Exit Strategy

Lead Image Credit - CapperPeter Eriksen examines how new technology can be used to increase runway capacity in situations where new infrastructure is difficult to build

Many airports are suffering from lack of sufficient runway capacity. In particular airports in the core area of Europe are struggling to increase runway throughput in order to meet future demand.

When airports fill up, airlines use bigger and bigger aircraft to accommodate demand. A clear example is the number of A380s planned to operate into London Heathrow, Frankfurt and Paris Charles De Gaulle. Bigger aircraft take up more space, require more Wake Vortex separation and need more time on the runway, so even maintaining the current runway throughput is a big challenge that require changes to current practices.

Airports, airlines, not to mention travelers, are suffering more and more from airports operating very close to their theoretical maximum capacity. Operations become less optimised, more expensive and the effect spreads to the entire network.

This situation worsens as more airports will meet their capacity limit in the coming years and the bottleneck is the runway more than any other factor.

Building a new runway in a populated area is very difficult, if not impossible, due to environmental or physical constraints. Airports have often been surrounded by populated areas, leading to solutions that are full of compromises and far from optimal, if a new runway finally is build.

Examples in Europe are Amsterdam Schiphol, where very long taxi distances lead to high emission and increased cost for the operators, or Frankfurt where the new runway only provides a limited capacity gain. But these airports at least managed to get a new runway constructed. The situation is worse in other areas of Europe, where the situation seems to be stuck and no solution at all can be agreed upon, such as for example the London area or Munich.

Better use of existing infrastructure: The only other possible solution is then to get more out of the existing infrastructure, even if that also can meet environmental constraints.

Most airports have already increased the number of runway entries and exits to the maximum. This is an expensive process, but can normally be done without too much external interference.

When you consider the number of aircraft you can manage on a given runway, you need to look at the spacing or separation between the succeeding departing and arriving aircraft. Four main criteria need to be addressed:  Runway Occupancy Time, Radar Separation on final approach and initial departure phase, Wake Vortex Separation and Performance limitations on the navigation infrastructure.

In principle, the most constraining of the four criteria will determine the required spacing between a given pair of aircraft.

There are two possible ways to reduce the spacing between a pair of aircraft. A reduction of the required separation, e.g. radar separation or Wake Vortex separation normally will lead to an increase in runway throughput, and this is going to happen in the future.

It is however also interesting to consider the buffers applied in the spacing, the ‘too much and not required’ spacing between two aircraft, why we have these buffers and how they can be reduced.

Reduction of separation minima: Optimisation of Wake Vortex separation, Time Based Spacing (TBS) and reduced Radar Separation on final approach is being addressed in the SESAR Research activities as well as in the ICAO RECAT activities. I will not go too much into details about this work here.

It is expected that the Wake Vortex activities can lead to runway throughput improvements of between 5- 10 per cent depending on the weather conditions. It is worth mentioning that the implementation of RECAT-1 in Memphis Tennessee by far exceeded the expectations and simulations made before the implementation.

Time Based Spacing on final approach can significantly reduce the delay build up during periods of strong winds. It is maybe not obvious, but headwind on final approach is the weather phenomena that create most delay in European airports.

During low visibility, when ILS CAT2/3 is in operation, the extended ILS protection zone increases the need for separation on final approach. Low visibility also increases the time needed on the runway. It is more difficult for the pilot to manage the breaking towards the runway exit not visible until the aircraft is very close or the line-up before take-off.

Introduction of GBAS approaches will eliminate the need for the increased protection zone and there are ways to help the pilot managing the breaking, but more about that later in this article.

Reducing buffers: If you combine the best controllers with the best pilots, accustomed to operating in a particular airport, the buffers become very small. London Gatwick is a good example of this. Most pilots fly in very often, the controllers are used to a very high throughput and you see a very slick operation. Constantly maintaining a runway throughput above 50 movements per hour in a mixed mode environment like Gatwick is absolutely brilliant.

But there is room for improvement. Simulations carried out by NATS and EUROCONTROL to validate TBS show that providing a spacing tool to the final approach controller reduces the buffers and leads to a more homogeneous delivery of traffic to the runway threshold. As the rules become more and more complex and dynamic, the need for such spacing tool increases. Also, the need for integrated Arrival and Departure managers will increase.

One of the big unknowns is the aircraft Runway Occupancy Time. Modern aircraft actually have a pretty good idea about the time needed from threshold to clear of the runway, or from start of acceleration to rotation, and it can vary a lot.

Measurements in one European airport show that the line-up time of A320 family aircraft varied between 25 and 45 seconds on the same runway entry. All measurements were made the same day.

The controller is estimating the time needed based on experience, but will for obvious reasons have to base his spacing on the bad performer. In particular in mixed mode operations, where the spacing between two arriving flights to make room for a departure, spacing will be determined and established five minutes before landing, this leads to big buffers and loss in runway throughput, often leaving a busy runway empty  for 15 seconds.

The AIRBUS Brake-to-Vacate application not only optimises the breaking, but will also be able to communicate the required time and the selected runway exit to the ground system. This will help reducing the actual time as well as the uncertainty, leading to reduction of the buffers. A similar application could be envisaged for line-up and acceleration for take-off.

We will most likely see more and more mixed mode operations in the future, as this in principle gives higher runway throughput due to less Wake Vortex constraint, so there is a lot of potential in better management of Runway Occupancy Times  including downlinked aircraft parameters.

A reduction of the average spacing between aircraft of four seconds will allow for an increase of runway throughput from 40 to 42 operations per hour, whereas a reduction of six seconds per operation will allow an airport like London Gatwick to increase runway throughput by five operations per hour.

The future spacing tool: A spacing tool will provide benefits even in a very basic form, but the vision for the future advanced tool, where you really get the maximum out of the runway, is an integrated Tower and Approach tool with individual displays tailored to the needs of the two controller roles.

The required spacing indicated to controller will include the Wake Vortex and Radar separation minima, possible spacing due to limitations of the used navigation system, e.g. CAT 2/3 minima for ILS aircraft, but not for GBAS aircraft, the planned Runway Occupancy Time of the preceding operation, ideally using Runway Occupancy Time linked from the particular aircraft, initially may be taken from a database. The tool will visualise the required departure slot between two arrivals as, again preferably downlinked from the aircraft itself.

Optimise the sequence and meet the plan: In addition to an increased runway throughput, the integration of air and ground data combined with advanced tools will increase the predictability, facilitating improved predictability for arriving and departing flights. Not only will this improve the performance in the entire ATM network, as the airport will live up the Network Operations Plan, it will also enable reductions in taxi-out times and airborne holding, leading to a better planning of gates and stands as well as the management of connecting passengers.

There has been some hesitation from ANSPs and airports to implement advanced tools, and for good reasons. In the current generation of systems, and the level of integration we have today, the benefits of such tools were limited.

With the increased demand for better use of available runway resources, and with the possible reduction in separation minima and buffers in a very dynamic environment, the time has come for the capacity constrained airport to carefully analyse the local situation and invest in the required infrastructure, including a far better integration of the available data.

The combination of these tools and increased quality of data received from the ATM network manager, the future aircraft capabilities and the new dynamic separation rules will assure the payback of the investment.

We will see a set of global harmonization of procedures and airborne capabilities, but the different lay-out and organization of the airports calls for a local implementation tailored to the particular need of the individual airport.

In an environment where new runways are so difficult to build, this seems to be the most efficient way to assure future traffic demand can be accommodated.

EriksenPeter Eriksen has joined Metron Aviation as an airport optimisation and ATM expert. Eriksen most recently led European air navigation agency Eurocontrol’s contribution to the SESAR Joint Undertaking in the airport domain.

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