The layman’s term “hitting an air pocket” is a good characterization of a situation in which the aircraft shakes or vibrates, creating discomfort; it’s similar to driving a car on a bumpy road. But how can there be bumps in the air?
The way that an aircraft moves in the atmosphere resembles the movement of a boat in water. If there are swirls and vortexes in the stream, the vessel’s path becomes uneven. In addition to the magnitude of the flow disturbances, also the size of the vessel has an impact on its rocking. Larger vessels and aircraft are less affected than small ones.
Turbulent weather feels uncomfortable, but is by no means dangerous. Aircraft have been built to last sudden strain caused by turbulence. For example, the wings are designed to be flexible, and they bob up and down. Due to this flexibility, the wings will not fracture or change shape, and neither will bumps destroy wing lift. In fact, a sudden gust may even increase wing lifting power.
Pilots attempt to predict and avoid turbulence, but sometimes there is no forewarning available. For this reason, the cockpit crew keep their safety-belts fastened at all times. I recommend keeping safety belts fastened in the cabin as well, even when the “fasten safety belt” sign is off. This way, you can avoid any bumps or bruises caused by sudden turbulence if standing on the aisle.
Young children often observe and mimic the reactions of their parents. You can teach your children to become more relaxed air travellers by taking a natural approach to turbulence yourself. Remember that children usually love being on a roller coaster, so you could try and point out the fun side of turbulence to them. You may feel butterflies in your stomach, but even severe turbulence will not damage the aircraft, as it has been designed to swim in these currents.
As pilots, we concentrate on flying the aircraft to its destination as smoothly as possible, so that passengers can focus on keeping their coffee in the cup. Here’s a tip for doing that: first sip any excess coffee, and then lift the cup off the tray table and keep it in your hand instead, but don’t use the armrest for support. Your hand on its own is surprisingly steady, and can usually keep your cup balanced without spilling the coffee.
Takeoff and landing more turbulent than cruise
Turbulence usually takes place when no coffee is served, during takeoff and landing. The air at lesser altitudes is “bumpier” than higher up. The reason for this is that the roughnesses of the earth’s surface, as well as temperature differences, cause plenty of flow disturbances in other words: turbulence in air masses at lower altitudes. Once again, air can be compared to water: a shallow stream has more swirls than a deep river.
There are several reasons for vortices in air currents. At cruising altitude, “air pockets” are usually caused by CAT, Clear Air Turbulence. It has been named thus, because the turbulence does not appear to be caused by anything visible to the human eye, for example clouds. CAT is caused when bodies of air moving at different speeds or heading in different directions meet and mix together. CAT is an ordinary atmospheric phenomenon, which usually occurs close to high mountain ranges and jet streams. Jet streams are fast-flowing (even over 300 km/h), thermal air currents found in the upper levels of the troposphere, which balance temperature differences in different parts of the world.
Pilots are informed about jet streams and any possible CAT on the route before departure, during their flight planning. The location of the streams can be predicted quite accurately, but the exact time of occurrence, duration, and severity of turbulence is still difficult to estimate due to its scattered nature.
Clear air turbulence is not visible to the naked eye or recognized by traditional radars, but modern Doppler radars are able to identify it. In the future, when the resolution and velocity detection of Doppler radars are further increased, measurements will become even more accurate.
Summers are more turbulent than winters
Frequent flyers have probably noticed that the air is “bumpier” in summer than in winter, when there are less warm currents. In addition, various weather phenomena, such as cumulonimbus clouds and weather fronts, cause vertical wind shifts in the troposphere. When this hits the aircraft, it abruptly changes the angle of attack “sensed” by the wing and causes turbulence. In other words, disturbances in air flow are experienced as turbulence in the cabin. Vertical wind shift and even ordinary wind can cause turbulence, which actually means rapid changes in air flow in relation to time.
Wake turbulence is an entirely different kind of turbulence. Just like boats, aircraft leave a wake or trail, but usually an invisible one. Wingtip vortices are tubes of air that trail from the tip of each wing, and are created as a side-effect of the wing generating aerodynamic lift. The heavier the aircraft, the bigger is its wake vortex as well. The wake vortices flow in a circular motion, anti-clockwise from the right wing and clockwise from the left, leaving a downwards sloping trail behind the aircraft. The wingtip vortices begin to recede, expand and fade with the wake, just like a boat’s trail in water. The duration of wake vortices depends on the evenness of the air mass. At high altitudes in even parts of the atmosphere, wake vortices can last for several minutes. When another aircraft hits this kind of vortex, it causes a sudden jerk, which may frighten passengers.
Aircraft have been divided into three groups based on wake vortex categories. Aircraft that weigh less than 7,000 kilograms are categorized as light. Private “small planes” usually belong to this category. Anything from fairly small propeller planes (for example the ATR) to aircraft as large as the Boeing 757 used by Finnair leisure flights, belong to the medium group. Finnair’s Airbus A330 and A340 aircraft, as well as the MD-11 in our use for a few months longer, all belong to the heavy category.
However, it is a good thing that several airports, such as Helsinki-Vantaa, treat the Boeing 757 as a heavy aircraft, due to its effective wing lift. What this means in practice is that longer intervals are left between take offs and landings. The intervals prevent the following aircraft from getting caught in the wake of a previous one, or at least provide enough time for the wake vortex to fade.
Wind shear is practiced in flight simulators
The most dangerous of all meteorological phenomena is wind shear. Wind shear refers to the quick variation of wind between two points either horizontally or vertically.
Wing lift is directly proportional to flying speed squared, and therefore, a reduction in speed results in a decrease in lifting power. In order for it to not decrease significantly, aircraft are flown 10-30% faster than required, to cover any ordinary changes in air flow. However, with wind shear this may not be enough. The change is so sudden and great that the aircraft may stall, which means that the air no longer flows along the wing, and lifting power is lost. Excess speed during approach should still be avoided, as this makes landing more difficult, and increases the landing distance.
Therefore, pilots practice for these rare situations in their simulator training, and handling wind shear is also one of the compulsory topics in annual refresher training and simulator flight inspections. Pilots practice scenarios that have led to accidents in the past in flight simulators, where there is no unnecessary risk.
In my experience, I can safely say that most situations could have been avoided by good decision-making and by taking the appropriate measures. Then again, most of these methods have been developed by analyzing accidents carefully, and learning from them. It is justified to claim that the losses of past accidents are part of the total cost of flight safety today. In any case, wind shear is an awe-inspiring meteorological phenomenon, which should not be taken lightly. The general rule at Finnair is to avoid flying to places where it has been detected. For this reason, our aircraft are equipped with Windshear Detection System radars, which can detect wind shear and even alert aircraft in advance.
To conclude, turbulence is an annoying but mainly harmless phenomenon. Flight safety and passenger comfort are vital for Finnair. This is why our pilots have been trained to anticipate and avoid the worst areas.
The next time your aircraft hits turbulence, tell your neighbours how to avoid spilling their coffee, and comment on how nice it is to get a free massage while seated – at least your back won’t go numb on a long flight! You might even get to enjoy a longer conversation with them, and have the time pass quicker.
Filed under: Safety | Tagged: CAT, clear air turbulence, turbulence, wake turbulence, windshear
