Turbulence is a significant factor in aviation, affecting aircraft operations in various ways. Turbulence is caused by irregular motion of air resulting from eddies and vertical currents. Understanding the different types of turbulence and their effects is crucial for ensuring the safety and efficiency of flight operations.
Clear Air Turbulence (CAT)
CAT occurs at high altitudes, typically above 15,000 feet. It is normally found near the jet stream and intertropical convergence zone (ITCZ).
Figure 1: Picture demonstrates the location of jet stream and ITCZ. [1]
CAT is caused by the vertical and horizontal wind shear near jet streams and ITCZ as shown in the picture. This natural phenomenon is invisible and can occur unexpectedly, making it difficult to detect and avoid. Pilots rely on weather reports, turbulence forecasts and local experience to anticipate CAT. However, sudden encounters of such turbulence cannot always be avoided.
Convective Turbulence
This type of turbulence is associated with convective weather phenomena, such as cumulonimbus clouds (CB) and thunderstorms. Within a cumulonimbus cloud, the updrafts and downdrafts of air moving can be very strong which brings severe turbulence. Pilots typically use weather radar to detect and avoid flying through CB.
Figure 2: Photo captures cumulonimbus outside an aircraft window. [2]
Convective turbulence can cause significant discomfort to passengers. Pilots should think ahead and monitor weather conditions during the whole flight. Weather avoidance strategies should be implemented to remain clear of convective turbulence when necessary.
Wake Turbulence
Wake turbulence is caused by the vortices generated at the wingtips of an aircraft as it produces lift. The strength of the turbulence increases as airspeed increases. These vortices can be particularly strong behind large, heavy aircraft and can pose a hazard to following aircraft, especially during takeoff and landing. Air traffic controllers manage spacing between aircraft to reduce the risk of wake turbulence.
Figure 3: This picture from a NASA study on wingtip vortices qualitatively illustrates the wake turbulence. [4]
Wake turbulence is also the type of turbulence that can be caused by wind turbines; though, it is relatively small in comparison due as they are driven by natural wind and would shut off when the ambient wind is too strong.
Mechanical Turbulence
Mechanical turbulence occurs when airflow is disrupted by obstacles such as mountains, buildings, or rough terrain.
Figure 4: Fluid dynamics lab experiment illustrates flow past a mountain-shaped obstacle. [6]
This type of turbulence is more common at lower altitudes. The most important thing to know is that mechanical turbulence caused by buildings is relatively smaller in comparison with the types of turbulence mentioned above. Computational Fluid Dynamics (CFD) analysis has shown that the potential impact of a development on wind conditions along a runway and its associated flight path is minor and not significant. [5]
Pilots need to be cautious during takeoff and landing phases in areas with known mechanical turbulence and are required to maintain good airmanship during these critical phases of flight.
Impact on Aircraft Operations
When a pilot anticipates they will encounter turbulence, particularly CAT and CB, they will keep the seatbelt sign illuminated to minimise discomfort and risk of injury. Additionally, pilots may be forced to deviate from their planned routes or altitudes to find smoother air, impacting flight efficiency by increasing fuel consumption and flight times.
Conclusion
Modern aircraft are equipped with advanced systems to help detect and manage turbulence, such as weather radar and turbulence prediction algorithms. However, pilot experience and training remain crucial in effectively handling turbulence. By understanding the different types of turbulence and their effects, pilots can make informed decisions to ensure the safety and comfort of their passengers while maintaining the integrity of their aircraft.
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References
[1] Published online at commons.wikimedia.org. Last accessed on June 1st 2024. Available at: https://en.wikipedia.org/wiki/Jet_stream#/media/File:Earth_Global_Circulation_-_en.svg
[2] Published online at commons.wikimedia.org. Last accessed on June 1st 2024. Available at: https://upload.wikimedia.org/wikipedia/commons/thumb/d/d0/Cumulonimbo_con_pileus._Volando_a_11.000_metros_sobre_Brasil.jpg/1024px-Cumulonimbo_con_pileus._Volando_a_11.000_metros_sobre_Brasil.jpg
[4] Published online at commons.wikimedia.org. Last accessed on June 1st 2024. Available at: https://upload.wikimedia.org/wikipedia/commons/thumb/f/fe/Airplane_vortex_edit.jpg/1920px-Airplane_vortex_edit.jpg
[5] Bagnara, A. (2024) ‘Gascogine Wood Sherburn in Elmet’, NOVA Fluid Mechanics Ltd
[6] Published online at commons.wikimedia.org. Last accessed on June 1st 2024. Available at: https://upload.wikimedia.org/wikipedia/commons/0/07/Lee_wave_GFD_lab.JPG
