Capacity assessments, another application of Fast-Time Simulation

In previous blogs we have discussed how Fast-Time Simulation is a very useful tool for analysing and comparing the performance of different scenarios quickly and inexpensively. FTS can support the selection of the best planned future endeavour, whether it be a new airspace design, a terminal expansion plan, an investment in airside infrastructure, infrastructure maintenance phasing, etc. A properly calibrated FTS model can produce accurate and objective predictions of the effect of planned developments on the daily operation of airports and airspaces.

This blog will focus on capacity and how FTS can be employed to determine the capacity of a system.

Capacity, an elusive concept

Capacity is not a clear-cut concept as it can be defined and understood in different manners, even more so when talking about complex systems. The capacity of an airport or an airspace is obviously not a simple count of how many aircraft or people fit within a certain space, but it is also not as simple as how many aircraft or people can be served at a given time (i.e.: the throughput of the system). As a result, several adjectives are usually attached to the word capacity in order to contain its meaning: sustained capacity, declared capacity and operational capacity, to name a few.

A key part of figuring out the capacity of a system is to identify the system’s constraints. The specific circumstances might vary but, in the context of an airport or airspace and in general terms, the capacity of the system will be constrained by the available technology and the safety, environmental and efficiency considerations. For a given technology level (i.e.: aircraft size, speed, air navigation system accuracy, availability, integrity, etc), safety is the number one limitation to capacity, and rightfully so. Safety manifests itself not only in the form of safe separation of aircraft but also in the design of safety procedures, redundancies, usage of certain technologies over others, management of ATCo and pilot workload levels, etc. Safety is of such importance that it creates the unbreakable rules of the system.

Then, as a sort of optimization levers, one can find the sometimes-opposing forces of environment preservation and efficiency of the operation. As an example, an airport operator aims to serve as many passengers as possible while maintaining an acceptable level of service (usually meaning reduction of flight delays and passenger waiting times). A solution could be to simply have more flights. However, there is a limitation on how many aircraft can land or take-off from a runway due to the required safe separation of aircraft, so increasing the number of planned departures in an hour might translate into long runway queues, unhappy passengers, and unsafe situations both on the ground and the airspace. There is also a limitation on the available airport infrastructure and on how many aircraft ATC can safely handle. The communities surrounding the airport may also impose limits on the number of  daily or hourly aircraft movements at the airport  due to aviation noise as well as emissions. Up in the airspace, the air traffic flows can become difficult to manage, increasing ATC workload, as a results of limited airspace, complex airspace design or the traffic mix. And the list goes on. So, while the runway(s) of an airport can handle certain number of movements per hour, the real capacity of the airport will never be as high as that figure.

We implement the capacity constraints into the FTS model as well as into the simulation output analysis. A critical analysis of the output of the simulation is a key part of our process.

To70’s approach

At To70 we work together with many operational experts, including air traffic controllers, pilots and aerospace engineers who support our FTS team in producing accurate models that resemble what is feasible and acceptable in the current air traffic ecosystem. Furthermore, when carrying out a capacity assessment project, we identify with our clients and other stakeholders the capacity constraints that can affect the specific airport or airspace in question and quantitatively determine acceptable levels of measurable KPIs. For instance, the capacity of an airport will be highly dependent, among other factors, on how many minutes of delay are acceptable; and in the case of airspace, the acceptable levels of ATC workload will play an important role in capacity definition.

Subsequently, we implement these constraint into the FTS model as well as into the simulation output analysis. A critical analysis of the output of the simulation is a key part of our process since the quality of a simulation model is highly dependent on the quality of the input used for it. Thus, we must identify with a critical eye and expert judgement whether the results obtained from the simulation are a reflection of the actual features of the system being investigated or if they are the result of an imperfect input. Additionally, we perform sensitivity analyses, not only to identify gaps in our model, but also to quantify how sensitive the results are to changes to the different variables of the system. The objective of this entire process is to provide our clients with meaningful capacity results that are sound and consider the full picture of the system.

About To70. To70 is one of the world’s leading aviation consultancies, founded in the Netherlands with offices in Europe, Australia, Asia, and Latin America. To70 believes that society’s growing demand for transport and mobility can be met in a safe, efficient, environmentally friendly and economically viable manner. To achieve this, policy and business decisions have to be based on objective information. With our diverse team of specialists and generalists to70 provides pragmatic solutions and expert advice, based on high-quality data-driven analyses. For more information, please refer to

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