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Lee Waves or Standing Waves
Mountain Waves is defined as oscillations to the lee side (downwind) of a mountain caused by the disturbance in the horizontal air flow caused by the high ground.
The wavelength and amplitude of the oscillations depends on many factors including the height of the high ground above the surrounding terrain, the wind speed, and the instability of the atmosphere.
Formation of mountain waves can occur in the following conditions:
- Wind direction within 30 degrees of the perpendicular to the ridge of high ground and no change in direction with height.
- Wind speeds at the crest of the ridge in excess of 15 kts27.78 km/h
7.71 m/s, increasing with height.
- Stable air above the crest of the ridge with less stable air above and below that stable layer.
Vertical currents within the oscillations can reach 2,000 ft/min10.16 m/s. The combination of these strong vertical currents and surface friction may cause rotors to form beneath the mountain waves causing severe turbulence.
Mountain Waves are associated with severe turbulence, strong vertical currents, and icing.
- Loss of Control and / or Level Bust. The vertical currents in the waves can make it difficult for an aircraft to maintain en route altitude leading to level busts and can cause significant fluctuations in airspeed potentially leading, in extremis, to loss of control. Loss of Control can also occur near to the ground prior to landing or after take off with a risk of terrain contact or a hard landing if crew corrrective response to a downdraft is not prompt.
- Turbulence. Aircraft can suffer structural damage as a result of encountering severe clear air turbulence. In extreme cases this can lead to the break up of the aircraft. In even moderate turbulence, damage can occur to fittings within the aircraft especially as a result of collision with unrestrained items of cargo or passenger luggage. If caught unaware, passengers and crew walking around the aircraft cabin can be injured.
- Icing. Severe icing can be experienced within the clouds associated with the wave peaks.
- Forecasting. Local knowledge of the conditions which tend to cause the formation of mountain waves enables forecasting of potential wave propagation.
- Cloud Formation. Lenticular Clouds (lens shaped clouds) can form in the crest of the mountain waves if the air is moist. Roll Clouds can also occur in the rotors below the waves if the air is moist. These clouds are a good indication of the presence of mountain waves but, if the air is dry, then there may not be any cloud to see.
- Restraint Systems. Passengers and flight crew should routinely wear their seat belts / harnesses when seated to provide protection against unexpected turbulence.
An aircraft tracking perpendicularly across or downwind of a mountain range or significant mountain ridge experiences a sudden loss of altitude followed by a significant and sudden reduction in airspeed during severe turbulence. Regaining the desired flght path may be difficult for a short period until the wave is exited.
- Airspeed. Reducing the aircraft speed reduces the risk of structural damage and reduces vibration making instruments easier to read in turbulence BUT beware the effect of vertical currents on airspeed and the risk of stalling the aircraft.
- Strap in. Aircraft commanders should ensure that if conditions in which there is a possibility of encountering mountain wave turbulence are envisaged en route that not only are seat belt signs selected on but that all cabin crew are instructed to cease service and secure their equipment and themselves in good time.
- Inform ATC. Notify ATC of mountain wave activity.
Accidents & Incidents
The following reports include reference to Mountain Wave activity:
- B735, Denver USA, 2008 (WX HF RE FIRE) (Runway Side Excursion During Attempted Take-off in Strong and Gusty Crosswind Conditions.)
- B733, en-route, north of Antalya Turkey, 2009 (LOC WX) (On 2 May 2009, a Boeing 737-300 being operated by French airline Europe Airpost on a passenger charter flight from Marseille to Antalya, Turkey was descending in day VMC towards destination when a sudden and severe turbulence encounter led to a temporary loss of control and stall. Recovery was achieved and none of the 115 occupants was injured and the aircraft was undamaged.)
- ^ Rotor: a turbulent horizontal vortex generated around the first trough.