Page 103 - Planning and Design of Airports

Air craft Characteristics Related to Airport Design 73

Wind Speed and Direction
Since aircraft depend on the velocity of air flowing over their wings
to achieve lift, and fly through streams of moving air, similar to ships
moving along water with currents, the direction and speed of wind,
both near the surface of airports and at altitudes have great effect on
aircraft performance.
As winds primarily affect the speed at which aircraft operate at
an airport, it is important to understand the basic difference between
two ways of measuring speed in an aircraft, groundspeed and airspeed.
The groundspeed is the speed of the aircraft relative to the ground.
True airspeed is the speed of an aircraft relative to the air flowing
over the airfoil, or wing. For example, if an aircraft is flying at a
groundspeed of 500 kn in air where the wind is blowing in the oppo-
site direction, known as a headwind, at a speed of 100 kn, the true air-
speed is 600 kn. Likewise, if the wind is blowing in the same direc-
tion, a tailwind, and the aircraft maintained a groundspeed of 500 kn,
the true airspeed would be 400 kn.
On the airport surface, the speed and direction of winds directly
affect aircraft runway utilization. For takeoff and landings, for exam-
ple, aircraft perform best when operating with the wind blowing
directly toward them, that is, with a direct headwind. Headwinds
allow an aircraft to achieve lift at slower groundspeeds, and thus
allow takeoffs and landings with slower groundspeeds and shorter
runway lengths. While wind blowing from behind an aircraft, that is,
a tailwind is preferable for aircraft flying at altitude, as they achieve
greater groundspeeds at a given airspeed, it is not preferable for take-
off or landing, for precisely the same reason. As such, airports tend to
plan and design runways so that aircraft may operate most often with
direct headwinds, and orient their primary runways in the direction
of the prevailing winds.
It is not very often the case that aircraft fly into a direct headwind
or tailwind. Moreover, it is quite common for an aircraft to takeoff or
land from an airport at such a time when the runways are not ori-
ented directly into the existing wind. When this situation occurs, air-
craft performance takes into consideration any effect of what are
known as crosswinds.
While operating in direct headwind, tailwind, or calm conditions,
the direction toward which an aircraft is pointing, or heading, is the
same direction as the aircraft is actually traveling, or tracking over the
ground. However, when operating with a crosswind, the aircraft
heading is different than its track. A common analogy to this situation
is the swimmer swimming across a river with a swift current. Even
though such a swimmer may be pointing directly to the opposite
shore of the river, he or she may end up farther downstream than
simply straight across the river, and to end up directly across the river,
the swimmer would have to point, or head, at some angle upstream.