Aviation Lingo: What is an ATC Slot Restriction?

by | Sep 6, 2016 | Aviation Lingo, Flying | 0 comments

You are running through the airport like your life depends on it. Your flight is going to depart soon and you are all the way on the other side of the airport. It will be tight, but you ask to all gods above for Usain Bolt speed and try to break a world record just to catch your plane.

You see the gate, you speed up and the gate agent lets you, board. Your heart is still racing and you feel like fainting. As soon as you board the plane and sit down you start catching your breath and relax. Phew, that was close!

Then over the PA system a strong and cracked voice thunders as if God itself was talking to you. You try to pay attention and this voice says something like “…this is your captain. We are all ready to go, unfortunately, we have an ATC Slot restriction and will have to stay here for another hour…”

Upon hearing this terrible news, you start thinking about all that precious time of holidays that you will lose, maybe you will also lose your transfer and other activities that you booked for that evening.

You get angry by the fact that you just ran half a marathon to catch this flight and now you have to stay put for another hour!? Also, what the heck is this ATC thing?

Flight Paths
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The sky is divided into various flight routes, some are restricted to military use, others for commercial flights carrying passengers and others are for aircraft carrying cargo.

An aircraft carry passengers will need a flight plan that shows the route that it’s meant to follow and the weather conditions along that route.

When the Air Traffic Controllers are on strike, the companies will try to get different routes to avoid the flight areas affected by the strike in an attempt to prevent cancellations. Unfortunately, there is a limited number of flights that a certain route can take, which means that there might be a slot restriction on the departure of your flight.

What Is a Slot
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A slot is basically a block of time assigned to a flight. There are two types of Slot: The Airport Slot and an Air Traffic Control (ATC) Slot.
Airport Slot:

Airports have a finite capacity for departures and landings, they also have a limited capacity of aircraft that can be on the ground at a given time.

Airlines that want to flight to an airport will need to buy slots to land on that airport, these are blocks of time that might be available. The schedule of a flight will be greatly impacted by these slots. If an airline wants to schedule a flight for 4:30 am and the airport is at full capacity at that time, the airline might need to negotiate and get assigned an earlier/later block time.

ATC Slot:

These will be the ones that will affect your flights the most. These slots are assigned to flights by the Air Traffic Control and they are used to control the flow of flights on certain routes.

Imagine that you have a square box and you can put four cubes inside the box without touching each other. If you put an extra cube, this extra cube will have to touch one of the cubes that were already inside the box.

The only way to put that fifth cube in the box is if you take out one that was already inside the box.

Now imagine that the cubes are aircraft and the box is a flight route. Everything in aviation is done to minimise risk, so to avoid a collision some flights will have to be delayed to keep the flight path safe – maintaining each aircraft at a safety distance between each other.

There are external factors that can greatly reduce the flow of moving aircraft on a certain flight path. Bad weather, too much traffic, strike action, military activity and even no parking space at the arrival airport will influence the flow of movement.

What Happens If We Miss Our Slot?
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If your flight is delayed for some reason and the pilots have not sent a READY message to the Air Traffic Control, you will miss that slot. Lucky for another aircraft waiting, because if they are ready to go, they will have your slot assigned to them.

The risk of missing a slot is quite stressful, because who knows if the next slot will be in 2-3 hours. The whole crew and ground staff want flights to departure on time because that means the day’s operation will run smoother.

Missing slots might cause the crew to run out of hours to work for the day and the flight might get delayed even further. In the worst case scenarios, the flight might get cancelled and rescheduled.

What Does a Ready Message Mean?
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You are sitting down on the aircraft, your breathing has calmed down but your heart is still racing. You are probably getting annoyed for making a fool of yourself by running like a mad man all over the airport.

The captain makes another PA informing you that they just sent a Ready Message and hope that the ATC slot will improve. Ready message? Weren’t they ready 10 minutes ago!?

Sending a READY message simply means that the aircraft is fully boarded, all bags have been loaded into the hold, all doors are closed, the airbridge/steps have been removed and a tug is in place to push back the aircraft so it can make its way to the runway.

So I’m Stuck Here For One Hour?
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Even if the ATC slot is of one hour, it doesn’t mean that you will have to stay put for one hour. There are a lot of flights departing and arriving at an airport through the day. Which means that some other aircraft might lose their slot and you might get clearance to take off.

In most cases, the ATC slot will always improve and sometimes an one hour slot might turn into just a 20 minutes one. Unless the weather is really bad or there is any strike going on, it’s most likely that your slot will improve.