Page 102 - Planning and Design of Airports

72 Airp o r t Pl anning

condition is one in which the actual temperature and pressure cor-
respond to the standard temperature and pressure at a particular
altitude. When reference is made to the temperature being “above
standard” it means that the temperature is higher than the standard
temperature.
As aircraft takeoff performance data is typically related to the
local barometric pressure and ambient air temperature, which in turn
affects the density of the air, a defined value known as density altitude
is often used to estimate the density of the air at any given time. Den-
sity altitude is a function of the effect of barometric pressure on air
density, defined through the measurement known as pressure alti-
tude, and the ambient temperature.
Assuming that at a standard day at sea level, where the elevation
above sea level is effectively 0, the density altitude on a standard day
would also be 0. If the barometric pressure was less than the standard
pressure of 29.92 inHg, the pressure altitude would be greater than 0.
Conversely, if the barometric pressure was greater than standard
pressure, the pressure altitude would be less than 0. This relates to the
fact that, when the atmospheric pressure drops, the air becomes less
dense, requiring a longer run on the ground to obtain the same
amount of lift as on a day when the pressure is high. Thus a reduction
in atmospheric pressure at an airport has the same effect on its air
density as if the airport had been moved to a higher elevation. Pressure
altitude is defined as the altitude corresponding to the pressure of the
standard atmosphere. Thus if the atmospheric pressure is 29.92 inHg,
the pressure altitude is 0. If the pressure drops to 28.86 inHg, the pres-
sure altitude is 1000 ft. This can be obtained from the formula relating
pressure and temperature. If this lower pressure occurred at a sea
level airport, the geographic altitude would be 0, but the pressure
altitude would be 1000 ft. For airport planning purposes, it is satisfac-
tory to assume that the geographic and pressure altitudes are equal
unless the barometric pressures at a particular site are unusually low
a great deal of the time.
Density altitude is defined as pressure altitude adjusted for tem-
perature. Similar to the effect of barometric pressure on aircraft per-
formance, if the temperature of the air was greater than standard
temperature, the density of the air would be lower and the density
altitude would increase, and if the temperature were lower than
standard, the density altitude would decrease. It is because of the
effect of both barometric pressure and ambient air temperature on
aircraft performance that airports located at high elevations, where
air pressure is generally lower than at sea level, and in locations
where the ambient air temperature often rises well above 59°F, are
airports constructed with longer runways, as longer runways are
required for aircraft to reach needed airspeeds to get sufficient lift for
takeoff, than at sea level elevations, or when temperatures are lower.