Understanding Turbulence

There are two main categories to learn when trying to understand turbulence; what is the degree of the turbulence and what is the cause of the turbulence. The degree describes the severity of the turbulence and how the airplane experiences the effects. The cause of the turbulence identifies whether the turbulence could have been expected and helps understand whether the pilot could have avoided or warned of the impending turbulence.

For there to be a cause of action the turbulence must be substantial enough to cause damage to persons or property. Additionally, there must be a causal link between the turbulence causing damage and the pilot’s failure to avoid or warn of the turbulence. In most cases, if the turbulence is light or moderate and the pilot had no warning that the turbulence was likely to occur, there is no cause of action. If the pilot was aware and improperly flew into known turbulence when able to avoid it or without warning passengers, a cause of action likely exists. If turbulence is severe enough and a pilot knew or the signs that turbulence was likely to occur is present and the pilot failed to see the signs, a cause of action likely exists regardless of whether the pilot knew or warned the passengers as the pilot should avoid such turbulence.

Degrees of Turbulence:

Light Turbulence
Light turbulence is a series of momentary, slightly erratic changes in your altitude or attitude. When in light turbulence, you might feel a slight strain against your seat belt. Small unsecured objects might get dislodged, but it would be easy to walk around the cabin with little or no difficulty.

Moderate Turbulence
Moderate turbulence consists of changes in your altitude or attitude, but your aircraft remains in positive control at all times. You will feel a definite strain against your seat belt. Unsecured objects in the cabin will become dislodged and walking will be difficult in your cabin.

Severe Turbulence
This is where a cause of action most likely begins to arise, as long as the pilot has the ability to avoid the turbulence, as discussed below. Severe turbulence consists of large, abrupt changes in altitude or attitude. The aircraft may be temporarily out of control, and you will be forced violently against your seat belt. Even lightly secured objects may become dislodged and you will be unable to walk or stand. Even if secured in your seatbelt, you may hit your arms, legs, and head against the people, seat, or walls around you.

Extreme Turbulence
Extreme turbulence is very rare, as it requires a pilot to fly directly into weather that is visible, both to the naked eye and on weather radar. A pilot is capable of avoiding such conditions in all cases. In extreme turbulence, the aircraft is violently tossed about and practically impossible to control. Extreme turbulence can cause structural damage or structural failure. This type of turbulence is so severe it can cause mid-air breakups of the aircraft leading to a crash.

Causes of Turbulence

The causes of turbulence is important to understand to know if the pilot should have anticipated the turbulence and failed to act properly by alerting passengers or avoiding the area.

Clear Air Turbulence
The Federal Aviation Administration (FAA) defines clear air turbulence (CAT) as “sudden severe turbulence occurring in cloudless regions that causes violent buffeting of aircraft.” The FAA adds that the CAT definition is most commonly applied to higher altitude turbulence associated with wind shear, which is a change in wind direction or speed over a specific distance.

Convective (thermal) Turbulence
Thermal turbulence occurs with localized columns of convective current (a rising column of warm air). These rising columns of air come from surface heating or cold air moving over warmer ground. This turbulence is often the most severe and is the most common type of turbulence to lead to a cause of action. Convective turbulence is usually at least moderate and the conditions are well understood by pilots. This type of turbulence should be avoided and a failure to do so usually results in a cause of action.

Frontal Turbulence
This occurs with the lifting of warm air by the sloping frontal surface of a cold air mass. It’s here that friction occurs between the two opposing air masses, producing turbulence in the frontal zone. When the warm air is moist and unstable, there can be a risk of thunderstorms, leading to more severe turbulence.

Mechanical Turbulence
Mechanical turbulence occurs when there is friction between the air and the ground. Found at low altitudes, this is often the result of irregular terrain and man-made objects. This irregular terrain (think tall buildings and mountains) causes the obstruction of airflow. The intensity will depend on the strength of the surface wind and the nature of the surface.

Mountain Wave
Mountain wave turbulence, sometimes described as a form of mechanical turbulence, occurs when strong turbulence occurs downwind from mountain ridges. Mountain waves are said to produce some of the most severe mechanical turbulence.

Wind Shear (including temperature inversions)
Wind shear is when there is a great change in the direction of winds aloft. This causes turbulence because the lift created by the aircraft is rapidly changed as the winds change from head winds, to cross, winds, to tail winds. This sudden change causes rapid changes in altitude and attitude resulting in turbulence.

Temperature inversions are zones of strong stability that prevent the mixing of the stable low layer with the warmer layer above. “The greatest shear, and thus the greatest turbulence, is found at the tops of the inversion layer,” Turbulence associated with temperature inversions often occur due to nighttime cooling of the Earth’s surface creating a surface-based inversion.

Jet stream turbulence is another term that can fall under wind shear. As you can tell from the name, this turbulence comes from jet streams, which are strong horizontal winds that follow a wavelike pattern as a part of the general wind flow. Jet streams turbulence occurs at altitudes as the jet streams are usually five to nine miles above the earth’s surface.

Wake Vortex Turbulence
Wake vortex turbulence is encountered when an aircraft follows or crosses behind another aircraft. Caused by wing tip trailing vortices generated by the first aircraft, this is the reason why planes have designated minimum separation distances. It’s also why the callsigns of larger aircraft add the term “heavy” or “super” as they another indication that a 747 or A380 (respectively) should be given sufficient space.