Wind, Turbulence & Clouds
Mountain flying represents a real challenge for most flatland pilots. At the same time the rewards are tremendous, offering very spectacular sceneries and views. It also means that the pilot needs training, develop skills and knowledge to fly safely in such an environment. We provide some basic knowledge on this subject, but this is no substitute for real training in the actual environment.
Make sure that you receive thorough training with a qualified instructor or experienced mountain pilot. Flying in mountain area's leaves little room for error. Effects of density altitude and degraded performance needs to be recognized. Get a good demonstration of soft, short field take-off and landings. Practice minimum radius turns, just in case you need one.
Mountain wind conditions
The weather can change very rapidly in the mountains and must be a serious consideration when flying in these conditions. At a very minimum an understanding of the major patterns is necessary for those only wanting to overfly any mountain ranges at higher altitudes as mountain waves sometimes extend to above the tops.
Wind is always present in the mountains and it helps to see airflow as water and think about how water would flow over the terrain. Where it would accelerate through passes and move over the valley floors, mix with other wind flows and tumbles over constructions. This will give you an rough idea where turbulence will occur and how to avoid or minimize it.
Downdraft and turbulence will generally be found on the lee side of structures and will usually increase when wind speeds are higher.
When the wind speed is below 15 knots the flow is rather predictable, above this speed the flow patterns become more difficult to predict. In this case you need to focus more on wind lanes on water surfaces or bush than on upper cloud movement or winds.
Up and downdraft
The strong vertical movements, especially on windy days, can be very daunting. These movements may exceed 3000 ft/m and most light aircraft are unable to climb out of a downdraft this strong. Move out of the down flowing air and try to locate the up flowing air, usually up- or downwind.
Turbulence and Maneuvering speed
Turbulence is created when an obstruction is placed in the path of the flow of air (wind). Obstructions like hills, buildings and trees produce small scale turbulence (eddies). The intensity of the turbulence varies with wind speed, direction and the size and shape of the obstruction. Wind shear (changes in speed and direction) is also reported in turbulence. Just maintain a safe margin above stall speed, below maneuvering speed (VA), and fly attitude rather than airspeed. Ride the waves.
Remember that VA is the speed above any full or abrupt control movement (by pilot or turbulence) can cause excessive stress on the aircraft structure and depends on the weight of the aircraft. So remain below VA. Only fly above VA when the air is smooth and never descend to an altitude when turbulence is expected and with an airspeed above VA.
VA depends on aircraft weight too: the lower the weight the easier it is to over stress it.
Ensure that all loose objects are stowed and that passengers have their seat belt on before flight into mountainous terrain. Keep in mind that passengers have a lower tolerance for turbulence than you as a pilot might have. When they become quiet it is time to pay attention to them. Be prepared for unexpected turbulence and please do inform (not scare) all onboard, make sure that the flight is enjoyed by all.
These waves are formed when the wind blows at or near right angles to a mountain range or high isolated peaks. The flow of air is disturbed as it must go over or around the obstacle. A wind of at least 15 knots increasing in strength with height and an unstable atmosphere at low levels with a stable layer at altitude together with sufficient moisture will result in standing Alto Cumulus lenticular clouds as a signpost for heavy turbulence.
These clouds have a stationary appearance but the winds are blowing continuously and the cloud is forming on the upwind side and dissipating on the downwind side. Below these lee wave clouds rotors usually are formed.
A rotor is a large closed eddy that forms in the lee of a mountain or obstacle and represent an area of severe turbulence. Up and downdraft are measured in excess of 5000 ft/m, avoid them when possible! Rotor clouds are formed when there is enough moisture in the air.
Clouds form when the temperature drops near the dew point and this change can occur rapidly in mountain area's due to the rising of the air. In good weather, thermal heating and cloud buildup are commonly found near the tops of ranges, upwind side. You will find these to be at their maximum between 2 and 3 pm and they usually disappear in the late afternoon.
A good example is the lenticular cloud. This cloud forms when there is sufficient moisture and the temperature drops towards the dewpoint by the upward movement of the air. The cloud dissipates when the upward movement of the air stops and descends again like a wave.
When the wind moves almost saturated air toward and over a ridge or mountain the air gets lifted orographically and depending on the height of the mountain and relative humidity, orographic clouds can form. Small islands in the ocean are known to have a cloud cap due to this effect. The above mentioned lenticular cloud is a form of orographic cloud.
The image to the right from the US, Dept of Agriculture shows it in detail.
Weather can change with subtle changes in wind speed and direction, it would be wise to keep an watchful eye on weather conditions when doing the preflight and during the actual flight itself. Be prepared to back out before any major problem present itself, basically: always have a way out.