Capacity Analysis Methods and Metrics

Main contributing authors:

Shanna Schönhals and Meiko Steen − Institut für Flugführung, TUBs

5.6.1 Overview

The assessment of expected capacity benefits of the operational scenarios (CONOPS) presented in Sections 3.1 to 3.5 has become an important part of wake vortex operational research. The results can serve as a figure of merit for the concept under study and are an input of the cost-benefit analysis.

Some years ago in 2005, the “European Wake Vortex Mitigation Benefits Study” (Galpin et al. 2005), also known as the Euroben study, investigated the impact of three concepts (ATC-Wake, CREDOS and TBS) on the capacity of a generic ATM system. This high level study indicated possible capacity gains but the significance of the results was limited to its context and assumptions.

Final figures can only be obtained through a more detailed analysis taking into account specific (site dependent) details, when all boundary conditions (uncertainties in weather and wake prediction, system complexity, operational aspects, proof of safety, …) are properly dealt with. Such assessments were conducted in the scope of projects described in the previous sections taking into account site and concept dependent details.

An important item in this investigation is the clear differentiation of tactical capacity gain (as a function of mainly weather and other factors), which allows to mitigate ad-hoc delays and hence environmental impact, and strategic capacity gain (as a function of revised separation applicable in all weather conditions or at least close to 100% of the time), which enables to increase the scheduled airport capacity. Some concepts explicitly do not aim for a capacity increase but try to recover lost capacity in unfavourable visibility (e.g. SOIA) or strong headwind conditions (e.g. TBS).

Various methodologies have been used in order to assess the capacity benefit of the specific concepts, such as analytical approaches, fast-time simulation and real-time simulation including human-in-the-loop approaches. So the results should also be regarded in the context of methodology used.

In order to avoid repetitive descriptions of the available concepts the results of capacity assessments have been included in the previous sections 3.1 to 3.5 together with the descriptions of the concepts. In this section, a definition of the capacity term will be given as well as a description of currently used capacity assessment methods.

5.6.2 State-of-the-art

5.6.2.1 Definition of Capacity

The term capacity in the context of an airport or airspace describes the capability of the facility to provide service within some period when there is continuous demand, e.g. the number of aircraft landings and/or departures per hour. A distinction has to be made between the maximum theoretical capacity of an airport and its practical capacity which is always smaller. Scheduled airport capacity is associated with the practical capacity due to the trade-off between maximum throughput and the acceptable level of delay.

Another aspect of capacity investigation is the division into strategic and tactical capacity gains. Strategic capacity is equal to the scheduled capacity of an airport and it is assumed that it is available for nearly 100% of operational time. Tactical capacity is available only when specific operating conditions are met (e.g. when crosswind conditions are favourable) and can be used to reduce delays or accommodate additional unscheduled flights.

It should be considered that capacity is characterised by the number of arrivals and departures on a runway. This should be kept in mind during the design of a concept and during the capacity assessment process.

5.6.2.2 Capacity Analysis Methods

In order to assess the capacity impact of a new operational concept, different methods can be applied that imply modelling the system under investigation, running simulations and designing metrics to evaluate the impact. These models, simulations and metrics vary in the level of detail and complexity. The following methods are usually used to perform capacity analyses, with increasing level of detail:

• Analytical models which can be applied to different airports and are used for high level analyses.

• Process or event driven simulations that make use of synthetic traffic scenarios and emulate basic functions.

• Fast-time simulations that take into account realistic traffic scenarios, ATC procedures and the airport and airspace layout.

• Real-time simulations that take place in the complete operational context including realistic aircraft behaviour and the entire ATM system with its interactions; human factors are also assessed via human- in-the-loop simulations involving air traffic controllers and (pseudo) pilots.

Examples for the use of these methods have already been given in the previous sections 3.1 to 3.5 where various methods have been used for capacity benefit assessments.

5.6.3 Research Needs

Based on the overview of the capacity benefit assessments presented above as well as on the discussions lead during the specific workshops, the following recommendation for further work in the area of capacity analysis can be given:

5.6.3.1 Influencing factors of capacity analysis

Factors influencing results of capacity analyses are manifold and should be taken into account when assessing a concept and creating benchmarks. Among those factors are:

• Influence of uncertainty (e.g. in weather prediction for wind/weather based concepts) • Stability of forecast and the persistence of operational modes

• Airport design and procedures • Traffic mix

• Impact on safety and the risk assessment

• Selected metrics to quantify capacity (e.g. influence of metrics related to payload/passengers)

As a consequence, methods to account for these influencing factors need to be refined (e.g. for the time when changes from the re-categorisation effort become effective). Various studies have shown that capacity benefit is above all airport specific; in order to produce comparable results and to transfer findings from one airport to another, a holistic methodology is recommended for future studies.

5.6.3.2 Finding the right capacity metric

The right capacity metric is still under discussion – aspects of this discussion are: • Relation of capacity metric to the aircraft type (payload/passenger)

• Weighting between arrivals and departures

5.6.3.3 Capacity gains of new and evolving concepts and systems

The capacity benefits for wake mitigation developed e.g. in SESAR WP6.8.1 and WP12.2.2 need to be assessed consecutively for each implementation phase. In order to provide realistic estimations, the methods

should be refined and real-world conditions should be taken into account. It should also be considered that there has to be a trade-off or a sound balance between capacity increase (referring to scheduled capacity) and the tolerated increase of delay occurring when the conditions of a concept cannot be satisfied. It should be considered to transfer some of the capacity gain into delay reduction.

The impact of future traffic growth but also other changes envisioned in the future air traffic system (advanced landing aids, continuous decent procedures, 4-D business trajectory etc) need to be considered during capacity assessments of any evolving wake mitigation concept.

For most of the concepts developed today, one of the central questions remains: What capacity increase is needed to adopt the concept or to justify the implementation of the concept by the stakeholders? Therefore, the communication between developers, authorities and operational stakeholders is one of the important challenges for the next years to find a common answer.