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g) The relative infrequency of airspace incidents and accidents precludes using data on
these events to generate probability distributions with any degree of accuracy.
However, the narratives of these events provide very useful qualitative information on
what can go wrong and how pilots and controllers react in such circumstances. These
narratives also give some insight into how different factors might interact in
unexpected ways.
h) Consideration should be given to more realistic modeling of aircraft performance.
Aircraft technology (including navigation and general aircraft performance
characteristics) has changed significantly in the past 30 years, warranting a more
accurate modeling of the aircraft maneuvering envelopes in the en route flight phase.
Given that aircraft have very limited maneuvering envelopes at cruising conditions, it is
important to include aircraft performance as a constraint variable in any future
collision risk model.
Wake vortices at high altitude and in the clean aircraft configuration are not well
understood. Nevertheless, there is sufficient anecdotal experience to suggest that this
factor might well play a role in future separation criteria applied to the en route
system. The team should identify possible avenues to model wake vortex as an
exogenous factor. There are several macroscopic wake vortex behavior models
developed by NASA that perhaps could serve as the basis of a modeling approach.
SEPARATION SAFETY MODELING
4-4
4.2 CONSIDERATIONS IN DETERMINING THE BENEFITS OF REDUCED
SEPARATION MINIMA
4.2.1 Potential Benefits of Reducing Separation Standards
A primary benefit that could be claimed for reducing separation minima in en route
airspace is increased airspace capacity. Increased capacity relates to greater aircraft
throughput and reduced delay, with consequent greater fuel efficiency. The questions are:
1) will reduced separation requirements actually improve airspace capacity, and 2) if so,
will this accrue to increased throughput and reduced delay in the entire NAS, or will other
bottlenecks in the system (e.g., limited airport and terminal airspace capacity) impose
restrictions that will cancel these benefits. If this is the case, the result may be that more
airplanes get through the en route airspace more quickly and fuel efficiently only to spend
more time in the terminal airspace or the feeder sectors.
4.2.2 Impact on Sector Capacity
Airspace capacity is limited not only by its usable/useful volume, but also by the workload
capacity of the controller. A controller team manning a sector can only provide service to
a given number of aircraft per unit of time, even though there may be enough airspace to
physically contain many additional aircraft while maintaining the required separation
between them. A way to overcome this problem is to divide the airspace into a greater
number of sectors. However, this is not a desirable solution as it requires more
controllers, more ATC equipment, more radio frequencies, and more work for the
aircrews in changing frequencies as they pass through more sectors.
Controller workload consists of much more than separation assurance. Each aircraft must
be radio contacted and radar identified. Aircraft must be given clearances and headings to
properly guide them through the airspace. They must be given hand-offs to other
controller teams as they approach sector boundaries. These are a few of the routine items
that are required even if there are no other aircraft to be considered.
On the other hand, as the number of aircraft controlled by the sector increases and as user
preferred routing becomes prevalent, controller workload may increase out of proportion
to the number of aircraft. Surveillance must be maintained on each to insure that other
aircraft are not potential collision threats. Controllers currently maintain aircraft at
horizontal separations greater than today’s minima. If separation minima are decreased,
controllers are likely to maintain aircraft at horizontal separations greater than the new
minima to ensure adequate warning time. These actual separations may be smaller than
those maintained today, depending on controller workload and airspace complexity.
One of the tools available to analyze controller workload is the FAA’s Sector Design
Analysis Tool (SDAT). SDAT provides data on the workload associated with a historical
sample of traffic data. It translates data recorded as controller/HOST messages in
Systems Analysis Recording (SAR) data into over a hundred different controller actions,
MODELING CRITERIA
4-5
either specifically identified in the data, or inferred from radar track data. The expected
number of required separation assurance actions is determined from a probabilistic analysis
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a concept paper for separation safety modeling(16)
