Page 143 - Planning and Design of Airports

112 Airp o r t Pl anning

of maintaining aircraft separations aircraft are classified by the FAA
as heavy, large, or small based upon their maximum gross takeoff
weight (MGTOW). Heavy aircraft are classified as those aircraft
which have a MGTOW of 300,000 lb or more. Large aircraft are as
those aircraft which have a MGTOW of in excess of 12,500 lb but less
than 300,000 lb. Small aircraft are as those aircraft which have a
MGTOW of 12,500 lb or less. Aircraft size is related to wake turbu-
lence. Heavy aircraft create trailing wake vortices which are a hazard
to lighter aircraft following them.
The minimum longitudinal separations en route are expressed in
terms of time or distance as follows:

1. For en route aircraft following a preceding en route aircraft, if
the lead aircraft maintains a speed at least 44 kn faster than
the trail aircraft, 5 mi between aircraft using distance measur-
ing equipment (DME) or area navigation (RNAV) and 3 min-
utes between all other aircraft
2. For en route aircraft following a preceding en route aircraft, if
the lead aircraft maintains a speed at least 22 kn faster than
the trail aircraft, 10 mi between aircraft using DME or RNAV
and 5 min for all other aircraft
3. For en route aircraft following a preceding en route aircraft, if
both aircraft are at the same speed, 20 mi between aircraft
using DME or RNAV and 10 min for all other aircraft
4. When an aircraft is climbing or descending through the alti-
tude of another aircraft, 10 mi for aircraft using DME or
RNAV if the descending aircraft is leading or the climbing
aircraft is following and 5 min for all other aircraft
5. Between aircraft in which one aircraft is using DME or RNAV
and the other is not, 30 mi
Minimum longitudinal separations over the oceans is normally
10 minutes for supersonic flights and 15 minutes for subsonic flights
but in some locations it can be slightly more or less than this value [3].
When the aircraft mix is such that wake turbulence is not a factor
and radar coverage is available, the minimum longitudinal separa-
tion for two aircraft traveling in the same direction and at the same
altitude is 5 nm, except that when the aircraft are in the terminal envi-
ronment within 40 nm of the radar antenna the separation can be
reduced to 3 nm. For this reason the minimum spacing in the termi-
nal area is 3 nm because the airport is almost always within 40 nm of
a radar antenna. Under certain specified conditions a separation
between aircraft on final approach within 10 nm of the landing run-
way may be reduced to 2.5 nm [3].
If wake turbulence is a factor, the minimum separation in the ter-
minal area between a small or large aircraft and a preceding heavy