Why am I overtaken by other aircraft before take-off?
 

I first wrote this post in.....
This thread
but thought others might be interested.

In terms of departure sequencing......

(Or, how I learned to lean back and relax as lots of other aircraft overtake me...)

At LHR specifically, but this holds for most major airports.

Building the 'ideal' departure sequence takes a lot of practice and is a complex task. There are various factors to consider:

Firstly, we separate for wake turbulence. At LHR we use a wake separation scheme called 'RECAT-EU' (RE-CATegorisation EUrope). This divides all aircraft into categories by weight and strength of wake turbulence:
Super: Basically A380 (and AN124s)
Heavy: 777, 787, A330, A350 etc
Upper: 767, 757, A300
Medium: A320, 737, A220, E190
Small: ATR72, Dash8, medium bizjets
Light: Small bizjets down to ultralights.

Timed departure separations are measured from the point at which the leader aircraft starts to rotate on take-off.

A Super followed by a Heavy is 1m40s, A Super followed by a Medium is 2m20s, a Heavy followed by a Medium is 1m40s, Upper followed by Medium is 1m20s etc etc.

Pairs of aircraft in the same category, or if the follower of a pair is in a heavier category, there is no wake separation requirement, so the follower can be cleared for take off as soon as the leader rotates (this will provide between 40s to 1min separation anyway due to hte time of pilot reaction and take-off roll.

Now route spacing...
LHR has 6 main departure routes. Taking the westerly runways (taking off towards Windsor):

• BPK (Brookmans Park) route turns NW to go towards Maindenhead, then NE to head towards Essex.
• UMLAT follows the same route towards Maidenhead, then heads directly N
• Compton (CPT) goes W over Windsor then Reading, then WNW towards S Wales
• GOGSI follows the CPT route to just E of Reading, then turns SW towards Southampton
• MAXIT turns SW to head towards Ascot, then heads towards Chichester
• Detling (DET) turns SE then E and heads towards Kent.

I've tried to colour code the above. Blue (BPK+UMLAT), Yellow (CPT+GOGSI) and Red (DET) routes can be considered independent, so can have a 1min separation between them (in good weather this can be reduced further to 'wheels-up' (40s to 1min) by the use of visual separation from the tower - i.e. weather good enough that we can see both aircraft far enough until they are 3 miles apart and/or 1000ft apart.
MAXIT is orange because it doesn't diverge enough from both the yellow and red routes to allow for 1m separation, but is 1m when paired with a blue route. We can still use visual separation in good weather, so a DET followed by a MAXIT can easily be 1min in good weather, as the DET route turns further than the MAXIT. But MAXIT followed by DET is almost always 2mins, because the geometry of the pair is poorer.

CPT/GOGSI followed by MAXIT or vice versa is usually around 1m10s-1m20s, as for these the controller will usually aim to get 3 miles separation.

A pair of aircraft in the same colour group will be around 1m40 (if on different routes, i.e. CPT then GOGSI), but a strict 2mins if the same exact route (CPT then CPT).

All of this needs an appreciate of aircraft performance, and a level of experience of airline SOPs to predict climb and acceleration performance, and an understanding of the headwind/tailwind/direction at various altitudes from the surface to 3000ft in order to minimise the separations applied.

Adding the wake turbulence separation to the route separation/spacing can result in the building of a sequence being quite the complex task. Hopefully you can appreciate now that if you're sat in a queue of DET departures (rush hour for these is 9pm!), you may see many aircraft overtake you and depart in the 2min 'gaps' in that sequence if they are CPTs, or northbounds (blues). The routes of the transatlatics can have a big impact on LHR departures, if it's a northerly day westbound, that helps, but over the years we have seen an increase in the southbound routes and a reduction in northbounds (exacerbated by the Russian invasion of Ukraine), so if all the USA flights are departting on CPTs or even GOGSIs that can slow the departure rate as there are fewer northbounds to go in the gaps.

To illustrate, if there are ten A320s waiting at the runway, in good weather, and there's a good mix of routes, then they could all be airborne in 8 or 9 minutes. In poor weather they might all go in 10 mins. If they are all going on the DET route, they will take 20 mins to all depart.

And I haven't even mentioned CTOT (Calculated Take Off Time - what you may hear on the PAs as 'ATC slot time) which are a 15min window within which a flight must get airborne. And then there can be MDI (Miminum Departure Intervals) placed on us by the ATC centre at Swanwick (usually for poor weather), which might change the spacing required between, say, all DET departures from 2mins to 4mins or 5mins apart.

A collection of various category aircraft, with various routes can present quite a difficult sequence in terms of determining the best order. Is it better to depart the Heavy BPK, then the Upper CPT, then the Medium MAXIT (to combine the wake and the route spacing in one?), but what about if the Medium MAXIT can't quite get to the holding point because of the Super DET in the way, so do I try and use another Medium MAXIT from an entry point further down the runway instead? But that one might not be ready for departure yet, and I can see one coming out with a tight slot time, so do I try to get that one through to the runway? But if I put the Super DET on next, that will take at least a minute to line up anyway as it taxies so slowly, and I won't need the full 2mins between the previous MAXIT and the Super DET because the MAXIT will easily out-perform it, so I can shave 10 to 20s off that, then I could use the Heavy CPT that's coming up from the other direction, then use the Medium MAXIT behind that one again using the route and wake separation at the same time, and I can see an inbound to T4 that is going to be crossing soon so I can use that gap to get that one across too, then I can use the Medium GOGSI behind that and I know that airline is usually a little bit slower to accelerate so I can squeeze a few seconds on that pair, then I have a Medium BPK who can go wheels up but is at the back of the queue so I'll take that one down a different taxiway....hmm, I wonder if someone will think it's jumping the queue?...... and so on and so on.....

Suffice to say that the departure runway task at LHR is one of the most challenging in UK ATC, and one that can take hundreds of hours of training in isolation to get good enough at to be permitted to 'go solo'.

MAXIT is a new one on me! I remember when MID became GOGSI (and GASGU for easterlies?) which took some getting used to.

Looks like Heathrow ATC are on long breaks today!! A great, well written explanation for us area controllers-thanks. lol

Thanks I really enjoyed that!

I have discovered an easy fix for the wake separation issue- you just start the day with all the small planes, then work through the uppers and the heavies and the rest in order of size so that all the A380s take off at 11pm.

NO PROBLEM YOU ARE WELCOME.
(currently available for complex air travel consultancy)

Like if you clicked on this thread expecting a juicy DYKWIA rant :D

(Great post Heathrow Tower)

I'll just date myself by saying I remember WOBUNs and WELINs and none of this new-fangled UMLAT... and what was that beacon they used on arrivals before there was BNN? Garston (Watford)?

It's part science, part game of chance (as in which hand the Air Deps controller is dealt), part black art. Lovely explanation.

May my SHTs always jump the queue and get intersections.

Usually, the aircraft with KARFA in 1F are given priority.

Brilliant stuff! Thanks so much Heathrow Tower!!

This is why I love FT - absolutely fascinating. Thank you.

I have always admired you lot for the complexities you have to manage. RAF ATC was much simpler … “Land in turn, 4 ahead” etc! Although a QRA scramble in the middle of a busy session coukl be stimulating. Or a Canberra in Singapore that had to be airborne within 5 minutes of ‘door closed’ due to heat/sun on the bubble canopy!

A great explanation, I think many (including me) knew that it was difficult for small aircraft to follow large aircraft but this puts it all in perspective. Thank You.

What a great post! I have always admired you ATCs for the complexities you have to be able to manage, and this puts it in a very understandable perspective.

I for one would love to hear more like this if/when you have time :)

Very interesting, thank you for posting. Out of interest are 747s Super or Heavy in this line up? (I assume there are still cargo flights). Also, in which category was Concorde?

Just ask ChatGPT and will tell you all the sequences. :D
very good explanation and article.

Very interesting - now I know that GOGSI is the reason why I have every evening a 380 and 350 towards South Africa directly over my house in Guildford :)

There must an approach towards LHR which overflies Guildford - I always see TAP airplanes coming from Portugal when I look out of my living room in PM - but hopefully this will be explained another time!