“Dear passengers, this is your captain speaking from the flight deck. We are fully ready but unfortunately, we got given a SLOT in about 45 minutes from now. Please setback and relax and we let you know if we will get any improvements.” Now people who are in aviation know that you just have a delay but have NO idea what a slot really is and why it is given so today I’ll try to explain you this.
A SLOT is a given time where an aircraft is allowed to take off. A plane departing at 1200 can have a SLOT in a period 5 minutes before and 10 minutes after often a slot is given. This is the time that the aircraft is allowed to take off by Eurocontrol (supervising the airspace for enough separation between airplanes in the air). Eurocontrol gives so-called CTOTs (calculated take-off times) also known as slots. So for this plane departing at 1200 could takeoff between 1155 and 1210 to integrate into the airway system.
If you have a normal flight and all goes well you never hear about these SLOT times, but for example, if the weather over a part of Europe gets worse the workload increases for Eurocontrol to divert them around these areas, a slot’s are given to other aircraft.
It happens that an aircraft misses a slot for different reasons, for example, passengers are too late or the luggage is not on time etc etc. Than a pilot has to ask for a new slot. When the pilot asked for this Eurocontrol puts in a new entry and the computer comes up with a new time for this flight where air airspace is quite enough again to allow this flight to take-off and depending on how busy it is you can get delays up to several hours even.
Now I can hear you ask yourself why don’t they let us wait outside the aircraft, it is more comfortable and I could go to the toilet or get a nice coffee instead of sitting in this Aircraft. I’ll explain why that is. A pilot can send out a “ready message” towards the tower. This means the aircraft is ready to go and all they need is an OK from the tower. In case there is a different flight who missed their slot, for example, they could take this slot. So the aircraft can still depart earlier than the given slot by Eurocontrol. (its a bit more difficult than this but in short it works like this.)
Slots given are not only given by Eurocontrol but can as well be for airports. Imagine if because of bad weather an different airport has to close, other aircraft have to divert towards the open airport. To lineup everyone in a good way, it needs time so you can even get Airport-slots for departure and arrival. Pilots usually don’t hear the reason for the slot times. They only hear something like KLM123, your new slot time is 45. This means instead of 1200 UTC the departure slot time is now at 1245 UTC. A ready message doesn’t guarantee that you depart earlier it only improves the chance and there is nothing much the pilots or the companies they work for can do about it.
If you have any questions regarding this subject please let me know below or send me a message with the contact form on this website.