In the previous section, when explaining the causes of turbulence, I didn’t mention Cumulonimbus clouds, so I’d like to add a supplement here.

Cumulonimbus clouds are visually spectacular and very attractive as a natural landscape, but for an aircraft, flying into a Cumulonimbus cloud is a very dangerous matter. This is because there are very strong updrafts and downdrafts interacting within Cumulonimbus clouds, and their energy can cause huge damage to the aircraft body or even lead to the disaster of hull loss and fatalities. Even without flying into the Cumulonimbus, just flying over the cloud layer or passing by the side can cause severe turbulence that injures passengers. In addition, hail in the clouds and lightning near the cloud layer can also cause damage to the aircraft body.

Pilots mainly use weather radar to avoid Cumulonimbus clouds. However, if they encounter a giant high wall standing in front of the airway, it is impossible to adopt the method of significantly changing the route introduced in the previous section. Therefore, the pilot will turn on the cabin seatbelt sign, requiring passengers and flight attendants to sit tight in their seats. At this time, the pilot mainly uses manual control, concentrating on finding gaps in the clouds, weaving left and right, trying to find airspace with less turbulence, until passing through the cloud area. Sometimes, after a struggle, the pilot safely passes through the cloud area without any turbulence inside. Conversely, passengers unaware of the inside story might complain, “You made us sit still in our seats, but it didn’t shake at all,” as if the pilot’s prediction was inaccurate. Little do they know that this is the result of the pilot’s hard work; everyone, please do not misunderstand.

The causes and shapes of Cumulonimbus clouds vary by region. Over continents and seas, in temperate and tropical zones, during day and night, pilots need to understand many meteorological conditions. For example, Cumulonimbus clouds in the continental United States are very distinctive. Moist air currents with a large amount of water vapor generated over the Gulf of Mexico meet the dry air in the central part of the continent, producing a single huge Cumulonimbus cloud with a diameter reaching 200 kilometers. For such a huge cloud layer, the flight route must avoid this area. Another example is the winter Cumulonimbus over the Sea of Japan. The cloud height is not very high, so its presence is not obvious when viewed from a distance. Sometimes there are large areas of stratus clouds over the sea, and Cumulonimbus clouds will hide in the stratus clouds. At this time, looking down from the sky, sometimes there are many holes in the clouds, which may be Cumulonimbus clouds.

Here are some additional supplements regarding pilot countermeasures for turbulence.

When turbulence occurs, the pilot needs to judge whether to continue flying at the same altitude, or Climb or Descent to a new altitude for cruise. At this time, the pilot needs to memorize the weather chart, especially the three-dimensional weather chart information, and correctly analyze the cause of the turbulence. Therefore, during the flight, pilots need to constantly pay attention to changes in wind speed, wind direction, and outside air temperature (OAT), observe the shape of external clouds, listen to ATC communications, and collect various information to make correct judgments.

For example, the narrow transition zone where cold and warm air masses meet, namely the “Frontal Surface,” has an inclined angle. Generally speaking, if the turbulence starts after the outside air temperature drops, descending the altitude will pass through the turbulence area as soon as possible. If the turbulence starts after the temperature rises, increasing the altitude will pass through the frontal zone as soon as possible.

Regarding the high-altitude Jet Stream, it will form a shape similar to a rapidly flowing air pipe. When flying into or out of this pipe, there will be some shaking, but generally speaking, once entering its center, it will be very stable without turbulence. However, when the wind speed exceeds 90 km/h, there is a high possibility of turbulence. At this time, the flight must be very careful, paying special attention to the Mach number display on the PFD (Primary Flight Display). Because the speed of sound varies with temperature, if the Mach number value starts to increase and decrease intermittently, it indicates that the temperature of the nearby air is beginning to change subtly, and turbulence may occur when the aircraft travels between air masses.

Observing the shape of the contrails in the upper sky is also very important. If the shape of the contrail does not change much for a long time after it forms, it indicates that the airflow at that altitude is stable, and the flight is also stable. If the shape of the contrail deforms and disperses quickly, then that altitude is very likely to experience turbulence.

When Cumulonimbus clouds appear on the route and avoidance measures are required, under radar control, a request for a new Heading is often made. Because the position of the Cumulonimbus cloud can be seen on the ND (Navigation Display), for example, if the pilot believes that a magnetic Heading of 250 degrees can avoid the Cumulonimbus, they will apply to the controller via ATC, such as “ABC Air 37, Request heading 250 due to cloud”. If the controller approves the request, they will often reply “ABC Air 37, Flying heading 250, report clear of weather,” asking the pilot to report the situation again after completing the avoidance. Therefore, after the pilot changes the route and confirms that there are no Cumulonimbus clouds ahead, they will report to the controller “ABC Air 37, Clear of weather”. If after completing the avoidance, you are ready to Turn right back to the original route, you can report “ABC Air 37, Accept right turn” to tell the controller that you are ready to Turn at any time, and the controller will then instruct the pilot with new heading or Waypoint information.

If the aircraft is flying over the ocean or in an area without radar control, the pilot can apply for an avoidance route using the method of deviating a certain distance from the route, such as “ABC Air 37, request deviate 10 miles right of track”, which means requesting to fly 10 miles to the right of the original track. The controller will reply with something like “ABC Air 37, 10 miles deviation right of track approved” to approve the pilot’s request.

刚刚虹桥落地，遇到了非常标准的夏季单体热雷暴⛈⛈ pic.twitter.com/b8zNWdFtFu

— 老堪不知道β-HMX是啥 (@candyshadow) July 19, 2017

In the 2.7 Takeoff Clearance section, it was introduced that modern passenger aircraft are equipped with the Traffic Alert and Collision Avoidance System (TCAS). Pilots can see aircraft flying around them and their altitudes through TCAS. Since the altitudes at which most aircraft fly are generally relatively stable, referring to this data is also a good method for judging cruise altitude.

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