Turbulence involves irregular air movement in any direction within flight. The effect of the turbulence depends on the type of turbulence and its severity as well as the aircraft involved.
Aside from severe turbulence in Cumulonimbus clouds or storms, low level turbulence close to the ground in the process of landing is considered the most dangerous form of turbulence.
The most common is called Wake Turbulence and is caused by other aircraft. This was covered in BAK.
For the navigation course you need to be aware of:
- Wake Turbulence from heavier aircraft
- Mechanical turbulence from buildings, mountains or other terrain and structures
- Thermal turbulence caused by atmospheric air movement
- Low level windshear cause by abrupt changes in wind speed or direction
You will have studied most of these in your pilot certificate.
When air moves over mountains it is forced up as the air hits the mountain. As it spills over the other side it causes a downdraft towards the next valley floor. The air then rises in pressure and rises just like a set of waves travelling across the land. These waves can extend for very long distances on the downwind side of mountains.
You should avoid mountain wave. When stuck in mountain wave with excessive downdrafts, making it difficult to maintain altitude, you should be flying slow in rising air and fast in falling air.
If the air coming over the top of the mountain is fast enough, the air breaks like a wave in the ocean as it spills over the mountain. This causes rolling air just like the white water of a broken wave. This is extremely dangerous to aircraft. Generally winds above 20 knots should be considered to have possible rotors.
The presence of lenticular (lense shaped on top and flat bottom ) clouds on the downwind side of a mountain is indicative of rotors,
As you fly more and more navigation you will be exposed to thermal turbulence where parts of the ground heat at different rates causing rising and falling columns of air at different speeds. These are very common on hotter days, particularly from around 11am to mid afternoon.
Turbulence Penetration Speed (Vb)
Each aircraft should have a turbulence penetration speed, published in the POH. Know this prior to takeoff.
Vb is generally 1.6 x your stall speed unless stated otherwise. An aircraft with a 44 knot stall speed would have a turbulence penetration speed of 70 knots. As you can see this is well under the published maneuvering speed of 1.95 x stall speed.
Flying too fast in turbulence can cause excessive load on the airframe and airframe failure, while flying to slow can cause a loss of control.