Page 105 - Planning and Design of Airports

Air craft Characteristics Related to Airport Design 75

crab angle x would be 10°10′. This crab angle is reduced to 0 just prior
to touchdown, so that the aircraft is appropriately pointed straight
down the center of the runway.
While aircraft operators are trained to safely operate aircraft in
these crosswind conditions, it is clearly desirable to minimize this
occurrence. Furthermore, the physical ability of an aircraft to prop-
erly land in crosswind conditions is limited by the aircraft’s weight,
landing speed, and existing winds. Often times, small aircraft cannot
safely land if crosswinds on a runway are too great. For this reason,
airports accommodating smaller, slower aircraft are often designed
with runways in several directions, to accommodate varying wind
conditions. As opposed to the primary runways that are oriented into
the prevailing winds, crosswind runways are oriented into the direc-
tion of winds occurring less frequently.
The FAA categorizes aircraft by the airspeeds at which they make
approaches to land at an airport, known as the Aircraft Approach
Category, and provides requirements to airports that runways be pro-
vided that allow for safe operation of the aircraft that use the airport
for at least 95 percent of the annual wind conditions at the airport.
The design process for estimating the number and orientation of pri-
mary, as well as crosswind runways based on the approach category
of selected aircraft is detailed in Chap. 6 of this book.

Aircraft Performance Characteristics

Aircraft Speed
Reference is made to aircraft speed in several ways. Aircraft perform-
ance data is typically made reference two airspeeds, namely, true air-
speed (TAS) and indicated airspeed (IAS). The pilot obtains his speed
from an airspeed indicator. This indicator works by comparing the
dynamic air pressure due to the forward motion of the aircraft with
the static atmospheric pressure. As the forward speed is increased so
does the dynamic pressure. The airspeed indicator works on the prin-
ciple of the pitot tube. From physics it is known that the dynamic pres-
sure is proportional both to the square of the speed and to the density
of the air. The variation with the square of the speed is taken care of by
the mechanism of the airspeed indicator, but not the variation in den-
sity. The indicator is sensitive to the product of the density of the air
and the square of the velocity. At high altitudes the density becomes
smaller and thus the indicated airspeed is less than the true airspeed.
If the true airspeed is required, it can be found with the aid of
tables. As a very rough guide, one can add 2 percent to the indicated
speed for each 1000 ft above sea level to obtain true airspeed.
The indicated airspeed is of more importance to the pilot than is
the true airspeed. The concern is with the generation of lift, specifically