streams”. The most dangerous type of turbulence is called “Clear Air Turbulence" (CAT). CAT cannot be seen and it is very difficult to forecast. One aircraft may experience it while another in the vicinity will not. The best source of information regarding CAT is communication with other aircraft flying on the same route.
Volcanic Ash Clouds The outpouring of volcanic dust into the atmosphere after a volcanic eruption can give rise to widespread areas of the dust suspended in the atmosphere which can persist for several days and extend for many miles from the originating volcano. The dust can cause jet engines to fail and is a real hazard to flight. Pilots will steer clear of these clouds wherever possible.
Sandstorms Sandstorms may also pose a hazard to flight. They can create visibility problems for take-off and landing. Lightning
The movement of rapidly rising and falling parcels of air inside storm clouds produces dense build-ups of static electricity of up to 100,000 volts per metre. Localized cells exist of up to 1,000,000 volts per metre. This magnitude of static charge is strong enough to overcome the normally poor conductivity of air, and is suddenly released. This is seen as a lightning discharge, which can occur from cloud-to-cloud or cloud-to-land.
Lightning rarely strikes an aircraft. When it does it can be alarming, but the damage caused is usually minor thanks to elaborate bonding structures which helps prevent the electrical discharge from penetrating the interior.
Winds
The result of different areas of pressure to equalize, create a movement of air called wind. Crewmembers should be aware of the two types of wind: the surface wind, and winds aloft.
The surface wind is the direction and velocity of the wind at the departure and destination airport. The surface winds dictate which runway will be utilized for takeoff and landing. It is possible that an air port will not be usable for takeoff or landing because the surface winds are outside the performance capability of the aircraft. Surface winds are expressed in com pass degrees and velocity, i.e. 150 at 15, meaning 150 degrees from true north at 15 knots.
Wind aloft is the direction and velocity of the wind at altitude. These winds create either a headwind or a tailwind affect upon the aircraft. The head wind component de creases the aircraft’s speed over the ground while the tailwind component increases the aircraft’s speed over the ground. That is why going Westbound in the U.S. normally takes longer than the same trip Eastbound, as the prevailing winds aloft are Westerly.
11 Aviation Basics
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