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Originally Posted by Duck1981
(Post 35125590)
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! |
See below. (Not to be used for real-world nav!!!)
The GOGSI is the white line, MAXIT the green. My blue arrows show how ATC may route aicraft once over 4000ft. Aircraft must stay on the published route up to 4000ft to respect the minimum noise route. https://cimg3.ibsrv.net/gimg/www.fly...632594f683.jpg |
Originally Posted by Duck1981
(Post 35125590)
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! https://cimg2.ibsrv.net/gimg/www.fly...f8c2f29f6f.jpg |
I have copied and pasted this excellent and generous posting to print so that Him Indoors can read this as well. I have learned much.
Thank you is inadequate but so very much appreciated. |
Originally Posted by flatlander
(Post 35125923)
So that's why we get photos of A380s looming over the A320 they are following? :)
More seriously, do you (ATC) have any influence over the choice of departure route, in particular when it doesn't increase the distance much? Can you, for example, influence the use of a northerly departure in the evenings for some flights? Or is this entirely the choice of the airline and Captain? For instance if you're flying from LHR to AMS, you'll depart on a BPK, head for Clacton, and join the the arrival procedure in to AMS. The routing document tells you that's the route between LHR and AMS, and that's what your meant to plan for and file. It used to be possible to have some tactical re-routing held in reserve due to airspace congestion, failures, breakdowns, weather, strikes, etc., and be deployable by ATC if available and relevant - if you use LHR-AMS as your example, LHR might have been able to depart it on a DET SID (South turning, rather than North), and from there proceed as though it's a flight out of Gatters to AMS. That did actually happen to me once some years ago on a BD flight to AMS where BPK departures were blocked for a significant period of time due to a line of storm cells passing to the North of the airfield. No idea if that actually happens or still can happen these days. The OP will know more! |
Originally Posted by Heathrow Tower
(Post 35123331)
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):
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'. As for the departure spacing, my tower wasn't busy enough to do that, but at the real busy ones, e.g. DFW, JFK, ORD, etc, ground control would taxi them out with the spacing needed by Local for the departure route, so that there would be no delay at the runway end, they would have been held before being put in the taxi stream. |
Originally Posted by NickB
(Post 35125745)
Thanks Heathrow Tower. Very interesting. The scheme seems to use a series of a variables in a systematic way, which suggests that it lends itself to automation. I assume that controllers would have the final say and ability to override any automated system but do the tools that you use suggest an optimised sequence based on these variables?
We have gone from wake separations that are either zero, 2mins or 3mins to zero, 1m20s, 1m40s, 2mins, 2m20s, 2m40s etc etc. The future is 'Pair-wise' separations which sees wake category-based separations replaced by indvidiual aircaft pairs each having separations (a 120x120 matrix). For example, 777-300 followed by A321 might be 1m20s, 777-300 followed by A320 1m30s, 777-300 followed by A319 1m40s etc etc. At that point no human could remember all the pertinent separation values. As has already been said, there is a Departure manager (DMAN) running in the background developing an 'ideal' departure sequence, which feeds back via the airport CDM system to publish Target Start Approval Times, but this uses historical average acheived departure separations, and average taxi times, and average holding point delay times, and doesn't take account of ground congestion nor any of the other myriad types of friction seen on the ground at LHR, so is very 'theoretical' in its application. A more tactical system in terms of departure separations is being developed and worked on, but the actual departure separations that a controller will aim for (apart from the binary wake separation minimums) are very judgement- and experience-led, which is very tricky to code into an automated system. |
Originally Posted by mjh0
(Post 35126352)
The Departure Procedures (aka "SIDs") feed into specific en-route airways. There are preferred routings between points (e.g. between airports, or between an airport and an upper airspace boundary) and these are designed to organise the randomness that might otherwise exist if everyone just flew in a roughly straight line, maximise use of the airspace safely, while also in theory making it manageable for the controllers.
For instance if you're flying from LHR to AMS, you'll depart on a BPK, head for Clacton, and join the the arrival procedure in to AMS. The routing document tells you that's the route between LHR and AMS, and that's what your meant to plan for and file. It used to be possible to have some tactical re-routing held in reserve due to airspace congestion, failures, breakdowns, weather, strikes, etc., and be deployable by ATC if available and relevant - if you use LHR-AMS as your example, LHR might have been able to depart it on a DET SID (South turning, rather than North), and from there proceed as though it's a flight out of Gatters to AMS. That did actually happen to me once some years ago on a BD flight to AMS where BPK departures were blocked for a significant period of time due to a line of storm cells passing to the North of the airfield. No idea if that actually happens or still can happen these days. The OP will know more! https://cimg7.ibsrv.net/gimg/www.fly...d53283f27b.jpg ...which shows the recommend/compulsory routes within each nation's airspace. In terms of influencing this, Heathrow ATC has a person embedded within HAL's Airport Operations Centre, who monitors the traffic situation a few hours to a few days in the future. If they determine there are going to be delays on certain routes, we might contact certain airlines and, for example, if the Dover sector will see delays tomorrow, we might recommend that some airlines who would normally route through it (to S Germany, Austria etc) re-plan and fly on an initially northbound SID via Essex and out over the North Sea. Yes, it's a longer flight time with more fuel burned, but it might mean zero delay instead of 30mins delay on the ground at LHR. It's not a mandatory process, the airline can say 'no', it's down to their commercial decision. |
Originally Posted by zdcatc12
(Post 35126383)
Thanks for this!!! I am a retired ATC from the US, FAA. It is interesting that we have different wake turbulence categories/times than you do. I thought that they were all ICAO, but I guess not. We have Small, Medium, Large, Heavy and Super Heavy. And the 757 is in a class by itself. Super is only the A380. Heavy is anything with a gross takeoff weight of over 300k lbs, appx 136k kgs.
As for the departure spacing, my tower wasn't busy enough to do that, but at the real busy ones, e.g. DFW, JFK, ORD, etc, ground control would taxi them out with the spacing needed by Local for the departure route, so that there would be no delay at the runway end, they would have been held before being put in the taxi stream. |
Sensational! Thanks for all the explanations!
[now I only need to manage to spot GDF once from the plane - it took me ages until I was able to spot my previous places in Earls Court and Richmond - the later was easy in the end though with the church above The Alberts] |
As someone who is just needing his Medical Clearance to start the education for ATC, this was EXTREMELY interesting to read! Thank you so much for the immensely detailed post, as well as follow-ups!
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A great insight, thanks very much!
You mention that departures on different routes don't have to follow the strict 2 mins - is reduction there purely based on the controller's experience or are there still set limits? For example, CPT/GOGSI hardly diverge but 3nm will still be there in trail if you departed them 1min apart; why the extra 40 seconds - is it to ensure the GOGSI turns South? Curious to see what the basis behind that is. |
Originally Posted by CarefreeBA
(Post 35127744)
A great insight, thanks very much!
You mention that departures on different routes don't have to follow the strict 2 mins - is reduction there purely based on the controller's experience or are there still set limits? For example, CPT/GOGSI hardly diverge but 3nm will still be there in trail if you departed them 1min apart; why the extra 40 seconds - is it to ensure the GOGSI turns South? Curious to see what the basis behind that is. For example, if you have a DET followed by a MAXIT, the DET runs early (2miles out) and turns left by around 120 degrees. The MAXIT first turn is about 3-4 miles out, and only turns by about 20 degrees. With DET then MAXIT, as soon as the DET is established in the turn, I can predict with accuracy the geometry of the pair in the future, and that barring the MAXIT actually following the DET route in error, the aircraft will always be diverging. i.e. the point below, DET aircraft at the blue dot, MAXIT aircraft at the red dot (still on the take off roll): https://cimg9.ibsrv.net/gimg/www.fly...92f446a38f.jpg With a MAXIT then DET, I have to monitor both aircraft more closely for far longer, until the MAXIT is maybe 5 or 6 miles out, and the follower DET is well into its own turn. Red dot MAXIT and blue dot DET again: https://cimg0.ibsrv.net/gimg/www.fly...34ba91a3ab.jpg |
A bit of a tongue-in-cheek observation / question of course but do any airlines get preferential treatment at airports? :D
Without fail at Gatwick, on a BA plane we usually get out to the holding point and seem to sit there for a minimum 10-15 minutes whilst a string of Easyjet planes go past us and take-off. Usually it seems at least 50% of these have arrived at the holding area after the BA one I am on - but then depart ahead of us. It can't be due to plane types for separation distance as they are all A32X series. Whether they are turning left or right on departure maybe? Plus regarding push-back times, one frequently will hear a member of the flight deck say "we are ready to go so are negotiating with air traffic for an earlier departure". Now to most the word "negotiate" means to trade something in exchange for something in return and a quick google of the meaning is: to transfer to another in return for something of equal value. Therefore what exactly does a member of the flight deck have to offer the fine chap in ATC in return for a better slot time? :confused: So maybe during the push-back time "negotiation" the Easyjet pilots have more up their sleeve to offer the ATC chaps and get the priority first onto the runway? ;) |
Quite a good example from Sydney of how it's possible to get loss of separation on subsequent departures where one aircraft turns inside another - multiple issues at play here, issues with the procedure as published (it didn't "fail safe"), poor performance assumptions by the ATCO, etc. - https://avherald.com/h?article=505e2c5d&opt=0
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