Making Trade-Offs

The complexity of making 'trade offs' repeats throughout the Planning Phase and the methodology identifies the Activities where such trade offs should to be considered.  Similarly, in the Design phase trade offs are revisited.

Activity 1, Operational Requirements, is when strategic trade-offs will need to be made.

The operational requirements and their prioritisation will, as the discussion on Consultation has implied, often be the net output of prolonged consultation and trading off of different needs and political pressures. The classic battle confronting high level policy makers today is the need for trade offs between economic expansion (air traffic growth) and sustainable development. Mostly, such trade-offs will have been achieved and reflected in the ‘received’ operational requirements, but an awareness of these is imperative for the team because these trade-off decisions re-surface repeatedly during project life-cycles.

Activity 3, Objectives, Scope, Resources and Timescales, is when the stakeholders will need to start considering the balancing of CNS/ATM needs against each other and the impact on cost.

The Trade-offs needed in Activity 3 speak to trade-offs between project objectives which are necessary at an operational level several layers below the trade-offs already made for the operational requirements.  Using an example, an airspace design team may wish to achieve three project objectives by a particular date (e.g. for all runways, new RNP APCH and new SIDS/STARs and in the terminal area, the inclusion of two new holding patterns).  As the objectives, scope and resource discussion evolves; it becomes obvious that achieving everything by the desired date is not possible.  Therefore the trade-offs begin, whilst keeping as targets safety and the project objectives.  What may emerge, is a modification to the project objectives as these are ‘reduced’ (traded off) due to limited resources; alternatively, a decision might be made to reconfigure the project, and to undertake it in phases (here available time is traded-off against the project objectives).  The frequent needs for trade-offs is the challenge in these kind of projects.  It should be repeated that safety is paramount and is never to be traded off.


Activity 5, Safety Policy and Planning and Performance Criteria, is the point at which the Airspace Design Team balances the 'needs' identified in the Operational Requirements against a set of deliverable performance and safety targets for the project.

When setting a project’s performance and safety criteria, clashes can occur between performance criteria and between performance criteria and safety criteria.  Often, it is not possible to meet all these criteria: reduced track miles by designing the shortest route (flight efficiency criteria) may clash with the environmental requirements (and associated criteria to reduce noise by 50% over a particular area.  Trade offs are inevitable in such situations, and the reliance on stakeholder consultation, particularly public consultation as regards neighbourhood nuisance is critical.  An example of a trade-off occurred at a European airport in the 2010s.  A trade off was made between number of movements per year (a quote was set) in order to reduce environmental impact.


Activity 6, Assumptions, Enablers and Constraints, is when the stakeholders clearly define the CNS/ATM capabilities needed to support the project and the cost effectiveness of the intended solution.

The evolution of an airspace concept brings together the use of all available ATM/CNS enablers, the management of constraints and the putting in place of mitigations to reduce risks or overcome constraints. This is not a forced reality but one that occurs almost naturally with the stepped technological improvements across the decades.

It is tempting, particularly when introducing new technological changes, to concentrate exclusively on the new technology being introduced.  What needs to be understood and remembered is that any new technology, process or procedure must take its place in a family of existing technologies, procedures and processes, each with their own strength and weaknesses.

A holistic, rounded view of ATM/CNS is essential particularly in increasingly privatised environments where costs play a more and more important role and the call for CBAs (cost-benefit analyses is constant. For this reason, trade-offs between ATM/CNS enablers is needed in order to meet performance targets, balance the costs and ensure safety. If a new navigation enabler is being introduced (e.g. PBN) and it is reliant on GNSS positioning source, the conceptual design will have to account for potential failure modes and mitigations.  This is where trade-offs become so important.  Because the new PBN enabler is anchored in the navigation domain, this does not mean that the mitigation of risk must also be provided by navigation.  It could be provided by Surveillance, for example, that in the event of a GNSS outage, the aircraft will be radar vectored; this assumes the surveillance source is not also dependent on GNSS.  At the operational front end, pilots and controllers do not think in terms of ATM or C-N-S when doing their job.  They use the suite of enablers and tools available.  For years, operational staff has known that if an aircraft has a radio failure (COM), it will squawk a particular SSR code (SUR) using a data broadcast (COM!).  The art of efficiency in concept design is to use and interleave the enablers so that the new technological inserts appear seamless.

The possibilities for trade-offs are extensive.  A procedure could replace the need for a new technology (or a more expensive version of a new technology).  An add on to an existing COM technology, for example, could be a solution for what SUR is attempting to solve.

The key message is that no enabler or party to the airspace concept development must think in isolation.  A rounded view is needed: it saves costs, increases efficiency and often makes for a better concept.


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