If I could sum up July in just one word, I would choose “slotty”. Slot restrictions because of the Air Canada Rouge incident at Gatwick (where thankfully the aeroplane landed safely), slot restrictions due to storms over Europe, and almost daily slot restrictions due to capacity at Gatwick.
You’ve probably been travelling as a passenger on an aeroplane and felt annoyed and frustrated when the welcome on board PA includes a phrase along the lines of “we have an Air Traffic Control slot today”. But what exactly is a slot restriction, and what can we do to mitigate its effect?
What Is A Slot Restriction?
A slot is a restriction placed on a flight by Eurocontrol with the aim of regulating the flow of air traffic either at the departure or arrival airport, or in the airspace the aircraft will occupy en route. It is not generated by nor influenced by the airline, although by updating the Estimated Off Blocks Time (EOBT) to ensure it is always accurate, the airline can attempt to minimise the delay generated. The slot restriction specifies a Calculated Take Off Time (CTOT) that must be complied with, although a window of -5 minutes to +10 minutes is permitted. Ever optimistic, we always try to manage the departure so we can push back and be ready for departure at the -5 end of the slot, but the window allows Air Traffic Control to sequence our departure more efficiently. A slot is usually generated for one of three reasons…
- The demand at the departure airport exceeds capacity. At any given time an airport has a “flow rate”, which is the number of movements per hour it can handle. Many factors can cause this to decrease – fog, high winds, and emergency traffic can all cause a dip in the flow rate. Airports plan their day assuming normal flow rate, so as soon as the rate dips, all subsequent arrivals and departures for that day are affected. A classic example of this is early morning fog. Passengers are often incredulous that delays persist long after the fog has dispersed, but at a busy airport there is little or no spare capacity built into the programme, so it’s impossible to catch up once the schedule becomes disrupted.
- The demand enroute is high. Sometimes air traffic units on the planned route are unable to accept the normal number of flights, for example due to storms requiring aircraft to reroute, so restrictions are placed to ensure the number of aircraft in a particular sector are kept to a lower number. This is where there can be an opportunity to improve on or eliminate a slot. By rerouting the aircraft it can be possible to avoid the affected airspace. This can be difficult in practice – with many aircraft affected simultaneously, the surrounding sectors might not have sufficient capacity for the increased traffic, and if even if a reroute is possible, if the slot has appeared late in the departure process and extra fuel is required, it might take longer to call the refueller back than to wait for the slot.
- The demand at the arrival airport exceeds capacity. Again, if the flow rate at the destination airport reduces, slot restrictions will appear. This can be the most difficult for passengers to understand, as only traffic going to the same destination will be affected. It can be a beautiful day at Gatwick, with other aircraft pushing back, taking off and landing normally, but if we’re flying to Venice and the flow rate there is reduced, we’ll have to sit on stand until we’re allowed to go.
When Do You Know If You Have A Slot?
There’s no hard and fast answer to this. Sometimes we’ll be notified of the slot when we report before a flight, and sometimes we’ll be ready to go and the flight deck printer will come to life and print something like this…
As soon as we become aware of a slot, we’ll do everything we can to minimise its impact.
Why Do You Board The Aircraft When You Know You Have A Slot?
This is probably the question that passengers ask most frequently. We do this because a slot is based on a Target Off Blocks Time (TOBT) – a target time to push back from stand. Once everybody is on board and we have the doors closed, we can bring the TOBT time forward and Air Traffic Control can put in a “Ready Message”, which informs Eurocontrol that we can push back immediately if required. This means if a gap opens up somewhere (for example another aircraft missing its slot), we can set off immediately. It also enables us to remote hold, which allows the ground crew to move on to other departures. Chances of a significant improvement vary depending on the reason for the slot, but your flight crew honestly do have your best interests at heart and we’re not shutting you in for any other reason!
What Happens If We Miss Our Slot?
The simple answer is that you get another slot. How long after the original slot this is depends on a number of factors. If we know we’re going to miss our slot, for example because of an engineering issue, we can telephone BA’s Air Traffic Management department. If we can give them an estimate of when we’ll be ready, they can obtain a new slot for us.
This Wouldn’t Have Happened If I’d Flown With (Insert Other Airline)
Actually, it almost certainly would. If there’s another airline with a flight scheduled to leave for the same destination at about the same time, you can be almost certain that they’ll have a similar slot. Of course if your flight has a slot caused by the destination airport and the flight on the next stand is going somewhere else, they may well push back as they might have a different slot restriction, or no slot at all. And if they are going to the same place, the flight was probably due to depart significantly earlier – they’ve waited out their slot and aren’t jumping the queue.
Why Can’t We Just Sneak Away?
Slots are very frustrating – everyone on board including your crew want to get underway, but they do serve a very important purpose. By pre-empting bottlenecks, delays can be absorbed on stand rather than in the air. Without them we’d fly to our destination only to find there was a backlog of arriving traffic. We’d have to enter the hold and, when the hold times were too long, we’d have no choice but to divert.
I hope that gives a little insight into the world of slot restrictions. As always, feel free to comment below with any questions / corrections!