Runways
A runway is a rectangular area on the airport surface prepared for the takeoff and landing of aircraft. An airport may have one runway or several runways which are sited, oriented, and configured in a manner to provide for the safe and efficient use of the airport under a variety of conditions. Several of the factors which affect the location, orientation, and number of runways at an airport include local weather conditions, particularly wind distribution and visibility, the topography of the airport and surrounding area, the type and amount of air traffic to be serviced at the airport, aircraft performance requirements, and aircraft noise .
Runway Configurations
The term ?runway
configuration? refers to th orientations of one or more runways on an airfield.
Many runway configurations exist. Most configurations are combinations of
several basic configurations. The basic configurations are (1) single runways, (2)
parallel runways, (3) intersecting runways, and (4) open-V runways.
Single Runway
It has been estimated that the hourly capacity of a
single runway in VFR conditions is somewhere between 50 and 100 operations per
hour, while in IFR conditions this capacity is reduced to 50 to 70 operations
per hour, depending on the composition of the aircraft mix and navigational
aids available .
Parallel Runways
The capacities of parallel runway
systems depend on the number of runways and on the spacing between the runways.
Two, three, and four parallel runways are common. The spacing between parallel runways
varies widely. For the purpose of this discussion, the spacing is classified as
close, intermediate, and far, depending on the centerline separation between
two parallel runways. Close parallel runways are spaced from a minimum of 700
ft (for air carrier airports) to less than 2500 ft . In IFR conditions an
operation of one runway is dependent on the operation of other runway.
Intermediate parallel runways are spaced between 2500 ft to less than 4300 ft . In IFR conditions an arrival on one runway is independent of a departure on
the other runway. Far parallel runways are spaced at least 4300 ft apart .
In IFR conditions the two runways can be operated independently for both
arrivals and departures. Therefore, as noted earlier, the centerline separation
of parallel runways determines the degree of interdependence between operations
on each of the parallel runways. It should be recognized that in future the
spacing requirements for simultaneous operations on parallel runways may be
reduced. If this occurs, new spacing can be applied to the same
classifications.
If the terminal
buildings are placed between parallel runways, runways are always spaced far
enough apart to allow room for the buildings, the adjoining apron, and the appropriate
taxiways. When there are four parallel runways, each pair is spaced close, but
the pairs are spaced far apart to provide space for terminal buildings. In VFR
conditions, close parallel runways allow simultaneous arrivals and departures,
that is, arrivals may occur on one runway while departures are occurring on the
other runway. Aircraft operating on the runways must have wingspans less than
171 ft (airplane design groups I through IV, see Table 6-2) for centerline
spacing at the minimum of 700 ft . If larger wingspan aircraft are operating
on these runways (airplane design groups V and VI), the centerline spacing must
be at least 1200 ft for such simultaneous operations . In either case, wake
vortex avoidance procedures must be used for simultaneous operations on closely
spaced parallel runways. Furthermore, simultaneous arrivals to both runways or
simultaneous departures from both runways are not allowed in VFR conditions for
closely spaced parallel runways. In IFR conditions, closely spaced parallel
runways cannot be used simultaneously but may be operated as dual-lane runways.
Intermediate parallel runways may be operated with simultaneous arrivals in VFR
conditions. Intermediate parallel runways may be operated in IFR conditions
with simultaneous departures in a nonradar environment if the centerline
spacing is at least 3500 ft and in a radar environment if the centerline
spacing is at least 2500 ft . Simultaneous arrivals and departures are also
permitted if the centerline spacing is at least 2500 ft if the thresholds of
the runways are not staggered . There are times when it may be desirable to
stagger the thresholds of parallel runways. The staggering may be necessary
because of the shape of the acreage available for runway construction, or it
may be desirable for reducing the taxiing distance of takeoff and landing
aircraft. The reduction in taxiing distance, however, is based on the premise
that one runway is to be used exclusively for takeoff and the other for
landing. In this case the terminal buildings are located between the runways so
that the taxiing distance for each type of operation (takeoff or landing) is
minimized. If the runway thresholds are staggered, adjustments to the
centerline spacing requirement are allowed for simultaneous arrivals and
departures . If the arrivals are on the near threshold then the centerline
spacing may be reduced by 100 ft for each 500 ft of threshold stagger down to a
minimum centerline separation of 1000 ft for aircraft with wingspans up to 171
ft and a minimum of 1200 ft for larger wingspan aircraft. If the arrivals are
on the far threshold the centerline spacing must be increased by 100 ft for
each 500 ft of threshold stagger. Simultaneous arrivals in IFR conditions are
not permitted on intermediate parallel runways but are permitted on far
parallel runways with centerline spacings of at least 4300 ft .
The hourly capacity of
a pair of parallel runways in VFR conditions varies greatly from 60 to 200
operations per hour depending on the aircraft mix and the manner in which
arrivals and departures are processed on these runways . Similarly, in IFR
conditions the hourly capacity of a pair of closely spaced parallel runways
ranges from 50 to 60 operations per hour, of a pair of intermediate parallel
runways from 60 to 75 operations per hour, and for a pair of far parallel
runways from 100 to 125 operations per hour .
A dual-lane parallel
runway consists of two closely spaced parallel runways with appropriate exit
taxiways. Although both runways can be used for mixed operations subject to the
conditions noted above, the desirable mode of operation is to dedicate the
runway farthest from the terminal building (outer) for arrivals and the runway closest
to the terminal building (inner) for departures. It is estimated that a
dual-lane runway can handle at least 70 percent more traffic than a single
runway in VFR conditions and about 60 percent more traffic than a single runway
in IFR conditions. It is recommended that the two runways be spaced not less
than 1000 ft apart (1200 ft, where particularly larger wingspan aircraft are
involved). This spacing also provides sufficient distance for an arrival to
stop between the two runways. A parallel taxiway between the runways will
provide for a nominal increase in capacity, but is not essential. The major
benefit of a dual-lane runway is to provide an increase in IFR capacity with
minimal acquisition of land.
Intersecting Runways
Many airports have two
or more runways in different directions crossing each other. These are referred
to as intersecting runways. Intersecting runways are necessary when relatively
strong winds occur from more than one direction, resulting in excessive
crosswinds when only one runway is provided. When the winds are strong, only
one runway of a pair of intersecting runways can be used, reducing the capacity
of the airfield substantially. If the winds are relatively light, both runways
can be used simultaneously. The capacity of two intersecting runways depends on
the location of the intersection (i.e., midway or near the ends), the manner in
which the runways are operated for takeoffs and landings, referred to as the
runway use strategy, and the aircraft mix. The farther the intersection is from
the takeoff end of the runway and the landing threshold, the lower is the
capacity. The highest capacity is achieved when the intersection is close to
the takeoff
and landing threshold.
Open-V Runways
Runways in different
directions which do not intersect are referred to as open-V runways. This
configuration is shown in Fig. 6-4. Like intersecting runways, open-V runways
revert to a single runway when winds are strong from one direction. When the
winds are light, both runways may be used simultaneously.
The strategy which
yields the highest capacity is when operations are away from the V and this is
referred to as a diverging pattern. In VFR the hourly capacity for this
strategy ranges from 60 to 180 operations per hour, and in IFR the
corresponding capacity is from 50 to 80 operations per hour . When
operations are toward the V it is referred to as a converging pattern and the
capacity is reduced to 50 to 100 operations per hour in VFR and to between 50
and 60 operations per hour in IFR .
Combinations of Runway Configurations
From the standpoint of
capacity and air traffic control, a single-direction runway configuration is
most desirable. All other things being equal, this configuration will yield the
highest capacity compared with other configurations. For air traffic control
the routing of aircraft in a single direction is less complex than routing in
multiple directions. Comparing the divergent configurations, the open-V runway
pattern is more desirable than an intersecting runway configuration. In the
open-V configuration an operating strategy that routes aircraft away from the V
will yield higher capacities than if the operations are reversed. If
intersecting runways cannot be avoided, every effort should be made to place
the intersections of both runways as close as possible to their thresholds and
to operate the aircraft away from the intersection rather than toward the
intersection.
Chicago's O'Hare Field,
with multiple parall runways. It should be noted that a large capital
improvement program is being undertaken to simplify the runway configuration,
by adding additional parallel runways and removing many intersecting runways.
This runway redesign is being done with the intention of improving the capacity
and efficiency of airport operations at the airport.
Runway Orientation
The orientation of a
runway is defined by the direction, relative to magnetic north, of the
operations performed by aircraft on the runway. Typically, but not always,
runways are oriented in such a manner that they may be used in either
direction. It is less preferred to orient a runway in such a way that operating
in one direction is precluded, normally due to nearby obstacles.
In addition to obstacle
clearance considerations, which will be discussed later in this chapter,
runways are typically oriented based on the area's wind conditions. As such, an
anal planning runways. As a general rule, the primary runway at an airport
should be oriented as closely as practicable in the direction of the prevailing
winds. When landing and taking off, aircraft are able to maneuver on a runway
as long as the wind component at right angles to the direction of travel, the
crosswind component, is not excessive. The FAA recommends that runways should
be oriented so that aircraft may be landed at least 95 percent of the time with
allowable crosswind components not exceeding specified limits based upon the
airport reference code associated with the critical aircraft that has the
shortest wingspan or slowest approach speed. When the wind coverage is less
than 95 percent a crosswind runway is recommended.
The allowable crosswind
is 10.5 kn (12 mi/h) for Airport Reference Codes A-I and B-I, 13 kn (15 mi/h)
for Airport Reference Codes A-II and B-II, 16 kn (18.5 mi/h) for Airport
Reference Codes A-III,
B-III, C-I, C-II, C-III
and C-IV, and 20 knots (23 mph) for Airport Reference Codes A-IV through D-VI .
ICAO also specifies
that runways should be oriented so that aircraft may be landed at least 95
percent of the time with crosswind components of 20 kn (23 mph) for runway
lengths of 1500 m more, 13 kn (15 mi/h) for runway lengths between 1200 and
1500 m, and 10 kn (11.5 mi/h) for runway lengths less than 1200 m .
Once the maximum
permissible crosswind component is selected, the most desirable direction of
runways for wind coverage can be determined by examination of the average wind
characteristics at the airport under the following conditions:
1. The
entire wind coverage regardless of visibility or cloud ceiling
2.
Wind conditions when the ceiling is at
least 1000 ft and the visibility is at least 3 mi
3.
Wind conditions when ceiling is between
200 and 1000 ft and/or the visibility is between . and 3 mi.
The first condition
represents the entire range of visibility, from excellent to very poor, and is
termed the all weather condition. The next condition represents the range of
good visibility conditions not
requiring the use of
instruments for landing, termed visual meteorological condition (VMC). The last
condition represents various degrees
of poor visibility
requiring the use of instruments for landing, termed instrument meteorological
conditions (IMC).
The 95 percent
criterion suggested by the FAA and ICAO is applicable to all conditions of
weather; nevertheless it is still useful to
examine the data in
parts whenever this is possible.
In the United States,
weather records can be obtained from the Environmental Data and Information
Service of the National Climatic Center at the National Oceanic and Atmospheric
Administration located in Ashville, N.C., or from various locations found on
the Internet.
Weather data are collected from weather
stations throughout the United States on an hourly basis and recorded for
analysis. The data collected include ceiling, visibility, wind speed, wind
direction, storms, barometric pressure, the amount and type of liquid and
frozen
precipitation,
temperature, and relative humidity. A report illustrating the tabulation and
representation of some of the data of use in
airport studies was
prepared for the FAA. The weather records contain the percentage of time
certain combinations of ceiling and visibility occur (e.g., ceiling, 500 to 900
ft; visibility, 3 to 6 mi), and the percentage of time winds of specified
velocity ranges occur from different directions (e.g., from NNE, 4 to 7 mi/h).
The directions are referenced to true north.
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