Turbulence is a common yet complex weather phenomenon that affects aviation. Broadly speaking, it refers to irregular air movements that cause erratic changes in the altitude and/or attitude of an aircraft. Let’s take a look at the various types of turbulence, their causes, and some notable incidents they have played a part in.
Just how turbulent is turbulence?
The International Civil Aviation Organization (ICAO) categorizes turbulence based on the intensity of its effects on aircraft and passengers. This categorization helps pilots and air traffic controllers assess and respond to turbulence.
Intensity Description Effects on Aircraft and Passengers
Light Turbulence Causes slight and erratic changes in altitude and attitude of the aircraft. Passengers may feel a light strain against their seat belts. Little or no difficulty in walking for crew members, and unsecured objects may be slightly displaced.
Moderate Turbulence Results in changes in altitude and/or attitude that are more intense than light turbulence, but the aircraft remains under control at all times. Passengers feel a more definite strain against their seat belts, unsecured objects are dislodged, and walking becomes difficult.
Severe Turbulence Causes large, abrupt changes in altitude and/or attitude, with large variations in airspeed. The aircraft may be momentarily out of control. Passengers are forced violently against their seat belts, unsecured objects can become airborne, and walking is impossible.
Extreme Turbulence Results in the aircraft being tossed about violently, making it practically impossible to control. May cause structural damage to the aircraft. Severe injuries to passengers and crew are possible if they are not securely fastened.
Clear Air Turbulence (CAT) Occurs without visual indicators like clouds, often associated with jet streams and atmospheric shear zones. Invisible and difficult to predict with conventional radar. Improved detection technology, such as Doppler LIDAR and scintillometers, aids in forecasting and detection.
ICAO Turbulence Reporting Requires pilots to report turbulence encounters using these categories to provide real-time data for immediate flight safety and long-term analysis. Helps other pilots anticipate turbulence and allows meteorologists to refine forecasting models.
Light Turbulence
Light turbulence causes slight and erratic changes in altitude and attitude of the aircraft. Passengers may feel a light strain against their seat belts. There is little or no difficulty in walking for crew members, and unsecured objects may be slightly displaced.
Moderate Turbulence
Moderate turbulence results in changes in altitude and/or attitude that are more intense than light turbulence but the aircraft remains under control at all times. Passengers feel a more definite strain against their seat belts and unsecured objects are dislodged. Walking becomes difficult for crew members.
Severe Turbulence
Severe turbulence causes large, abrupt changes in altitude and/or attitude, with large variations in airspeed. The aircraft may be momentarily out of control. Passengers are forced violently against their seat belts, and unsecured objects can become airborne. Walking is impossible. Severe turbulence is generally quite rare.
Extreme Turbulence
Extreme turbulence results in the aircraft being tossed about violently, making it practically impossible to control. This category of turbulence may cause structural damage to the aircraft. Severe injuries to passengers and crew are possible if they are not securely fastened. Extreme turbulence is, as named, extremely rare.
What are the different types of turbulence?
1. Mechanical Turbulence
Mechanical turbulence occurs when airflow is disrupted by obstacles on the ground, such as mountains, buildings, or trees. As air moves over these obstructions, it becomes turbulent, creating eddies and vortices. This type of turbulence is typically encountered during takeoff and landing when the aircraft is closer to the ground. Pilots are trained to anticipate mechanical turbulence in areas with significant topographical features or large urban environments.
A well known example of mechanical turbulence can be seen at London Heathrow Airport (LHR), where large buildings to the side of the runways can cause aircraft to be blown slightly off track right before landing. The best way to observe this is to watch one of the brilliant live stream shows presented by Big Jet TV.
2. Mountain Wave Turbulence
This is a similar phenomenon to mechanical turbulence. Mountain wave turbulence arises when stable air flows over mountain ranges, creating waves that can extend for hundreds of miles. These waves can produce severe turbulence, both in the lee of the mountains and at higher altitudes. Mountain wave turbulence is particularly dangerous because it can occur unexpectedly and even affect aircraft flying at cruising altitudes. Pilots crossing mountain ranges, such as the Rockies or the Alps, are trained to recognize and avoid areas prone to mountain wave activity. Mountain wave turbulence can be identified by observing particular cloud formations on the lee side of the mountain, such as lenticular clouds. Mountain waves can be particularly dangerous for small aircraft at low altitude and low speed.
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A notable incident involving mountain wave turbulence occurred in 1966, when BOAC Flight 911 broke apart in mid-air over Mount Fuji in Japan. The aircraft encountered extreme turbulence, leading to structural failure. This tragic event resulted in the loss of all 124 passengers and crew on board and underscored the deadly potential of mountain wave turbulence.
3. Thermal Turbulence
Thermal turbulence, also known as convective turbulence, is caused by the uneven heating of the Earth’s surface. When the sun heats the ground, warm air rises, creating thermals. These rising pockets of warm air can cause turbulent conditions, especially during the early afternoon, when solar heating is at its peak. Pilots often experience thermal turbulence when flying over land masses with varying terrain and during summer months. Glider pilots, in particular, rely on thermals to gain altitude (but must also manage the turbulence they create). Thermals (and thermal turbulence) are best identified by looking for cumulus clouds (or any clouds with a lot of vertical extent) as these indicate the vertical movement of air below.
4. Wake Turbulence
Wake turbulence is generated by the passage of an aircraft through the air, creating wingtip vortices. These vortices can be particularly hazardous for smaller aircraft following larger ones, especially during takeoff and landing. Wake turbulence is a leading cause of loss of control incidents in the vicinity of airports. Air traffic controllers implement separation standards to mitigate the risks associated with wake turbulence, ensuring safe distances between aircraft.
Aircraft are classified as light, medium or heavy, and are either separated by radar distance, or by time.
Radar distance separation
| Leading aircraft | Following aircraft | Separation distance |
|---|---|---|
| Heavy | Heavy | 4 nautical miles |
| Heavy | Medium | 5 nautical miles |
| Heavy | Light | 6 nautical miles |
| Medium | Light | 5 nautical miles |
Time separation
| Leading aircraft | Following aircraft | Separation when departing | Separation when arriving |
|---|---|---|---|
| Heavy | Medium | 2 minutes | 2 minutes* |
| Heavy | Light | 3 minutes | 2 minutes* |
| Medium | Light | 3 minutes | 2 minutes* |
One of the most well documented accidents caused by wake turbulence occurred in Mexico City in 2008, when a Learjet45 aircraft encountered severe wake turbulence on approach to the airport. The crash caused the deaths of 9 people on board and a further 7 people on the ground. Pilots are routinely trained to avoid and handle wake turbulence, and The German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) is currently in the second phase of testing a wake turbulence warning system in flight experiments using its Advanced Technology Research Aircraft A320, D-ATRA.
5. Frontal Turbulence
Frontal turbulence occurs near weather fronts, where air masses of different temperatures and densities meet. This turbulence can be quite severe, especially near cold fronts, where the colder, denser air forces the warmer air to rise abruptly. Pilots often encounter frontal turbulence during transitions between air masses, particularly in the vicinity of weather systems and during the changing seasons.
6. Clear Air Turbulence
Clear air turbulence (CAT) is the turbulent movement of air masses in the absence of visual clues such as clouds. It typically occurs at high altitudes, near the jet streams, where bodies of air moving at different speeds meet. This type of turbulence is particularly dangerous because it is invisible and difficult to detect with conventional radar.
What effects do turbulence events have on an aircraft?
In short, not a great deal.
Whilst turbulence can feel unsettling, especially to nervous flyers, it is important to emphasize that severe turbulence events are generally quite rare. Pilots carry out an extensive weather brief before a flight which identifies areas of known turbulence en-route, which are avoided if possible.
Where turbulence is unavoidable, you can be assured that the aircraft is designed to withstand it. In fact, aircraft wings are designed to flex well beyond anything that could be induced during a turbulence event. The most common damage to an aircraft in severe turbulence is usually in the form of broken cabin fixtures and fittings. Passenger injuries can occur during sudden turbulence events, such as that of SQ321, and as such it is highly recommended that you keep your seatbelt fastened when seated.
The swept wings fitted to passenger jets also aid the situation. As swept wings generate less lift in general, so do they also provide less lift when a flight is disturbed, meaning the aircraft returns to a stable condition more easily.
Turbulence is a known variable in commercial aviation. Pilots are trained to recognize and avoid obvious turbulence, and to react appropriately to unexpected turbulence events. Have you experienced turbulence? Let us know in the comments.
Cover photo: cwbspotter, JetPhotos.
