DALLAS – The role of a pilot is to get his or her aircraft and passengers safely and efficiently to their destination, on time and without incidents.
Most passengers dislike turbulence; it is uncomfortable and is the main source of anxiety for nervous flyers (and non-fatal injuries). There are four main types of turbulence:
- Clear Air Turbulence. This is caused when large air masses of high or low pressure meet, or high-level jet streams. CAT can be avoided by planning a route to avoid these areas.
- Wake Turbulence. This is caused by following a preceding airliner that has disturbed the air, just like the visible wake from a large boat. Wake is avoided by the lateral spacing between aircraft, flying at different levels, or offsetting our track.
- Mechanical Turbulence. This is the name given to the turbulence created when strong winds flow over mountains. This causes waves of turbulence up to five times the mountain’s height and for hundreds of miles, depending on the wind strength. Again, this can be avoided by planning a route around the mountains or other areas where this type of turbulence can be expected.
- Weather-related Turbulence. This is created in the presence of cumulonimbus, or thunderclouds. They’re the huge white puffy clouds that change shape and size as they grow in width and height, sometimes to altitudes well above the normal cruising altitude of airliners. We avoid these by simply flying around them, or delaying our take-off or landing if they’re directly over the airport.
Cumulonimbus clouds contain powerful vertical shafts of air and moisture. Rising air (updrafts) lifts huge amounts of moist air into the atmosphere, which condenses to form these clouds. When the interior of the cloud becomes completely saturated, it forms rain and hail, which falls within the cloud, causing large downward shafts of air (downdrafts).
The combination of multiple updrafts and downdrafts within the cloud is what will cause Light, Moderate, or Severe turbulence.
During the day, we can see these clouds from hundreds of miles away and plan an alternative route around them, but what about at night?
Updrafts and downdrafts within a cloud cannot be seen, but we know the downdrafts contain water (rain or ice) The aircraft radar sends powerful radio beams ahead of the aircraft, if those beams hit something (water) they’re reflected back to the aircraft radar. The stronger the radar signal return indicates the concentration of water.
The time the signal takes to return, allows the radar to calculate the distance of these various returns. The concentration of water particles is shown on our navigation display (ND) as Green-Light, Amber-Moderate, or Red-Severe. If the distance between any of these is close, this could indicate Extreme turbulence and this is depicted in Magenta.
The two Airbus A350 radar systems are particularly sophisticated, continually scanning the sky ahead, above, and below our aircraft. The returns are shown in a horizontal view, but also in a vertical view, showing clouds above or below our cruising level.
If our route takes us above the storm clouds, this is shown in a “ hatched” format to remind us that we will be passing over a convective cloud and therefore may encounter some light chop. If we can navigate a route around the Green, Amber, and definitely the Red, you won’t spill your drink and you will arrive calm and relaxed.
Featured image: Otherwise known as “The King of Clouds,” cumulonimbus clouds exist through the entire height of the troposphere, usually characterized by their icy, anvil-shaped top. Photo: metoffice.gov.uk