For passengers, turbulence can be quite scary as they feel the plane moving around without knowing what’s going on. In order to feel more relaxed, it’s nice to know what is turbulence and how it’s caused. How do pilots cope with turbulence?
In short, turbulence is rapid change of altitude and direction of the plane and it is caused by airmasses with different temperature, speed and air density that meet each other.
You will find turbulence usually in three different circumstances: Turbulence caused by cumulonimbus clouds, Wake Turbulence from other airplanes and Clear Air Turbulence.
Cumulonimbus clouds are the thunderstorm clouds. Inside these clouds, the air moves vertically up and down and if you enter these clouds, the plane will move along with the air around it, giving the plane a shaky vertical movement.
Wake turbulence is caused by other airplanes that are passing through the air. The engine’s exhaust gasses are hot and make the air turbulent. This is called jetwash and may cause a crossing aircraft to change altitude or heading momentarily.
Another component of wake turbulence are wingtip vortices, which are created by the pressure difference when an airplane’s wing generates lift. The pressure difference that was created above and below the wing, makes the air move in a circular direction. These vortices can stay up to 3 minutes and move downwards.
Therefore, it’s important for take off and landing, to keep a minimum time 2-3 minutes between two aircraft, depending on the weight categories of both airplanes.
Clear Air turbulence is the most difficult turbulence to predict as it is not visible on the airplane’s weather radar. You find clear air turbulence near mountains or when flying close to the tropopause. Our atmosphere consists of different layers. The lower layer is called the Troposphere. The second layer is called the Stratosphere. Between these two layers is the Tropopause, usually around 10km of height, (Flight Level 330) around the same height of the airplane’s cruise levels.
The tropopause is usually very turbulent as it’s the middle between the two layers that have different temperatures and air density.
On low levels you find clear air turbulence around mountains. When the wind encounters the mountain, the air lifts or descends. This can cause dangerous situations for aircraft at low levels. When the wind comes from the top of the mountain, it will freefall downwards and cause downdrafts and then updrafts. We call this phenomena wind shear.
Turbulence is categorised in three levels: Light turbulence, where the turbulence is uncomfortable for some people. Moderate turbulence, where writing and walking is difficult and lose objects will move significantly. Severe turbulence, where all crew needs to sit down immediately as it may cause heavy injuries to all people that are not seated with their seatbelts fastened.
So how do pilots avoid turbulence? In the case of cumulonimbus clouds, we use our weather radar that indicates areas of heavy clouds. When we approach such cloud, we will fly a different heading to fly well around the cloud, preferably at the upwind side as the air on the downwind side of the cumulonimbus cloud may be turbulent as well.
For wake turbulence, it’s important to follow the established rules and keep enough time after another airplane has taken off.
As clear air turbulence is the most difficult to predict, it is also the most difficult to avoid. When we encounter clear air turbulence, we will make a PIREP, a pilot report, to the Air Traffic Control and tell the flight level and intensity of the turbulence. We then ask if we can climb or descend to another flight level where no turbulence has been reported.
If we are approaching a runway and the wind is very gusty and mountains are around, we expect wind shear. When we encounter wind shear on landing, we immediately discontinue the landing and make a go around, as that is always the safest thing to do in case of windshear.