Page 282 - Planning and Design of Airports

Geometric Design of the Airfield 243

Mix Index * Exit Range from Arrival Threshold
0–20 2000–4000
21–50 3000–5500
51–80 3500–6500
81–120 5000–7000
121–180 5500–7500

∗ Mix Index is equal to the percentage of Class C aircraft plus three an aircraft with
a maximum certified takeoff weight in excess of class D aircraft, where a class C
aircraft is an aircraft with a maximum certified takeoff weight greater than
12,500 lb and up to 300,000 lb and a class D aircraft is an aircraft with a maxi-
mum certified takeoff weight in excess of 300,000 lb.
Source: Federal Aviation Administration [5].
TABLE 6-26 Exit Range Appropriate to Runways Serving Aircraft of Different
Arrival Mix Indices, ft

operations with aircraft approach category A and B aircraft, an exit
located between 1500 and 2000 ft from the landing threshold is rec-
ommended.
Planners often find that the runway configuration and the loca-
tion of the terminal at the airport often preclude placing the exits at
locations based on the foregoing analysis. This is nothing to be
alarmed about since it is far better to achieve good utilization of the
exits than to be too concerned about a few seconds lost in occupancy
time.
When locating exits it is important to recognize local conditions
such as frequency of wet pavement or gusty winds. It is far better to
place the exits several hundred feet farther from the threshold than
to have aircraft overshoot the exits a large amount of time. The stan-
dard deviation in time required to reach exit speed is on the order
of 2 or 3 s. Therefore, if the exits were placed down the runway as
much as two standard deviations from the mean, the loss in occu-
pancy time would only be 4 to 6 s. In planning exit locations at
specific airports, one needs to consult with the airlines relative to
the specific performance characteristics of the aircraft intended for
use at the airport.
The total occupancy time of an aircraft can be roughly estimated
using the following procedure. The runway is divided into four com-
ponents, namely, flight from threshold to touchdown of main gear,
time required for nose gear to make contact with the pavement after
the main gear has made contact, time required to reach exit velocity
from the time the nose gear has made contact with the pavement and
brakes have been applied, and time required for the aircraft to turn