You've spent the last few hours getting to the airport, trailing around the terminal, queuing to get on the aircraft and now you're finally settled into your seat as you taxi towards the holding point. Straining to see out of the window you try and count the number of aircraft ahead of you attempting to work out how long until you'll be in the serenity of the sky, getting excited as you approach the front. The one ahead enters the runway and you hear the dull rumble as they apply the power knowing you're next, then all of a sudden another aircraft appears and lines up, then another and one more. You start thinking to yourself that this isn't fair, you were there first why aren't you going?
Why did that flight takeoff before us?
While it is a simple question, it is far from simple to answer it! There are multitude reasons which must be considered when deciding the departure order from the direction the aircraft will fly to how big they are, from restrictions on the flight to weather. To make things more challenging none of these reasons work in isolation, each one impacts the others making the decision who goes next a multifaceted challenge. These rules which prescribe the minimum time between successive departures is referred to as separation. In this post I'll explain the two most common factors and expand to others in later posts so you'll hopefully have a little more understanding why 4 aircraft may go before you or why that one random aircraft comes from the back and takes off before everyone else.
I would like to note this is a fairly simplistic version to make it accessable for all, but if you have any further questions feel free to ask.
The reason we don't depart aircraft in a first come first served basis is that we're attempting to maximise the use of the runway and to 'loose/waste' the least amount of time. In an ideal world an aircraft would takeoff, immediately another would takeoff and this would be repeated continually. If an aircraft has to wait on the runway for a period of time rather than departing immediately it is said that you're wasting/losing time. As I will explain in the future paragraphs/posts you will see that there are times when you can't help but lose some time, but the idea of the game is to order the traffic in such a way that this happens the least amount of times as possible. Airport schedules are based on a nominated minimum departure rate, or number of aircraft getting airborne per hour, therefore if the controller is unable to order the traffic to attain this rate it may introduce delays for traffic ready to go at the gate. In order to add some perspective at busy airports they need to depart 50+ aircraft an hour and therefore you're only able to loose time a maximum once every 10 minutes.
Here is probably a good place to highlight that all departure separations are based on the time an aircraft takes off. So if a separation requires 1 minute spacing that means that the 2nd aircraft needs to get airborne 60 seconds after the time the first one got airborne. Therefore you have to be a little creative with when you roll aircraft. For example if BAW123 got airborne at 12:14 (that's 12 minutes, 14 seconds past the hour) the next aircraft should not be airborne until 13:14, subject to it been a 1 minute separation. So if the next aircraft to go is a jumbo, you can roll it almost immediately as you know their roll will be at least 1 minute, however if the next to go was a Citation you will have to wait a little bit as their takeoff roll may only be 30 seconds. In that case you many not roll the citation until 12:44 so it would be airborne at 13:14. Simple eh?!
The first separation we will consider is route separation, or simply the direction the aircraft will fly after takeoff. If aircraft took off and flew in whatever direction they liked it would be near on impossible to know how long you needed to wait until it was safe for the next aircraft to depart. In order to solve this issue major airports have SIDs (Standard Instrument Departures) or set routes that aircraft follow after takeoff. Below is a fictional example of 4 routes out of an airport, A, B, C and D. An aircraft wanting to go North would fly the red route, A; one wanting to go towards Spain would fly the green route, C.
Since we now know exactly which way aircraft are going to fly once they've taken off we can work out how long must elapse between successive departures to ensure they're separated. This information is then put into a matrix table as below:
Starting on the left you choose the departure routing of the first aircraft, then move across to the routing for the second aircraft and read off the minutes required between them. A followed by C would be 1 minute, B followed by C would be 2 minutes, D followed by D 3 minutes, etc.
In order to loose the least amount of minutes we try to order the aircraft so that they're always following one which allows a 1 minute separation. For example if you had aircraft on the routing A A C D B at the hold the difference between doing them as they turn up or ordering is as follows:
A (2) A (1) C (2) D (1) B = 6 mins.
A (1) D (1) B (1) A (1) C = 4 mins (2 min saving).
But it's not that easy.... it's not just between the one just airborne and the next one to depart we need to separate, but also from the one that went before that one too. For example at the hold is D B B A D
D (1) B (2) B (1) A (1) D = 5 mins.
B (1) A (1) D (1) B (1) D = 4 mins (1 min saving)
Do you see the issue though? We only achieve 2 minutes between our two D aircraft whereas the minimum is 3. So
D (1) B (1) A (1) B (1) D = 4 mins like before, but now we have achieved our minimum 3 mins between D's and 2 mins between B's.
In these examples we're just working with 5 aircraft, as you can imagine if you extrapolate these savings/losses over the course of an hour, morning, day, etc the massive difference it can make to the efficiency of the airport by just changing the order they depart in subtly.
Aircraft are grouped into one of 6 categories based on their takeoff weight for wake turbulence purposes and there are minimum time intervals that must elapse between departures of particular groups.
For example 2 minutes must elapse between a heavy aircraft followed by a medium, but there is no wake turbulence spacing required if it is a medium followed by a heavy.
Similarly to route separation it makes sense to depart aircraft after each other which have no wake turbulence separation requirements, this is why you'll often see a lot of small aircraft go in a row followed by lots of large aircraft.
Now if you remember how I said each separation was intertwined, this is where we can get clever. If you have a heavy departing on route A and a medium aircraft on route A you will require 2 minutes for wake turbulence separation, but it also means you achieve 2 minutes for route separation too. It's almost like a 'freebie' loss of time as you've achieved two separations in one movement.
I think that's enough to ponder for now! Next post we'll start to look at slots and departure restrictions and how they impact on the 'perfect' order.
As I said at the beginning this is a very basic overview so that those with little aviation knowledge can hopefully understand, but if you want further information or questions please feel free to get in touch.