What is a slot?

You’ve checked in on time, made it through security, found the gate, queued to get on board, struggled with the overhead lockers and settled back into your seat. Then nothing happens. Finally you hear the captain saying, “We’re waiting for a slot." Why? What is a slot and why can’t you take off as soon as you are ready?

We’re not talking about airport slots here

An airport slot gives an airline the right to operate at an airport at particular times. They are used when the airport is constrained, either by the runway throughput (such as Heathrow) or by the available parking space (such as at a number of Greek island airports). These slots may be traded and can be very valuable – one was sold in 2016 for a record amount of $75 million.  

However, here we are looking at Air Traffic Management slots, issued by EUROCONTROL as Network Manager as part of its flow and capacity management role.

Avoiding congestion in the skies

The Network Manager covers the whole of Europe – from Ireland to Armenia and from Morocco to Finland. It handles over 10 million flights a year with summer peaks of over 34 thousand a day. That means an aircraft is taking off or entering European airspace every three seconds. Inevitably, there are bottlenecks. These may be in particularly busy bits of airspace or at some airports at some times of day. Any disruption, such as a runway out of action, fog, a thunderstorm or technical failure, can result in difficulties. 

The Network Manager receives flight plans for all the commercial flights in its area and also receives the declared capacity limits for air traffic control centres and airports across the continent. So for example if an airport has snow or fog, it may reduce the rate at which aircraft can land. This is called a regulation. 

The Network Manager then looks at the whole picture and problem areas are identified – where the demand is greater than the capacity.

One solution of course is to see if the capacity can be increased. So if a controller is covering a large area and the traffic is expected to be at the limit of how many aircraft can safely be handled at a single controller position, then that area might be split into two sectors, with two controllers/teams of controllers each covering part of the traffic.

However, there are limits to this process as sectors need to be large enough so controllers and pilots are not continuously handing over from one sector to another. You also need to have enough trained controllers to handle all the sectors. There are about 1,750 possible sectors across Europe.

The Network Manager will work with the local Air Navigation Service Provider to identify capacity constraints, sometimes well ahead, in order to optimise the sector configuration. One good example of this occurred recently when a new ATC system was being introduced in the southwest of France, in Bordeaux Area Control Centre. Neighbouring areas made sure that they had enough controllers to cope, with Spain providing summer levels of capacity in November. The result was that relatively few delays occurred.

The slot

If capacity is still less than demand, then the demand needs to be managed. For an en-route sector, this is a question of safety. Overloading a controller by giving him or her too many aircraft at once is just too risky, particularly if the sector has complex traffic, with criss-crossing aircraft that need to climb and/or descend. For an airport, it is possible to manage incoming traffic by putting them into holding patterns (‘stacks’) – each aircraft flying in ovals and gradually descending while waiting their turn. However, that wastes fuel and is environmentally undesirable.

Demand is managed by giving the aircraft a slot stating when it can take-off – the ‘Calculated Time of Take Off’ or CTOT. Normally, the aircraft should take-off within 15 minutes of the time stated in its flight plan but, if a slot is necessary, then this window is shrunk – to within five minutes before the CTOT or within ten minutes after the CTOT. If the aircraft can’t achieve this take-off time, then it has to reapply for a slot.

An alternative is for the airline to change its route so as to avoid the congested area. This may take a little more time but could be better for the airline than incurring a delay. It is their choice but the Network Manager may propose alternatives so as to help ease the traffic flow.

Who decides on which flight is delayed?

As you might expect, airlines are very keen to make sure there is no discrimination and so we have developed open and fair algorithms. So it doesn’t matter if you are in an A320 or an A380, or whether you’re flying national airline, low-cost or charter, you will be treated the same.

I still see aircraft flying holding patterns. Why?

There are a number of reasons for this. The first is that sometimes regulations are necessary at very short notice. So if the weather becomes unexpectedly worse, or there is an incident on a runway (such as a burst tyre) or if the airport has to change the direction of landing because of a shift in the wind direction, then any aircraft already in the air, heading for that airport, may have to wait before they can land.

A second reason is that some airports are operating so close to capacity that it is very important for them to be able to sequence arriving aircraft as efficiently as possible. Smaller aircraft (such as an A320) behind a big aircraft (such as an A380) have to leave a bigger gap because of the wake of the aircraft in front. So it’s much better to group arriving aircraft according to their size.

Delays – three questions and many answers

Delays – three questions and many answers

Finally, the Network Manager flow management slots only apply to aircraft departing from a European airport. Any aircraft coming from outside Europe (that’s about one in eight aircraft landing at European airports) just file their flight plans and the Network Manager has to manage the rest of the flights taking into account this ‘Out Of Area’ traffic. But for some airports, such as London Heathrow, this proportion is much greater (more than one in three for Heathrow!) and so stacks become inevitable, especially as some times of day, such as early morning.

ICAO, the International Civil Aviation Organization, is looking at moving to Global Flow Management but that is still quite a long time away.

The importance of predictability

Even with detailed flight plans and flow management in operation, a controller still cannot be sure of exactly how many aircraft will be in his or her sector at any particular time. One aircraft may have departed slightly early or have received a more direct routing – or it may be flying slightly faster than expected; while for another aircraft it may be the reverse. The altitude of the arriving aircraft may also be slightly different than planned; for example, if they received an early instruction or permission to climb.

As a result, Air Traffic Control will build in a safety margin so that they are sure that any particular sector will not be overloaded. It we could improve the predictability of traffic – and the confidence in the forecasts – then these safety margins could be reduced – enhancing the capacity of the system and resulting in fewer delays.

So that is what we are doing. Here there are a number of approaches. First, we are connecting up more and more airports to the network so that the Network Manager gets a much more accurate forecast of when an aircraft will depart. This also helps the destination airports as they receive more accurate estimates of when inbound aircraft will arrive.

Second, we are exchanging real time operational data with other parts of the world. We’ve already been doing this for years with the USA, so we can see an aircraft heading towards Frankfurt when it is still over, say, Ohio; we are now in the process of extending this.

Third, we are working with pilots and controllers to encourage flight plan adherence. That means getting the aircraft to file flight plans and then stick to them – not asking for or receiving that early climb which might cause problems in another sector later in the flight.

The future of slots

Ideally, we wouldn’t have slots at all. Aircraft would fly accurate 4D trajectories, updated and de-conflicted automatically in real time. Controllers would monitor the aircraft and ensure they are safe but would do much less active passing on of instructions. This is central to the SESAR operational concept.

One stepping stone towards this is to move the control from the departure airport to where the problem actually is. So if there was a problem over, say, Luxembourg, an aircraft would receive a target time for its entry point into this sector. The pilot would adjust the take-off time and speed to arrive on time. This would be a much more accurate and targeted approach and one trial has been performed with a ‘window’ of just plus or minus ten seconds!

However, in the near term, the use of flow management in general, and slots in particular, is set to grow with many more areas of the world encountering the levels of congestion we see here in Europe. It is now twenty years since central flow management was implemented in Europe and there have been huge savings in terms of delays and fuel burn, as well as major environmental benefits. 

So next time you are waiting for a slot, remember that it’s better to be on the ground waiting rather than in the air and burning excess fuel unnecessarily!

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