The Indian monsoon is here and with it, has arrived the season of in-flight turbulence! Experiencing turbulence during a flight can be alarming, especially for those of us who fly infrequently. For the pilots and crew, turbulence is business as usual. Normally, upon experiencing turbulence, you will just receive a message to return to your seats and fasten your seat belts.
But turbulence may not always be so innocent. This year in May, 30 passengers were reportedly injured in an Etihad 1330-200 airliner as it flew through severe turbulence over Indonesia. The plane, however, landed safely. This incident poses the question of whether turbulence could trigger a worse accident or possibly lead to an air crash!
An aircraft’s resilience is built into its design
Aircrafts are designed and engineered to withstand enormous amounts of stress and strain. A huge margin is built into their design itself so that severe turbulence cannot test the limits. So, it is a bleak possibility that pilots will ever experience the level of turbulence that can bend a wing spar.
The wings of an aircraft are designed to hold out against a load that is about 1.5 times of what they experience in flight. During aircraft testing, the tips of the wings are flexed up to 90 degrees. They are meant to flex and bend a little as a rigid wing would break easily. Even skyscrapers are designed this way!
Turbulence is a disturbance in the air, not very unlike the movement of waves and sea currents. If there were no obstacles, the air would flow smoothly, but as it flows over mountains and man-made structures, the flow of the air is disrupted. This is very similar to when waves hit a sea wall and break up!
Therefore, it is fairly common to experience such turbulence if you land or take off from an airport that is close to a hilly terrain. Turbulence in higher altitudes is also caused due to weather conditions with the pressure differential disrupting the air flow. You may have heard of the term “air pocket” which basically means that the aircraft follows the direction of turbulent air which may be side to side or up or down. This leads to a quick drop in altitude causing you to lift from your seat or be pressed into it. When you are seated inside, these movements can feel amplified even though the aircraft hasn’t moved as much. Light, moderate, severe, and extreme are the four qualitative measures used to describe turbulence. Extreme turbulence can cause accidents as well, but such a scenario is very rare.
The ‘Swiss Cheese Model’ of analyzing aircraft accidents
The ‘Swiss Cheese Model’ is popular method of analyzing air accidents. In this method, slices of Swiss cheese, a kind of cheese that has holes, are lined up in a stack. Each slice represents a defence against an accident that may happen. The holes in the slices represent the weaknesses in this defence. When the holes line up perfectly, an accident occurs. An accident can also occur when there is no defence in place to prevent the accident, that is when the slice is missing entirely.
So, too many holes and the probability of an accident goes up! Unfortunately, human error can be another contributing factor besides turbulence. Back in 1966 when a pilot diverted his flight off the planed path in order to show Mount Fuji to his passengers, the high speed winds (about 140 miles per hour!) blowing off the mountain caused an air crash and killed everyone on board.
One of the ‘Swiss Cheese’ layers of defence to prevent such accidents from happening is flight planning. This essentially means that flight routes are carefully designed to minimize risks and pilots are trained to understand turbulence. By changing the planned flight route, the pilot removed a Swiss Cheese layer , exposing the aircraft to a greater risk.
One of the holes (weaknesses) in the Swiss Cheese layer in the above example was the aircraft design itself. It could not take the gust load when it encountered a turbulence over Gotemba City.
Pilots radio ahead to ask if any turbulence has been experiences and also tend to report the incidents. Turbulence can happen very quickly, which is why the passengers are asked to keep their seat belts on. A good thing is that modern day air planes are much more resilient and can withstand higher standards of strain!