ranges. The Perlan Project is working to demonstrate the viability of climbing above the tropopause in an unpowered glider using lee waves, making the transition into stratospheric standing waves. They did this for the first time on August 30, 2006 in Argentina, climbing to an altitude of 50,671 ft (15,447 m). The Mountain Wave Project of the Organisation Scientifique et Technique du Vol à Voile focusses on analysis and classification of lee waves and associated rotors.
The conditions favoring strong lee waves suitable for soaring are:
- A gradual increase in windspeed with altitude
- Wind direction within 30° of perpendicular to the mountain ridgeline
- Strong low-altitude winds in a stable atmosphere
- Ridgetop winds of at least 20 knots
The rotor turbulence may be harmful for other small aircraft such as balloons, hang gliders and para gliders. It can even be a hazard for large aircraft; the phenomenon is believed responsible for many aviation accidents and incidents including the in-flight break up of BOAC Flight 911, a Boeing 707, near Mt. Fuji, Japan in 1966, and the in-flight separation of an engine on an Evergreen International Airlines Boeing 747 cargo jet near Anchorage, Alaska in 1993.
The rising air of the wave, which allows gliders to climb to great heights, can also result in high altitude upset in jet aircraft trying to maintain level cruising flight in lee waves. Rising, descending or turbulent air in or above the lee waves can cause overspeed or stall, resulting in mach tuck and loss of control, especially when the aircraft is operated near the "coffin corner".
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