Page 279 - Planning and Design of Airports

240 Airp o r t D e sign

in providing an understanding of the significant parameters affecting
location, their usefulness to planners has been limited because of the
complexity of the analyses and a lack of knowledge of the inputs
required for the application of the models. As a result greater use is
made of much more simplified methods.
The landing process can be described as follows. The aircraft
crosses the runway threshold and decelerates in the air until the
main landing gear touches the surface of the pavement. At this
point the nose gear has not made contact with the runway. It may
take as long as 3 s to do so. When it does, reverse thrust or wheel
brakes or a combination of both are used to reduce the forward
speed of the aircraft to exit velocity. Empirical analysis has revealed
that the average deceleration of air-carrier aircraft on the runway
is about 5 ft/s 2.
In the simplified procedure, an aircraft is assumed to touch down
at 1.3 times the stall speed for a landing weight corresponding to 85
percent of the maximum structural landing weight. In lieu of com-
puting the distance from threshold to touchdown, touchdown
distances are assumed as fixed values for certain classes of aircraft.
Typically these values range from 500 to 1500 ft from the runway
threshold. To these distances are added the distances to decelerate to
exit speed. These relationships may be approximated by Eqs. (6-6)
and (6-7).

D = D + D (6-6)
td e
where D = distance from the runway threshold to the exits
D = distance from the runway threshold to the point where the
td
aircraft touches down
D = the distance from the touchdown point to the exit
e
V − V 2
2
D = td e (6-7)
e
a 2
where V = aircraft speed at touchdown
td
V = exit speed of the aircraft
e
a = deceleration of the aircraft on the runway
Although approach and touchdown speeds vary, they can be
approximated for locating exit taxiways. At predominantly air carrier
airports air traffic control authorities request general aviation aircraft
to increase their speeds above normal to reduce the wide range in
speed between air carriers and general aviation. At these airports,
the normal approach speeds for general aviation are probably not
applicable.
If it is assumed that the distances to touchdown are 1500 ft for air
carrier aircraft and 1000 ft for twin-engine general aviation aircraft, a