Why can't airplanes avoid turbulence?

Turbulence

Every pilot can explain to his passengers that turbulence has nothing to do with a vacuum and that the term “air hole” is therefore nonsense. Abrupt vertical accelerations in three-dimensional space are part of flying and for some pilots even have the appeal of a fairground ride. However, before you invite friends and relatives to a sightseeing flight, you should consider that they usually have less fun with the wobbling. The question of experiences in the roller coaster or on the high seas must be allowed, because the causes of sensitive reactions of the stomach are the same on land, at sea and in the air.

The discomfort is triggered by contradicting information from the balance organ in the semicircular canals of the ear and eyes. Getting used to it helps, however, so that an unpleasant experience on the first flight does not have to lead to flight abstinence. But even those who tolerate the bobbing will usually find a smooth flight more pleasant - even if it is because you hit all the switches straight away and can eat and drink without accidents. Anyone who knows the causes of turbulence has the chance to do so to avoid. Basically, a distinction can be made between two different pathways: horizontal air currents and vertical updrafts and downdrafts. High wind speeds alone - i.e. the rapid horizontal movement of air - do not initially affect the comfort of flying at all. Even in a jet stream that sweeps high over the Atlantic at more than 500 kilometers per hour, the planes lie calmly like an air mattress on a bathing lake. It becomes problematic when the rapidly moving air mass borders on something that is not moving. It can wobble briefly when entering and exiting the jet stream. One then speaks of Clear Air Turbulence (CAT). The cause is edge eddies that form at the boundary between fast air currents and stationary air mass, such as the eddies on the bank of a mountain stream.

Vortices close to the ground

The same applies to the friction layer close to the ground between the flowing air mass and the ground. Even over a flat surface like water or the flat Netherlands, high wind speeds at low altitudes lead to a stubborn flight path. The turbulence in hilly or even mountainous terrain becomes much more intense when the air flow can be deflected from obstacles in the ground up to great heights and swirled accordingly.

Even an approach over hilly terrain to runway 28 in Nuremberg, which has elevations of up to 2000 feet above the ground, makes even airliners dance at high wind speeds. Salzburg or Innsbruck, with their location on or in the high mountains, have completely different requirements when there is a storm. Even with a wind of 20 knots at peak height, a trip to the Alps is hardly recommended: What is part of everyday flying in the flatlands can cause downdrafts between mountain and valley, which a small aircraft with its low climbing performance can no longer escape.

The higher the mountain flanks, the higher the layer of air above it, which is also influenced. In principle, however, the following applies: The higher you fly, the sooner the air is free of turbulence caused by ground friction. An extreme case of wind deflection by obstacles is the leeward wave, which can form with stable stratification and stronger winds - often on mountain ranges perpendicular to the direction of the wind. The updrafts of a wave can reach up to 40,000 feet - they have carried glider pilots up to this altitude. The typical lens cloud (Altocumulus lenticularis) forms at the vertex of the wave.

One's Freud ’...

Even flying through these strong, but unchanged updrafts can be uncomfortable. But it is really turbulent on the leeward side of the wave: There rotors, huge gust rollers form, which aircraft can turn upside down. If there is enough moisture, rotors can also be seen in small cumulus clouds that form in and above them.

Without any wind, the instability of the air mass influences the calmness when flying. The clear weather on the back of a cold front, which is actually ideal for flying, is typical. But when the infiltrated, unstably stratified cold air begins to heat up, the different warming of the air, for example over cornfields or quarries, creates columns of updraft, at the upper ends of which cumulus clouds can form. This is the thermals that glider pilots are enthusiastic about. They turn their circles in it and thus gain height. Outside the updraft areas, the air sinks again.

The ascent to calmer heights can be blocked by clouds

Motorized pilots crossing these updrafts and downdrafts experience them only as constant blows, alternating from below and above. The escape upwards is also the way out here: thermal updrafts are limited in height. If the air mass is dry, no clouds form (blue thermals), and VFR can also be easily avoided in calmer waters. Cumulus clouds, on the other hand, can make the ascent impossible. In the case of highly moisture-labile stratification, the updrafts are so strong that cumulonimbus clouds (Cb) form - often together with thunderstorms.

In the vicinity of these clouds, underneath and above all in them, the elevator really goes off. Updrafts of over 60 knots are opposed to downdrafts of the same speed. Anyone who gets in here is literally on a suicide mission. This turbulence can burden aircraft beyond all structural limits. In addition, severe damage is often caused by hailstones, which in a Cb can grow to the size of tennis balls. We have no business here - a minimum distance of 10 to 20 miles should be observed.

It gets better with age

If you just manage to reach your destination before a thunderstorm is approaching, you shouldn't feel safe. Squalls in front of a front can create tremendous turbulence. Microbursts, which can hardly be controlled with a sink rate of up to 5000 feet, are particularly dangerous. Rain curtains that do not reach the floor or swirls of dust on the floor are suspect. If the tower is constantly changing wind directions and strengths, extreme caution is required.

Changes in wind direction and speed can also occur at inversion limits as well as due to the terrain due to local wind systems. Wind shear is when the airspeed indicator oscillates more than 15 knots or when the rate of descent oscillates more than 500 feet per minute when the boat is stable. If wind shear is to be expected, the power on approach must not be reduced in the event of an increase in speed. A short-term increase in speed, at least up to the maximum speed for the respective flap position, is accepted in order to build up reserves for the subsequent loss of speed. The best way to deal with turbulence is to avoid it. They not only burden the aircraft, but also its occupants. Little consolation: The tendency to be sensitive to movement decreases with advancing age.

Text: Helmuth Lage, Photos: Rolf Stünkel, Helmuth Lage (2), Richard Heinrich

  • Turbulence
  • turbulence
  • Leeward turbulence
  • Vortices close to the ground
  • Cumulus
  • Cumulus clouds
  • Cumulonimbus
  • Gust roller
  • Gusts
  • Inversion layer