The Hindu Business Line : Air turbulence can be dangerous at times

Generally, no. But if you ever find yourself in an aircraft trying to land in big storm you do have reason to be concerned, says Mr N. Jayachandran, an expert in air safety.

Making a presentation during a weekly talk programme hosted here by the Kerala State Centre of the Institution of Engineers, he said aircraft manage to make hundreds of safe landings and takeoffs every day in bad weather.

Still, if unusually bad weather is predicted for your destination, delay your flight. But most likely the airline will delay it - or divert it - for you.

A plane moves through air that is itself moving. Moving air flows in ways that are quite similar to moving water, only we cannot see the currents. Air currents vary, and flying rapidly from one current to another is what leads to the feeling of turbulence.

Some people call this "hitting an air pocket", but this description is a misnomer. The jolt comes not from falling into a "hole" in the air but from crossing a barrier between different currents.

There are several causes of turbulence: Convective currents result from the sun heating the ground, causing air to rise. As the air rises, it cools and forms clouds - those fluffy cumulus clouds are boiling with activity inside. Hence pilots always look for smooth air above the clouds where the convection stops.

After sunset the air is generally much smoother because of a lack of convective activity. But other forms of turbulence can occur even at night: Wind shear occurs at the boundary between winds that differ in speed or direction, or both.

A common occurrence near temperature inversions and at the border between weather fronts, this kind of turbulence is most likely encountered in a large aircraft at high altitudes.

Although turbulence is generally not dangerous, it cannot be said that it is never dangerous. The first real danger of turbulence is of structural failure of the aircraft.

The second one is of passenger injury. When an aircraft flies into downward-moving air, it will drop height with the air. Anything not securely attached to the plane - such as passengers who are not strapped to their seats - can get thrown around the cabin.

The third danger of turbulence is altitude loss near to the ground. Sudden altitude changes can also be a consequence of flying in turbulent air. When you are thousands of feet above the ground, a few hundred feet of altitude loss doesn't count for much.

But if the plane is only a few hundred feet above the ground, as when it is in the process of taking off or landing, a few hundred feet of altitude loss can make all the difference in the world, terribly so.

Severe turbulence near the ground is usually the result of one of two things. Wake turbulence occurs when an aircraft leaves a trail of disturbed air behind it simply as a result of its flying through the air.

The turbulence poses no danger to the aircraft itself, but any other aircraft following too closely behind it can fly into the turbulent air and lose control. For this reason, air traffic controllers maintain strict limits of spacing between departing and arriving aircraft.

The fourth real danger of turbulence is that wind changes near the ground. If the aircraft is just about to land and suddenly the wind changes to a tail wind, the plane can actually get "blown" right off the end of the runway.