CHOOSING A TRAFFIC SAMPLE

Selection of a traffic sample that is most representative of the traffic within a Terminal Airspace is best achieved by combining statistical analysis with ATC experience and by looking beyond the information available. Two elements of the traffic sample are to be distinguished, which for convenience, will be described as Traffic Distribution over Time and Geographic Distribution of traffic. An appreciation of both elements is crucial to choosing a representative traffic sample.

Traffic distribution over Time

As regards Time, a feasible starting point is a snapshot analysis of the number of movements through the Terminal Airspace by month so as to determine the regularity of the resultant graph.

Using the sample graphs below (of three fictitious Terminal Airspaces) the following information can be drawn: Where Terminal “A” has a graph that is characteristic of large Terminal Airspaces in the core area, Terminal “B” is typically representative of summer holiday resorts and Terminal “C” typical of winter holiday (ski) resorts.

Whilst in the case of Terminal “A” it is obvious that one day’s traffic (the traffic sample) should be selected from one of the busier months, airspace design planners for Terminals “B” and “C” may wonder whether selecting one day during the busiest month truly constitutes a representative traffic sample. Because two busy months of the year may not be ‘representative’, airspace designers from these two Terminal Airspaces would do well to select two traffic samples i.e. one day from the busy months and one day from the quieter period.

The advantages reasons for this are two fold:

 to enhance the potential to apply the Flexible Use of Airspace concept

 if the geographic spread of the traffic is significantly different during the ‘quiet’ and ‘busy’ months, it may be necessary to create two sets of Terminal Routes;

Comment: Is it viable to create two (or more) sets of Terminal Routes to accommodate

significant changes in traffic density and/or distribution? Opinions diverge as to how significant changes in the operating environment should be accommodated. Whilst one view holds that an ‘unstable’ or ‘changeable’ airspace structure is to be avoided, the opposing view contends that it is not only possible but desirable to use airspace in a flexible manner. Whatever the philosophy followed, designers should ensure that the design fully supports safe and orderly air traffic management.

Geographic distribution of traffic

Having selected a one-day traffic sample from a Time perspective, it is necessary to determine the geographic distribution of this traffic with a view to identifying the predominant and secondary traffic flow(s). To this end, the traffic sample needs to be analysed using, for example, a spreadsheet.

"B" TERMINAL AIRSPACE MOVEMENTS

0 100 200 300 400 500 600 700

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC MONTH F L IG H T S P E R DA Y Y2002 Y2001

"A" TERMINAL AIRSPACE MOVEMENTS

0 200 400 600 800 1000

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC MONTH F L IG HT S P E R DAY Y2002 Y2001

"C" TERMINAL AIRSPACE MOVEMENTS

0 50 100 150 200 250 300

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC MONTH F L IG H T S P E R DA Y Y2002 Y2001

Because traffic data files contain information on each flight, flights can be sorted in several ways, e.g. –

 Terminal Area entry “point” (in the case of arrivals) and Terminal area exit point (for departures).

 origin (in the case of arrivals) and destination in the case of departures;

Sorting the geographic traffic distribution by origin and destination so as to identify the raw demand is only necessary when (i) doubt exists that the current En-Route ATS route network is not sufficiently refined thus making it lightly that some aircraft are not on the most direct route or, (ii) in the case of futuristic design projects for new airports where part of the exercise is trying to develop an entire airspace organisation on a clean sheet. The diagrammatic representation of raw demand is not nearly as clean as that of entry/exit point.

Given that the thicker lines in the above diagram represent routes of heavier (raw) demand, it is possible to ascertain – by comparing the location of existing Terminal entry/exit point [black circles above] in relation to these lines –whether these points have been placed effectively.

In those instances where En Route airspace designers alter their route network within the greater EUR ARN so as to minimise the differences between the raw demand ‘tracks’ and actual traffic routeings, it is not necessary for Terminal Airspace design planners to undertake the ‘raw demand’ exercise – providing that En-Route or Terminal Airspace design is undertaken collaboratively as a matter of course.

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5.3.6 Determining the Predominant & Secondary Runway(s) In Use

By and large, the predominant and secondary runway(s) in use are usually easier to identify (e.g. either because environmental requirements or weather phenomena dictate runway use). Whilst ‘predominant runway in use’ is a relative term (as is ‘major traffic flow’), a predominant runway is one that is used most of the time. Usually stated as a percentage e.g. 80% (which equals 292 days a year), it may be said that RWY20 is used 80% of the time, and RWY02 20% of the time. At multiple-runway airports, this ‘predominance’ may be distributed among several runways e.g. e.g. RWY20 is used 80% of the time by arriving aircraft, and RWY 18 is used 90% of the time by departing aircraft.

5.3.7 When to Identify Assumptions, Constraints & Enablers

As shown in the above diagram, Assumptions, Enablers and Constraints are identified at different stages of the design process. Constraints and Enablers enter the design process during the critical review of the Reference Scenario where the constraints and enablers refer to the Reference Scenario. The Assumptions are identified prior to commencing the conceptual design -–and these are verified at different stages of the process. During the design process i.e. the conceptual design of Routes, Holds, Structures and Sectors, constraints, mitigation and enablers are identified. In some cases, a Cost-Benefit analysis may be required.

5.3.8 Area Navigation as an Enabler

Whilst communication, surveillance and navigation are all vital elements to be considered in the design of a Terminal Airspace, the importance of navigation into the design equation has increased through the application of area navigation (RNAV) in Terminal Airspace.

In order to design RNAV Terminal Routes (e.g. SIDs/STARs for use by RNAV-equipped aircraft in Terminal Airspace), coherency is required between:

 the availability of a navigation infrastructure that supports the standard of RNAV to be employed. (Navaids can be ground- or space-based or self- contained on-board the aircraft); and

TRAFFIC