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4.2.4 SIMILARITIES AND DIFFERENCES
Whilst (design) assumptions can be viewed as ‘uncertainties’ which have been elevated to ‘facts’ to be used as a basis for the design, the role of enablers is to mitigate against constraints which have been identified. An example can be used to illustrate this difference: Suppose that a designer wishes to design RNAV routes up to the final approach fix in a Terminal Airspace. Because Terminal RNAV Routes with waypoints having a level restriction below MSA or MRVA may only be designed for use by aircraft which are certified for P-
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RNAV operations, an assumption can be created that aircraft operating within the Terminal Airspace are appropriately certified. [Note: this sort of assumption should only be made if the design team is sure that aircraft are appropriately certified]. In seeking to design the route based upon this assumption, the designer identifies a constraint viz. that the navigation infrastructure is inadequate and therefore does not allow the design of a necessary STAR route. This constraint could be mitigated against by the installation of a new DME pair in the Terminal Airspace and the enabler would be an enhancement of the navigation infrastructure – see Table 4 - 1. As shown in the diagram below, the means by which the enabler is achieved/provided (functional and technical requirement) usually falls outside the scope of the design team’s work. In view of the costs which enablers sometimes incur, a Cost Benefit Analysis (CBA) may be required to determine whether the benefits provided by the enablers outweigh the costs. If this is not the case, it may be necessary to identify alternative mitigation. – See Part B.

Figure 4 - 3: Constraints – Mitigation - Enablers
CONSTRAINTS  MITIGATION  ENABLERS 
High Terrain on final approach RWY X  Increase ILS angle by 1°?  Specification change for ILS 
Multiple airports within close proximity with poor co-ordination agreement  Letter of Agrrement  EUROCONTROL DOC The Cross-Border Common Format Letter of Agreement 
Aircraft Performance Mix limits capacity  Design different SIDs for high and low performance aircraft.  Airspace Design 
Aircraft Navigation Performance Mix limits capacity by increasing ATC workload  ATC system modification to allow FDPS/RDPS to show aircraft navigation capability  Software Application Change 
Inadequate Navigation infrastructure  New DME at Location A  Enhance NAV infrastructure 
High mix of IFR-VFR movements limits capacity  SEGREGATED VFR/IFR ROUTES  Airspace Design 
Fixed-wing/Rotor craft mix increases approach workload and complexity  Separated routes based on aircraft category  Airspace design 
TSA which adversely affects traffic patterns  Airspace sharing arrangements  Flexible Use of Airspace Concept and EUROCONTROL DOC The Cross-Border Common Format Letter of Agreement 
Poor Radar Coverage prevents route placement in part of the Terminal Airspace  Improve Surveillance capability  Enhance Radar infrastructure 
　
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