Table 4 - 1: Constraints, Mitigation and Enablers
Edition: 2.0 Released Issue Page C-4-5 Amendment 1 – 17/01/05
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In order to identify and select assumptions, constraints and enablers, a comprehensive inventory of ATM/CNS elements is needed as well as expert input from, for example, meteorologists and pilots. Although it is possible for a Terminal Airspace design team to formulate the ATM/CNS parts of the inventory based upon their expert knowledge of local conditions, an inventory ATM/CNS completed in this manner is likely to be incomplete. This is because discrepancies frequently exist between what designers believe and what exists (see Example below). For this reason, it is necessary to determine from the outset what elements of ATM/CNS exist and are published in state-originated documents such as the Aeronautical Information Publication (AIP) and what factors can reasonably be assumed.
To these ends, thorough data collection of ATM/CNS elements is needed. If undertaken correctly, this data collection will reveal what can be assumed (assumptions), what is not available or inhibiting the design (constraints) and what is required to make the design workable (enabler). Importantly, assumptions, constraints and enablers should be linked to a certain date (i.e. ‘date stamped’) so that the design team may quickly identify the (time) period ascribed to a assumptions (or constraints/enablers) should it become necessary at later stage.
In order to illustrate the differences which can exist between perception (on which assumptions are frequently based) and reality of local conditions referred to above, the following examples are provided.
Example: A Terminal Airspace contains four DME stations. A fifth, located in an adjoining State, is assumed to be within the range of most aircraft departing from RWY23 at the only airport within the Terminal Airspace. Based on this belief the designers include the availability of this ‘cross-border’ DME in their assumptions when designing a SID (intended to be flown by P-RNAV equipped aircraft) from RWY 23. What the designers have not realised, however, is that the co-ordinates of this fifth DME are not WGS84 compliant (which is pre-requisite for SIDs designed for use by P-RNAV equipped aircraft). (Data collection obtained from an official source such as the AIP of the neighbouring State would reveal this shortfall.) Thus the ‘assumptions’ has turned out to be a constraint requiring mitigation.
Example: Having tested various sectorisation options, designers decided upon a combined functional/geographic sectorisation option because (a) it was the most efficient and (b) it did not require extra working positions and allowed them to make use of the existing three. When seeking to implement these new sectors, however, they were informed by ATC system specialists that the current ATC system was incapable of functional sectorisation and that it was no longer possible to modify the system software. (In this instance, the input of an ATC systems expert during the design phase would have prevented this option being chosen).
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