Page 120 - Planning and Design of Airports

Air craft Characteristics Related to Airport Design 89

For example, for an aircraft operating on a relatively short runway,
a lower V may be selected, which will allow for a shorter accelerate-
1
stop distance, but would require at least some clearway to allow for
the aircraft to safely climb out to 35 ft. Conversely, for relatively long
runways that may have obstacles near the runway’s end, or for run-
ways with less full-strength pavement but a stopway at the runway
end, a higher V may be selected, to allow for steeper climb-out under
1
engine-failure conditions, and the ability to accommodate a longer
accelerate-stop distance.
Thus, one can see that the regulations pertaining to turbine-
powered aircraft offer a number of alternatives to the aircraft opera-
tor. It should be emphasized that the takeoff distance and the takeoff
run for the engine-failure case must be compared with the corre-
sponding distance for the normal all engine takeoff case. The longer
distance always governs. A further discussion of these concepts is
presented by ICAO [1].
Both aircraft operators and airport planners are interested in
clearways, because clearways will, for a fixed available length of run-
way, allow the operator additional gross takeoff weight with less
expense to airport management than building full-strength pavement
would require.

Wingtip Vortices
Whenever the wings lift an aircraft, vortices form near the ends of
the wings. The vortices are made up of two counter-rotating cylin-
drical air masses about a wingspan apart, extending aft along the
flight path. The velocity of the wind within these cylinders can be
hazardous to other aircraft encountering them in flight. This is par-
ticularly true if a lighter aircraft encounters a vortex generated by a
much heavier aircraft. The tangential velocities in a vortex are
directly proportional to the weight of the aircraft and inversely pro-
portional to the speed. The more intense vortices are therefore gen-
erated when the aircraft is flying slowly near an airport [52]. The
winds created by vortices are often referred to as wake turbulence or
wake vortex.
Once vortices are generated they move downward and drift lat-
erally in the direction of the wind. The rate at which vortices settle
toward the ground is dependent to some extent on the weight of an
aircraft, the heavier the vehicle the faster the vortex will settle.
About one wingspan height above the ground the vortices begin to
move laterally away from the aircraft, as shown in Fig. 2-10. The
duration of a vortex is dependent to a great extent on the velocity of
the wind. When there is very little or no wind they can persist for
longer than 2 min. As a result of these tests, the FAA and ICAO
divide aircraft into three classes for the purposes wake-turbulence
separation minima.