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of the observed track data. SDAT has an assigned number of seconds to complete each
action, and the total number of work seconds is plotted and summed over the sample
period.
A sample of SAR data for a selected group of sectors over a number of busy periods can
be used to estimate the amount of work (i.e., aircraft handled without error) that a
controller team can accomplish in a given period of time in each sector. This can be
related to workload capacity for a given mix of traffic.
Assuming that separation distances are reduced for a historical sample of traffic, the
revised separation assurance workload can be re-computed using SDAT and the resulting
impact on total workload can thus be estimated. This will provide an estimate of how
reduced separation requirements will affect controllers’ ability in a specific sector to
handle increased numbers of aircraft. This process must be repeated for a number of
different sectors of different types to obtain a complete picture of the effects of reduced
separation.
It is vital that information be obtained from working controllers regarding the factors that
should be considered in determining the impacts of reduced separations and the controllers
own expectations of what those impacts would be. Both face-to-face exchanges with
controllers and a survey could be conducted. The survey should include problem sectors,
namely those that are frequently determined to be “red sectors” in the monitor alert,
and/or those that experience a high number of operational errors. This could provide
valuable insights to guide the analysis.
There are a number of controller work actions not captured in the SAR messages that
should be considered. One is the assistance given to airline pilots seeking to change
altitude to reduce turbulence. Although this is an optional service, it does occupy a
significant amount of a controller’s time.
4.2.3 Impact on Total NAS Capacity
Aircraft delay can be defined as the time required for an aircraft to fly from gate-to-gate
minus the time the flight would have taken in the absence of restrictions imposed by the
NAS. These restrictions can include those resulting from airport and airspace congestion,
adverse weather, noise abatement procedures, airspace restrictions, etc. Delay due to
weather is referred to as weather-related delay, and the like.
To estimate the potential reduced delay benefit resulting from reduced separation, all
restrictions that currently exist can be assumed to remain in force except for any increase
in sector capacity resulting from the reduced separation. A delay model that is capable of
relating a postulated increase in sector capacity to the total NAS capacity, including the
SEPARATION SAFETY MODELING
4-6
effect of limited airport and terminal airspace capacity, is needed. It is yet to be
determined whether any existing model has this capability.
Consider, for example, the National Airspace Simulation Performance Capability
(NASPAC), a model that considers both airport and sector capacity. Sector capacity is
modeled as both an instantaneous airborne aircraft capacity and as an hourly throughput
capacity. Airport capacity is measured as an hourly throughput value that is dependent on
the arrival/departure mix.
It is uncertain, however, whether NASPAC will have the capability to determine how
limitations on airport capacity impact improved en route airspace capacity. This model
could perhaps provide a first order approximation to the NAS capacity issue. This
analysis will be particularly challenging for NASPAC as future collision risk modeling
scenarios will likely involve aircraft-to-aircraft separations instead of the typical airway-toairway
separations. Given NASPAC’s flight route definition structure, i.e., predefined
link-node structure elements, it is possible to speculate that Free Flight operations
probably will be represented at a medium level of detail in this model. NASPAC will have
to be examined more closely to see if it can adequately perform this task.
Developing input data requires significant effort. Sector capacities currently used in
NASPAC have been derived from operational values used for traffic flow purposes. They
have not been validated, nor do they consider the mix of traffic or weather conditions. A
detailed study of how reduced separation minima affect airspace capacity could be
conducted, using, for example, SDAT, with the SDAT output then used in NASPAC.
This is feasible only on a sample basis of several centers over several hours. But it would
provide a reasonable estimate of the kind of benefits that would accrue in total system
capacity through reduced separation minima.
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a concept paper for separation safety modeling(17)
