Turbulence is one of the most common and unpredictable weather phenomena that affect aviation. It is caused by irregular and chaotic movements of air masses, resulting from various factors such as atmospheric pressure, jet streams, mountains, thunderstorms, and temperature inversions. Turbulence can range from light to extreme, depending on the intensity and duration of the air disturbances.
What is turbulence?
Turbulence is defined as the irregular motion of the air resulting from eddies and vertical currents. It may be as insignificant as a few annoying bumps or severe enough to momentarily throw an airplane out of control or to cause structural damage. Turbulence is associated with fronts, wind shear, thunderstorms, etc. Turbulence is usually classified into four categories: light, moderate, severe, and extreme. The degree of turbulence is determined by the nature of the initiating agency and by the degree of stability of the air.
- Light: Slight erratic changes in altitude and/or attitude or a slight bumpiness. Occupants of the airplane may feel a slight strain against their seat belts.
- Moderate: Change in altitude and/or attitude, but the aircraft remains in positive control at all times. Occupants will feel a definite strain against their seat belts and unsecured objects will be dislodged.
- Severe: Large and abrupt changes in altitude and/or attitude. Aircraft may be momentarily out of control. Occupants of the airplane will be forced violently against their seat belts.
- Extreme: Aircraft are violently tossed about and practically impossible to control.
What Causes Turbulence?
There are various types of turbulence, each with different causes and characteristics. Some of the most common types of turbulence are:
Clear Air Turbulence (CAT)
This is the sudden and severe turbulence occurring in cloudless regions that causes violent buffeting of aircraft. CAT is most commonly associated with wind shear, which is a change in wind direction or speed over a specific distance. CAT is often the culprit behind moderate to severe injuries, as it can occur so suddenly that flight crew do not have time to instruct passengers to buckle up.
Wind Shear
Wind shear is the change in wind speed and/or direction over a short distance in the atmosphere. Wind shear can be caused by jet streams, temperature inversions, thunderstorms, or mountain waves. Wind shear can produce turbulence and affect the performance and stability of the aircraft.
Convective (Thermal) Turbulence
Convective turbulence caused by localized columns of convective current, which are rising columns of warm air due to surface heating or cold air moving over warmer ground. Convective turbulence can produce bumps and jolts, especially in the lower levels of the atmosphere.
Wake Vortex Turbulence
Wake Vortex caused by wing tip trailing vortices generated by aircraft. These vortices are regions of low pressure and high velocity that trail behind the wing tips of an aircraft. They can persist for several minutes and affect the following aircraft, especially if they are lighter or slower. Wake vortex turbulence can be particularly severe and has contributed to incidents and accidents.
Mechanical Turbulence
Mechanical turbulence caused by friction between the air and the ground, especially irregular terrain and man-made obstacles. Mechanical turbulence creates eddies and swirls in the lower levels of the atmosphere. The intensity of this turbulence depends on the strength of the surface wind, the nature of the surface, and the stability of the air.
Frontal Turbulence
Frontal turbulence caused by the interaction of air masses with different temperatures, pressures, and densities at a front. Frontal turbulence can be enhanced by wind shear, convective activity, or mountain waves. Frontal turbulence can occur at any altitude and can be severe or extreme.
Important Documents & Tools for Operators
There are several documents and guidelines that provide information and advice on how to avoid turbulence for operators. Some of them are:
AC 120-88A – Preventing Injuries Caused by Turbulence
This is an advisory circular issued by the FAA that provides information and practices that can be used to prevent injuries caused by turbulence. It also suggests some components of standard operating procedures and training for crewmembers, aircraft dispatchers, managers, and others associated with flight operations under 14 CFR part 121.
AC 00-30C – Clear Air Turbulence Avoidance
This is another advisory circular issued by the FAA that describes various types of clear air turbulence, some of the weather patterns associated with them, and turbulence reporting systems and networks. It also includes information on turbulence forecasts and products.
IATA Expands Turbulence Aware Platform
This is a press release by the IATA that announces the expansion of its Turbulence Aware platform, which pools anonymized turbulence data from thousands of flights operated by participating airlines. The platform enables pilots and dispatchers to choose optimal flight paths, avoiding turbulence and flying at optimum levels to maximize fuel efficiency and reduce CO2 emissions.
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