WX BRIEF

Flying in turbulent skies

How to turn wind buffets into a breeze.

By Karsten Shein
Comm-Inst, Climate Scientist


NetJets Europe Beech 1900D lands at LTN (Luton, London, England). Notice aircraft has right wing down as pilot crabs to handle crosswind.

As the new year of 1998 ap­proach­ed, a Boeing 747-122 operating as United Airlines flight 826 was winging its way from NRT (Narita, Tokyo, Japan) to HNL (Honolulu, Oahu HI).

About 90 min into the flight, while cruising at FL 310, the flightcrew began to en­coun­ter some minor clear air turbulence (CAT), although no turbulence Sigmets had been issued for their route.

While they directed passengers to fasten their seatbelts due to the bumpy air, the force with which the extreme turbulence slammed the 747 was far stronger than anticipated. In the subsequent few moments, the aircraft experienced an upward load of 1.8 G, a 0.1 G lateral load and 15 kts of windshear.

Within seconds, a –0.8 G load resulted as the turbulence pushed the aircraft down some 100 ft. The 2 wave encounters lasted only a few seconds, but 161 passengers and crew sustained injuries, including one who later died. Warning systems also suggested the possibility of structural damage to the aircraft.

In this incident, the pilots did everything right in order to ensure the safety of their aircraft and its occupants. Unfortunately, despite their best efforts, the accident highlights the difficulty we still face when it comes to forecasting and steering clear of turbulent air.

What is turbulence?

Turbulence has been known to aviation since early balloon pilots felt the jostling of the wind or the rising air from heat-producing fallow fields beneath them, but as balloons move easily with air currents, strong turbulence was relatively unknown unless the balloon drifted too close to a thunderstorm.

As aviation developed into powered flight, the concept of turbulence as an adverse effect on flight became more important. Pilots flying near convective activity reported sudden unexpected drops in altitude.

These were given the lay term “air pockets,” as if the aircraft had suddenly encountered a space with no air to keep it flying. Only later, when fluid dynamics theories were applied to the atmosphere, did it become clear that these air pockets were in fact the force of the air coming at the aircraft from an unexpected direction.

Weather model chart of forecast winds aloft around the jet stream at 250 mb (about FL 350) over the North Pacific on Mar 1, 2009. The white areas (T) are jet streaks, where winds are forecast to 150 kts or more. Those areas also correspond well with regions of moderate to extreme turbulence (B).

To understand turbulence, however, one needs to know that all turbulence is due to atmospheric shear.

In other words, what we encounter as turbulence is nothing more than us flying through a boundary between air moving in one direction at a certain speed and air moving either in another direction or at another speed—or both.

Along that boundary, known as a shear boundary, the air flow creates friction and mixing. To visualize a shear boundary, simply watch a lake or ocean on a windy day.

The friction created between the moving air and the underlying water results in waves on the water surface. The air is moving fast by comparison with the water—and, as the wind gets stronger, so does that difference, and the resulting waves get bigger.

The amplitude of the waves defines a broader area of affected space known as a shear zone. In the atmosphere, the differences in atmospheric flow speed and/or direction between adjacent bodies of air will also result in waves that tend to amplify with increasing shear strength.

A shear boundary where there is a difference of only 3 kts will likely have only light chop, while one with a 20-kt change may have large waves and strong turbulence. Another consideration is that a shear boundary can occur in any direction—horizontally, vertically or both.

In situations where one air flow sits atop another, the shear and the waves tend to be horizontal. In convective situations, such as near fronts or thunderstorms, there will be a primary vertical component since the air is rising.

Horizontal shear is more likely to cause vertical motions, while vertical shear is more often the culprit in uncommanded rolls. However, the actual orientation of the wave, and the manner in which your aircraft encounters it, are what dictate the effect the wave will have on your aircraft.

Most likely you will encounter a combination of both vertical and lateral forces, especially with stronger waves. This is especially true when the waves devolve into turbulent eddies.

Eddies are vortices created as the forces associated with the shear continue to deform the wave into more of a spiral. We can see this in the turbulence created by wake vortices from landing aircraft.

Turbulence types

Turbulence can be divided into 2 basic types—thermal and mechanical. The terms refer to how the shear zone was created in the first place—knowledge that can be helpful in determining where turbulence may be encountered and how strong it may be.

Thermally-induced turbulence is by far the most common type of turbulence encountered in aviation. It’s the bumpiness you feel as you descend the glidepath over freshly plowed fields on a warm summer afternoon.

It’s also the type of turbulence you’d encounter in the vicinity of a convective cell. In general, thermal turbulence is simply the product of uneven heating of Earth’s surface by the Sun. Darker patches of the surface absorb more radiation than lighter ones, and become warmer.

The warmth is transferred to the overlying air which, as it heats, expands and becomes less dense than the air surrounding it. Unless it is capped by an inversion or some other block above it, the warmed air will rise.

As it moves upward, it creates a shear boundary with the air through which it is rising. In an unstable situation such as this, the surface air does not have a chance to gain too much heat energy before it rises and cools.

Thus, ascension is relatively gradual and the resulting turbulent ripples are not going to be too great. However, when the unstable surface air is prevented from rising by conditions above it, it will continue to heat until it can overcome the barrier keeping it near the ground.

When the “cap” is broken, the air will rise very rapidly—up to several thousand feet per minute—and the waves and eddies of the resulting vertical shear zone may become intense. In addition, when sufficient moisture is present in the surface air, a capped situation often results in the development of a thunderstorm.

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