Turbulences: what causes them and are they dangerous?

Turbulence, also known as “air pockets”, is the dread of air passengers not only because it makes their flight uncomfortable but also, and above all, because it is seen as potentially very dangerous.

But what exactly is the situation? In this article we will explain how turbulences are formed, whether they are really a danger to aircrafts and passengers and how pilots deal with them.

What causes turbulences?

Ladies and gentlemen, we are about to enter a turbulence area“. Nobody likes this announcement which means at best some uncomfortable moments and at worst some (unjustified) scares. But behind this generic term lie several realities.

Convective or thermal turbulences

This is the most frequent case and is based on a physical and meteorological phenomenon. Warm air is less dense and lighter than cold air so it will tend to move upwards.

When it is warm on the ground the air is heated and will tend to rise. Logically, the cold, heavier air will descend. The warm ascending air flows and these cold descending air flows create streams that can vary the altitude of the aircraft by a few dozen metres at most.

In the case of downward flows, which are the most frequently encountered, there is an impression of falling, hence the frequently used expression ” air pocket “. A totally inappropriate expression because there is no pocket or hole, just a flow of air that drives the plane.

Turbulence is often attributed to clouds but this is not entirely true. The meeting of warm and cold air flows does create clouds, but it is the flows that impact the flight of the aircraft. On the other hand, the presence of convective clouds (cumulus, cumulonimbus) is a sign that makes it possible to anticipate the presence of turbulences.

On the other hand, when hot air rises very quickly it condenses and forms droplets, each droplet emitting heat. When a large number of droplets form at the same time, this generates a thunderstorm and releases a large amount of heat all at once, which amplifies the phenomenon even more.

Mechanical turbulences

Mechanical turbulence occurs when the wind flow is disturbed by objects. These objects can be natural (landforms, mountains) or man-made (buildings).

When the wind blows over a mountain range, it follows the shape of the peaks and makes waves, creating swirling winds.

This phenomenon is known and can be anticipated. However, depending on the geographical context and the strength of the winds, it can propagate hundreds of kilometres from the landform that generated it and surprise pilots.

Clear air turbulences

Not a cloud in sight, blue sky as far as the eye can see and yet the plane is experiencing turbulences. This is called clear air turbulences.

Clear air turbulence occurs when winds travelling at different speeds meet and form whirlwinds.

The best-known example is that of jet streams, which are very high altitude winds that circulate around the earth at very high speeds (over 300 km/h). When, for example, cold air from the Arctic meets warm air from the south, it creates significant turbulence.

The problem with this turbulence is that it is totally invisible, unpredictable and cannot always be avoided.

In the United States, a 2018 NTSB report shows that in 28% of the cases reported between 2009 and 2018, the turbulences were clear air turbulences and therefore the crews could not anticipate them.

Wake turbulence

A flying aircraft generates turbulence, especially at the wing tips, which create vortices by pushing the air away. This is the reason why in flight there is a safety distance between two aircraft and why two aircraft cannot take off and or land immediately after each other.

Turbulence can have several levels of severity

Before asking about the real risks of turbulences, it is important to know that not all turbulences are created equal. They are classified according to their level of severity in 4 categories.

Light turbulence: the aircraft is subject to slight random changes in attitude, bank or heading. At this stage it is possible to move in the aircraft without problems.
Moderate turbulence: the aircraft is subject to slight random changes in attitude, bank or heading and its speed is also affected. It becomes difficult to move around in the plane.
Severe turbulence: the aircraft is subjected to sudden changes in trajectory or altitude and the pilot may temporarily lose control. It is impossible to move around in the plane.

Extreme turbulence: the pilot can no longer control the aircraft and the aircraft structure may be damaged to the point of failure.

Can turbulences be predicted?

You partially got the answer above: yes and no.

Yes, because if turbulences cannot be detected per se, it is possible to distinguish signs of turbulence such as clouds thanks to weather radars.

No, because some turbulence such as clear air turbulences are not detectable.

However, there is still a way for a pilot to anticipate them: alerts and radio messages from other aircrafts flying ahead on the same route and informing those behind of the conditions encountered.

Should turbulence be avoided?

Here again the answer is nuanced.

For safety reasons the pilot will make every effort to avoid severe and extreme turbulence. As for the rest, it’s up to him, but in general everything will be done to maximise passenger comfort: just because turbulences are not dangerous for the aircraft doesn’t mean that they are not uncomfortable or even dangerous for the passengers.

This brings us to the most important issue: the danger of turbulences.

Are turbulences dangerous and can they cause a crash?

We had to look hard because even we were surprised by the information we found: turbulences have never caused a plane to crash. Never.

Why ?

Because they cause at worst a fall of a few tens of metres, for an aircraft flying at 10,000m this is negligible. But it is also the reason why pilots are much more vigilant during low altitude phases like landing.

Secondly, because modern aircraft are designed to withstand much higher stresses than they would in flight. Their wings are designed to support 1.5 times the normal weight of the aircraft. And they are flexible so that they can absorb shocks. Thus, the wings of an A380 can undergo a vertical deformation of 6.80m without any risk for the aircraft. Some passengers are worried about the wings moving, but this is actually a good thing: they would break if they were too rigid.

Look at the treatment an A350 undergoes to check the strength of its wings.

There are, however, air crashes that have been wrongly associated with turbulences.

In 2001 American Airlines Flight 587, from New York to Santo Domingo, crashed in Queens shortly after take-off from JFK. The investigation proved that he had encountered wake turbulence from a Japan Airlines B747 that took off just before him. Are turbulences responsible? Well, no. The pilot was surprised and violently moved the rudders in one direction and then in the other, which resulted in an excessive load on the wings, leading to the engines being ripped off. The investigation finally concluded that if the pilot had not done anything, everything would have been fine.

In addition, it was an A300 of old design, an aircraft that no longer flies today.

Another example: in 1966 BOAC Flight 911 (the forerunner of British Airways), a Boeing 707 crashed near Mount Fuji, again after encountering turbulences. The wind destroyed the tail in mid-air, resulting in the crash. But here again there is a human error: the pilot deviated from his flight plan to show the passengers Mount Fuji and the plane should never have been there. And again, this is an aircraft of very old design, far from current safety standards.

In both cases it was not the turbulence that caused the crash but the actions of the pilot.

But beware: just because turbulences are not a danger to the aircraft does not mean they are not a danger to the passengers! You have seen that even with moderate turbulence it becomes difficult to move around in the aircraft.

At 900km/h a bum” of 20cm is enough to make the meal trays take off! So between the objects that can be thrown and the passengers who neglect to put on their seatbelts and can be thrown to the ceiling, the risk is real!

If we look at the DGAC’s report on aviation safety in 2021 in France (which distinguishes between aircraft registered abroad and in France), we see that turbulence-related incidents are a very small part of the incidents reported.

The 2017 report gives equally interesting figures.

These figures from the BEA show that in 10 years there have been only 35 incidents involving French aircrafts, 23 of which caused 51 injuries.

A study by the same BEA in 2008 showed that between 1995 and 2007, 48 turbulence-related incidents were recorded, resulting in 28 serious injuries and one death, mainly due to falls and projections in the cabin.

These figures confirm our point: turbulences are not so much a question of safety for the aircraft as of safety and comfort for the passengers.

A phenomenon that will increase in the future

On the other hand, we can expect an increase in both the quantity and intensity of these events in the coming years due to global warming. According to a 2018 Science Advances report, a 2-fold increase in CO2 levels would result in a 149% increase in the average intensity of high-altitude wind shear. An increase in the number of storms is also to be expected.

Again, this presents absolutely no risk to the aircraft but much more discomfort to the passengers.

Bottom line

Turbulence has never directly caused the loss of an aircraft but is more of a comfort issue for passengers, or even a safety issue when they do not follow simple instructions such as keeping their seatbelts fastened at all times.

A phenomenon that needs to be demystified.

Image : Turbulences by diy13 via Shutterstock

Plane Vortex by hlopex via Shutterstock

Bertrand Duperrin
Bertrand Duperrinhttp://www.duperrin.com
Compulsive traveler, present in the French #avgeek community since the late 2000s and passionate about (long) travel since his youth, Bertrand Duperrin co-founded Travel Guys with Olivier Delestre in March 2015.
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