Page 110 - Planning and Design of Airports

80 Airp o r t Pl anning

conditions are temperature, surface wind, runway gradient, altitude
of the airport, and condition of the runway surface.

Field Elevation
All other things being equal, the higher the field elevation of the air-
port, the less dense the atmosphere, requiring longer runway lengths
for the aircraft to get to the appropriate groundspeed to achieve suf-
ficient lift for takeoff. This increase is not linear but varies with the
weight of the aircraft and with the ambient air temperature.
At higher altitudes the rate of increase is higher than at lower
altitudes. For planning purposes, it can be estimated that between
sea level and 5000 ft above sea level, runway lengths required for a
given aircraft increases approximately 7 percent for every 1000 ft of
increase in elevation, and greater under very hot temperatures
those that experience very hot temperatures or are located at higher
altitudes, the rate of increase can be as much as 10 percent. Thus,
while an aircraft may require 5000 ft of runway to takeoff at an air-
port at sea level, the same aircraft may require 7500 ft or more at an
airport 5000 ft above sea level, especially during periods of high
temperatures.

Surface Wind
Wind speed and direction at an airport also have a significance on
runway length requirements. Simply, the greater the headwind the
shorter the runway length required, and the greater the tailwind the
longer the runway required. Further, the presence of crosswinds will
also increase the amount of runway required for takeoff and landing.
From the perspective of the planner, it is often estimated that for
every 5 kn of headwind, required runway length is reduced by
approximately 3 percent and for every 7 kn of tailwind, runway
length requirements increase by approximately 7 percent. For airport
planning purposes runway lengths are often designed assuming calm
wind conditions.
Runway Gradient
To accommodate natural topographic or other conditions, runways
are often designed with some level of slope or gradient. As such, air-
craft operating for takeoff on a runway with an uphill gradient
requires more runway length than a level or downhill gradient, the
specific amount depending on elevation of the airport and tempera-
ture. Conversely, landing aircraft require less runway length when
landing on a runway with an uphill gradient, and more length for a
downhill gradient.
Studies that have been made indicate that the relationship
between uniform gradient and increase or decrease in runway length
is nearly linear [55]. For turbine-powered aircraft this amounts to 7 to
10 percent for each 1 percent of uniform gradient. Airport design