Turbulence

"Any fool can carry on but a wise man knows how to shorten sail in time."

-Joseph Conrad-

Introduction

Turbulence is a natural phenomenon in which every plane is built to fly. It cannot cause a plane to fall. Turbulence is air movement that normally cannot be seen. It may occur when the sky appears to be clear and can happen unexpectedly. It can be created by any number of different conditions, including atmospheric pressures, jet streams, mountain waves, cold or warm fronts, or thunderstorms.

For our purposes, we'll look at what you need to know about meteorology, and of course turbulence which is a part of that. So are thunderstorms which aircraft can be directed to avoid, or, if over an airport or nearby, will keep aircraft on the ground, or circling some distance away, which may serve to delay your flight. Movement of air depends on heating of the surfaces during the day, the ability of the land to hold the heat at night, local terrain which can cause the air to tunnel between mountain ranges for instance or cause "eddies" as they roll over the mountain tops.

Causes of Turbulence

To understand and predict turbulence in advance the pilot should know the occasions where he should expect turbulence. Following are the causes of turbulence.

1. Thermal turbulence. Heat from the sun makes warm air masses rise and cold ones sink. Some surfaces absorb and radiate energy at a much higher rate than others. This creates a light form of thermal turbulence. Common when two areas of land heat at different rates (such as a large parking lot next to a field). When an aircraft flies into this it may experience vertical shifts as it moves over the two areas boundaries. This is also called convective turbulence

2. Jet streams or Clear Air Turbulence. Jet streams are fast winds over slow winds in neutral or unstable environment will cause turbulent eddies within clear atmosphere (no cloud signs), often found in the vicinity of the jet stream core. There are several notable problems with clear air turbulence. It cannot always be foreseen so there is no warning. It is usually felt at its mildest in the flight deck and is generally more severe in the aft section. It can occur when no clouds are visible. It is common at high altitudes, where cruising airline suddenly enter turbulent areas. Turbulence is the leading cause of in-flight injuries. There are countless reports of occupants who were seriously injured while moving about the passenger cabin when clear air turbulence is encountered.

3. Mountain turbulence. Air passes over mountains and causes turbulence as it flows above the air on the other side. This type of turbulence is dangerous for aircraft flying in hilly terrain and landing at airfields at high terrain areas. It is mostly found on the lee side of the mountain and can range from 500 feet to few thousand feet above the mountain top.

4. Wake turbulence. Wake turbulence is turbulence that forms behind an aircraft as it passes through the air. This turbulence includes various components, the most important of which are wingtip vortices and Jet wash. Jet wash refers simply to the rapidly moving gases expelled from a jet engine; it is extremely turbulent, but of short duration. Wingtip vortices, on the other hand, are much more stable and can remain in the air for up to three minutes after the passage of an aircraft. Wingtip vortices make up the primary and most dangerous component of wake turbulence.

5. Microburst. Microburst are strong downward rush of winds from a mature thunderstorm. They cause serious takeoff and landing hazards. The aircraft is most vulnerable close to the ground and operating near stall speed. The size of the event is typically less than 4 kilometers across. Microburst are capable of producing winds of more than 100 mph causing significant damage. The life span of a microburst is around 5-15 minutes. The turbulence caused by this downward column of air is called microburst. This phenomena is similar to the turbulence experienced crossing close to the thunderstorm.

Different intensities of turbulence

Turbulence can be divided in different categories according to it severity. They are classified so that it is easier for the pilots to report turbulence to air traffic control and device the corrective actions accordingly. The different intensities are

Light turbulence. It briefly causes slight, erratic changes in altitude and/or attitude.

Light chop. It is slight, rapid and somewhat rhythmic bumpiness without noticeable changes in altitude or attitude.

Moderate turbulence. It is similar to light turbulence, but greater intensity. Changes in altitude/attitude occur. Aircraft remains in control at all times. Variations in indicated air speed.

Moderate chop. It is similar to light chop, but greater intensity. Rapid bumps or jolts without obvious changes in altitude or attitude.

Severe turbulence. This includes large, abrupt changes in altitude/attitude. Large variation in indicated airspeed. Aircraft may be temporarily out of control.

Extreme turbulence. In this type, aircraft is violently tossed about and is impossible to control. This may cause structural damage.

Effect of speed on turbulence

The reactions inside aircraft vary from occupants feeling slight strain against their seat belts and unsecured items being slightly displaced, through to occupants being forced violently against seat-belts, and unsecured items being tossed about. (Imagine what it would be like if you were not wearing a seat belt!). Knowing the causes and intensities of turbulence now we will understand the effect of speed and use of controls and pilot techniques when the aircraft encounters turbulence.

Why to reduce speed in turbulence

An aircraft can encounter turbulence with or without any warning.

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