The capacity of a system of runways is determined by the capacity of the individual
runways that area available for simultaneous use. The previous chapter described
methods for computing the theoretic maximum throughput capacity (MTC) for the
individual runways. This chapter describes how the capacity for a system of runways is
determined.
The capacity of multiple runway systems is determined by the number of runways
available for simultaneous use.
(1) wind direction and magnitude
(2) independence of runways
(3) Noise and/or environmental constraints
Wind Direction and Magnitude
Winds are the primary determinant of the number of runways available for simultaneous
use. Aircraft can use a runway for takeoff or landing only when the crosswinds are within
prescribed limits and tailwinds does not exceed 5 – 6 knots (9-11 km/h). The prescribed
limits for crosswinds are summarized in the table below:
Aircraft Type Maximum Crosswind Allowed
Reference field length < 1200m 10.5 knots
1200m > reference field length >1499m 13 knots
16 knots
Reference field length > 1500m 20 knots
At airports that experience frequent periods of sustained strong winds from several
different directions, the number of runways aligned with the wind direction to meet an
availability of 95% can be high. In contrast, several major airports operate with an
availability in excess of 95% with single direction runways.
The capacity of a runway system is primarily determined by the number of runways
available for use at any given time and. This is known as the runway configuration and is
determined by wind direction and magnitude, the geometry of the runways and
noise/environmental considerations.
Independence of Runways
In addition to the number of active runways, the capacity of the runway system is also
determined by the independence of the available runways. Independent runways operate
without restrictions. Dependent runways operate with restrictions determined by the type
of operations on adjacent runways.
There are two type of dependence:
(1) parallel runway dependence, and
(2) intersection runway dependence.
Dependence and Independence of Parallel Runways
Parallel runways are a common geometric configuration used at major airports. By
duplicating the number of runways in a given direction, the capacity of the runway
system is increased when prevailing wind require use of these runways.
Operations on parallel runways can be grouped into three categories:
(i) dependent close parallel runway operations,
(ii) dependent medium spaced parallel runway operations, and
(iii) independent parallel runway operations.
The degree of dependence for mixed arrival/departure operations for parallel runways in
summarized in Table 1.
For close parallel runways, when the runways are used exclusively for arrivals or
exclusively for departures, they must operate as dependent runways and the separation
rules for single runway operation apply. For example, for arrivals, sequential aircraft
landing on each runway must have the same separation requirement as if they were
landing on the same runway.
When close parallel runways are used for departures and arrivals, they must operate as
dependent runways and the single runway rule applies. The landing aircraft must be at
least 2nm from the departure runway when the departure begins and may not touchdown
before the departure has left the runway. One small variant is that a departing aircraft
does not have to wait until the landing aircraft has cleared the adjacent parallel runway
before initiating the departure roll.
When one medium parallel runway is used for arrivals only and the other for departure
only, the runways can also operate independently.
When parallel runways are more than 4300 ft apart, the runway can be operated
independently. In some cases, the FAA has authorized the use of parallel runways spaced
3400 ft apart to operate as independent runways when the Air Traffic Controllers have
use of the Precision Runway Monitor (PRM) system. The PRM provides improved
accuracy radar surveillance data to the controllers to ensure safe separation.
The discussion above is based on the assumption that the parallel runway thresholds are
aligned. When the runways are staggered such that the runway thresholds are offset, then
the “effective separation distance” between runway centerlines is adjusted. Foe example,
when arrivals are to the “near end” runway and departures on the “far end” runway, the
2500 ft separation requirement is reduced by 100 ft for each 500 ft of threshold offset
down to a minimum of separation distance of 1200 ft. For example, when a runway offset
is 1000ft, a separation of 2300 ft between runway centerlines is equivalent to a 2500 ft
separation distance. When departures are to the near end and arrivals to the far end, the
separation distance is reduced.
Intersecting, Converging, or Diverging Runways
Intersecting runways are constructed to support multiple prevailing wind directions using
restricted real-estate. Runways are considered to intersect when their tarmac crosses each
other. Runways converge or diverge when the projections of their centerlines converge or
diverge.
The rules that affect capacity vary from airport to airport and from country to country for
intersecting, converging and diverging runways and are too variable to generalize in this
text, but some basic principles apply.
Noise Considerations
Environmental considerations, especially noise has an important influence in determining
runway system capacity. Air traffic controllers choose runway configurations to
minimize noise over populated regions. For example, at Los Angeles airport, shown in
Figure 2, assuming wind conditions are calm and operations can occur from either end of
the runway, ATC can choose to operate the runways by landing from the east and
departing towards the ocean (west). In this way, the population to the east of the airport is
subject to the idle thrust noise from descending aircraft, and not the Maximum takeoff
thrust required for departures.
There are three types of noise-related restrictions and configurations are usually due to:
Noise mitigation for densely populated areas
Short-term and long-term goals for runway utilization. The objective is to
distribute noise among neighboring communities in a way that is considered fair
by the parties involved.
Minimize continuous exposure of any single community to noise on any
particular day. For example, in Boston/Logan no runway can be used continually
for more than 4 h in any single direction and no runway can be used for more than
24 h in any 72-h period.
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