Some turbulence during a plane trip is an experience we are all familiar with. Scary, annoying, but practically always harmless. This was not the case for the patients on Singapore Airlines flight SQ321 which left London on 20 May for Singapore: sudden and serious turbulence caused one death and over 30 injuries among 211 passengers and 18 crew members, forcing the they change course to land in Bangkok and allow the injured to receive immediate medical attention. How was this possible, and how common is it for turbulence to cause such serious damage?
What is turbulence?
An airplane in flight moves within a fluid, the air, and is therefore subject to the action of the currents and vortices that it finds in its path. These are what cause turbulence, moving the airplane and making things and people inside it bounce. They are classified according to intensity into light, moderate, severe and extreme. And they are progressively less common, the higher we go in intensity: according to 2017 research, only 3% of the atmosphere at cruising altitude contains areas of mild turbulence, 0.4% moderate or worse, and just 0.1% severe or extreme.
This is if we are talking about unpredictable turbulence. In fact, there are several known causes of in-flight turbulence that are actively tracked and avoided by pilots. The most obvious are storms, which with their powerful winds can easily toss even the heaviest of aircraft. Next come the mountains, close to which powerful vortices form which can reach several kilometers in width. And therefore the jet streams, species of rivers of air that flow from west to east in the troposphere.
When turbulence can be predicted, it is very rarely dangerous, because pilots do everything possible to avoid it or minimize its effects, and passengers are warned in advance to fasten their seat belts and get into a safe position. However, it is not always possible, because sometimes turbulence, as we were saying, is unpredictable. And although rarer, it is these “clear air turbulences” that cause the majority of accidents reported by airliners, because they affect unprepared passengers and flight attendants, and can cause injuries and even deaths by banging objects and suitcases here and there, or causing falls and other types of accidents. Apparently, it was precisely in one of these clear air turbulences that the unfortunate Singapore Airlines flight SQ321 also encountered.
How many accidents do they cause?
The obligatory premise is that air accidents are always extremely rare, and deaths and injuries are an insignificant fraction compared to the millions of passengers who travel on scheduled flights every year. The most common are accidents on the runway, at departure or landing. But when some accident occurs during the cruise phase, then the main cause is actually turbulence. According to data from the US Federal Aviation Administration, between 2009 and 2021, turbulence in American skies caused just 163 serious injuries (with a prognosis of more than 48 hours), and a handful of deaths.
Severe turbulences, which are those in which truly dangerous accidents can occur, are, as we were saying, extremely rare. But apparently, on the increase: a study has in fact calculated that on the North Atlantic route they would have grown by 55% between 1979 and 2020. Another side effect of global warming, which we can't help but get used to.