Radar ATCO Sequencing

[theATCO]

This question is directed mostly towards radar ATCOs, working at busy units, where minimum spacing between aircraft on final approach is required to keep the flow going. What kind of techniques are you using to consistently achieve the exact minimum spacing required? Are you working a lot with "hard" speed control (where aircraft are instructed to fly at an exact IAS) or more with soft speed control (e.g. "speed 210 kt or less) and possibly more margin for compression?

How do you decide on the precise time to base an aircraft, behind number one, on final? Are you using the distance measuring tool between the aircraft and turn base at a specific distance (e.g. from downwind to base when the distance between the two aircraft is the minimum required on final) or are you using prediction lines to judge it?

At very busy units, when you are consistently required to achieve minimum spacing on final, I would assume that the required minimum separation might not always be achieved. What are the consequences of a loss of wake turbulence separation between aircraft on final approach? Immediate go-around or just a simple "caution wake turbulence recommended spacing 4 miles - are you happy to continue"?

Regards!

[Packer27L]

What kind of techniques are you using to consistently achieve the exact minimum spacing required?
Practice makes perfect.

[broken headset]

Good vectors and hard speed control. First a/c on final 160kts or more until 4 DME subsequent a/c exactly 160 to 4, last one (if there is a gap) 160Kts or less. 180Kts on base, 210Kts with about 25-20nm to go. Don’t lose vortex or it’s coming back to you.
experience helps with the compression after the first a/c passes 4DME (speed control finishes at 4) as there are too many variables wind a/c type etc

[broken headset]

Sometimes I use the vector lines, mode S speeds, ERBM lines, sometimes you don’t at all. Having a FIN controller (director) helps make the gaps more consistent when R/T loading is high.

[HEATHROW DIRECTOR]

Most radar ATCOs I've met (and me!) can't explain exactly how they achieve maximum landing rates but it gets easier with time (or so they say!).

[aaesteve]

The mystery of eyeballing. You can master it but you can’t understand it.

[HEATHROW DIRECTOR]

I don't know what vector lines, mode-S speeds and ERBM lines mean. We seemed to cope with paper strips and a felt tip pen!

[chevvron]

I don't know what vector lines, mode-S speeds and ERBM lines mean. We seemed to cope with paper strips and a felt tip pen!
What happened to the much vaunted 'CAAS' then?

[360BakTrak]

Most radar ATCOs I've met (and me!) can't explain exactly how they achieve maximum landing rates but it gets easier with time (or so they say!).

I agree! Working parallel runways with 3nm staggered spacing; no particular rules only guidelines and practice makes perfect. Trust in TCAS :E

[Talkdownman]

What kind of techniques are you using to consistently achieve the exact minimum spacing required?
The Number One Directors put the Number 2 under pressure by hollering "turn 'em in"...

[flightcatcher]

It comes with practise. What helped me (and sometimes still does) is to ensure vertical separation exists before turning one onto a tight base behind another on final. That way, if you've misjudged it, at least you have separation, and you can then sort out the spacing by taking it through the loc / slowing it down earlier etc.

As to a spacing which reduces below wake turbulence minima...if radar are still working the following aircraft, it's easy - break it off and re-sequence. The minima are there for a reason, and numerous fatal wake encounters over the years (and around the world) are why I have no time for the expression "are you happy to continue?" when they haven't been provided with the correct, safe spacing by radar...

[Get me some traffic]

A very respected Fleet Captain of a much missed UK airline once said in a Controller/pilot forum "in a capacity strapped scenario if you don't get the odd go-around, you're not trying hard enough." True comment in the early 2000s. Is this true today?

[Del Prado]

A very respected Fleet Captain of a much missed UK airline once said in a Controller/pilot forum "in a capacity strapped scenario if you don't get the odd go-around, you're not trying hard enough." True comment in the early 2000s. Is this true today?

A very wise mentor of mine once put it like this:
If someone is landing 40 aircraft per hour and you can sit down and knock a quarter of a mile off the spacing then over the next hour you’ll save 10 miles. That’s three extra movements. Even if you have two go arounds that’s still a net increase of one.

[Packer27L]

A very wise mentor of mine once put it like this:
If someone is landing 40 aircraft per hour and you can sit down and knock a quarter of a mile off the spacing then over the next hour you’ll save 10 miles. That’s three extra movements. Even if you have two go arounds that’s still a net increase of one.
A wise mentor indeed. And the guys/gals at Swanwick can actually do spacing to an accuracy of 1/4 mile. That’s why they’re the best in the business 👍🏻

[kcockayne]

A wise mentor indeed. And the guys/gals at Swanwick can actually do spacing to an accuracy of 1/4 mile. That’s why they’re the best in the business 👍🏻
That won't stop them being replaced by a computer !

[theATCO]

Thanks for all the replies guys!

It's interesting to hear how other people are nailing that spacing and it seems like I'm not the only one who can't explain it more than "eyeballing the relative distances and speeds".

In regards to the loss of wake turbulence separation I've heard of busy units where apparently it has become as standard practice that you, up to a certain point, only warn an aircraft of wake turbulence (let's say when the loss of separation equals to less than 0.5 nm) and ask them if they want to continue. However, if the loss of separation will be greater than this the aircraft has to be taken around for another approach. I would imagine that a procedure like this would have to be approved by the "appropriate authority" and described in the MATS part 2. Has anyone else heard of this?


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[Del Prado]

.......... only warn an aircraft of wake turbulence (let's say when the loss of separation equals to less than 0.5 nm) and ask them if they want to continue..........


​

I would suggest it’s a bit more nuanced than that.
If there is a loss of wake turbulence separation of up to 0.5 nm and you’ve taken remedial action (such as further speed reduction or there’s a significant ‘pull out’ due wind gradient) then it may be prudent to caution the pilot as to loss and ask if they wish to continue.
I don’t think anyone would condone habitually busting vortex spacing by up to 0.5 nm and just asking if they want to continue.
In either case, a loss of 0.5 or greater should result in the following aircraft being repositioned and as you say, this would be apparent in local orders or practices.

[360BakTrak]

I believe LHR tower advise arrivals if the wake turbulence separation reduces to 1/2nm below minimum; any further loss results in being broken off the approach. I stand to be corrected on that.....memory not wot it used to be! And it wasn't FIN habitually breaking the minima, but various factors can cause it occasionally (pilots slow to fly given speeds, changes in wind, etc).

[HEATHROW DIRECTOR]

I think the final director will notice the erosion so won't need to be told! You are right; there are so many factors that can cause the spacing to erode so achieving max runway utilisation is a hard job,

[Gonzo]

It all depends if the lead is inside or outside 4DME.

And yes, for LHR certainly all these procedures were in the MATS Part 2.

The new version of TBS has complicated things slightly.

Before TBS, it was pretty simple. If the lead was outside 4DME and wake turbulence spacing was infringed, Director had to rectify. If the lead was inside 4DME, then tower gave a caution to follower when that infringement reached 0.5nm.

Don't forget though, this was with procedures that only provided wake turbulence separation to 4DME, not to touchdown.

[Gonzo]

Prior to the most recent TBS version, which includes Optimised Runway Delivery (i.e. an indicator is presented to Director which is threshold wake separation + expected compression), then the wake separation was delivered to 4DME. so a 4nm H-H separation minima was applied when the leader got to 4DME, with compression reducing that as the leader reduced speed. There was no wake separation applied once leader got to 4DME. Tower would give the caution at 0.5nm compression.

With current TBS we separate to threshold.

[2 sheds]

So what you are saying, Gonzo, is that previously, under commercial pressure, reduction of required safety minima was officially brushed under the carpet with a few weasel words about delivery and compression! :-)

2 s

[Gonzo]

Those procedures had been in place at LHR since well before my time (don’t forget, we used to have LHR-specific separations too (off the top of my head, H-L was 7nm, not 8nm as in rest of UK) and were proven to be safe. LHR did not have any greater rate of wake encounter than anywhere else, as far as I know.

If you take the RECAT EU threshold separations, and add on the compression, it’s striking how similar those distances are to our previous 4DME separation distances. :}

Introducing RECAT and TBS v2 was a long term goal agreed by UK CAA and NATS to move to separation to threshold.

I prefer to think of it as LHR was running a long term trial of RECAT EU threshold separations. :ok:

[Brian 48nav]

Excuse my ignorance, please explain to this long retired 'old fart' what TBS is.

For what it's worth I found that spending 90 minutes as LL/LHR No2 Director, when you might turn 60 or so inbounds onto the ILS LOC', could be absolutely shattering. Having also been a LATCC controller I would say that once you had hacked how to do LL No2 it was easier than TMA, but the concentration required made it a very tiring position.

[360BakTrak]

'Time Based Separation' I think Brian.

[Brian 48nav]

Thanks 360BT - Blimey, what will they think of next?

[Gonzo]

Brian, have a look at NATS.aero/tbs

TBS v1 took the UK 6 category wake separation that you would recognise, and used that distance as a baseline for a headwind of 5kts. This is then translated into time separation, for example 90s for a 4nm separation.

The TBS tool calculates the headwind component on final approach by using the Mode S data downlinked from each aircraft.

It then calculates what distance would result in a 90s gap given current headwinds. So in a 15kt headwind, it might be 3.5nm, in a zero wind day it might be 4.3nm.

An indicator is then depicted on radar 3.5nm or 4.3nm behind the leader of the pair in question for FIN to reference.

Yes, you read that right, if the wind is less than a 5kt headwind, we actually increase separation over the distance based separation.

Only wake separations we’re provided with indicators.

TBS v2 incorporated RECAT (RECATegorisation of wake)....so 757, 767, A300 are what is effectively a ‘lower heavy’ category, and their new separations.

it also incorporates Optimised Runway Delivery, which forecasts the expected compression between the pair of aircraft based on average speed profiles inside 4DME, so FIN sees an indicator which is threshold separation + compression, and Tower sees an indicator at threshold separation only.

TBS v2 is also doing the same in terms of modifying according to headwind component.

TBS v2 also provides indicators for certain pairs where runway occupancy is greater than wake separation, or where wake separation requirement does not exist.

Hope that makes sense!

[kcockayne]

All very mind boggling ! I just wonder what used to happen in the days before Wake Turbulence Separation existed. To my recollection; I can't remember vast numbers of aeroplanes hurtling to the ground because they were too close to each other !

[DaveReidUK]

All very mind boggling ! I just wonder what used to happen in the days before Wake Turbulence Separation existed. To my recollection; I can't remember vast numbers of aeroplanes hurtling to the ground because they were too close to each other !

Probably because when wake turbulence rules were introduced in the 1970s, there weren't vast numbers of aeroplanes around.

Though by then, there had already been fatalities as a result of encountering WT.

[Brian 48nav]

Thanks Gonzo

I didn't understand a word of that - perhaps I need a months course at the college on full allowances to get to grips with TBS! Oh, I forgot - no college, no allowances these days!

[kcockayne]

Probably because when wake turbulence rules were introduced in the 1970s, there weren't vast numbers of aeroplanes around.

Though by then, there had already been fatalities as a result of encountering WT.

I generally accept what you say, Dave; but, I was vectoring plenty of a/c back then in pressurized environments (Aberdeen & Jersey - & others were doing the same at Heathrow & Gatwick) without the benefit of Wake Turbulence Sepn., & the regular separation was 3 nm (no WTS). I can’t remember any WT inspired incidents though. Maybe my memory is failing.

[NotaLOT]

Brian, have a look at NATS.aero/tbs

TBS v1 took the UK 6 category wake separation that you would recognise, and used that distance as a baseline for a headwind of 5kts. This is then translated into time separation, for example 90s for a 4nm separation.

The TBS tool calculates the headwind component on final approach by using the Mode S data downlinked from each aircraft.

It then calculates what distance would result in a 90s gap given current headwinds. So in a 15kt headwind, it might be 3.5nm, in a zero wind day it might be 4.3nm.

An indicator is then depicted on radar 3.5nm or 4.3nm behind the leader of the pair in question for FIN to reference.

Yes, you read that right, if the wind is less than a 5kt headwind, we actually increase separation over the distance based separation.

Only wake separations we’re provided with indicators.

TBS v2 incorporated RECAT (RECATegorisation of wake)....so 757, 767, A300 are what is effectively a ‘lower heavy’ category, and their new separations.

it also incorporates Optimised Runway Delivery, which forecasts the expected compression between the pair of aircraft based on average speed profiles inside 4DME, so FIN sees an indicator which is threshold separation + compression, and Tower sees an indicator at threshold separation only.

TBS v2 is also doing the same in terms of modifying according to headwind component.

TBS v2 also provides indicators for certain pairs where runway occupancy is greater than wake separation, or where wake separation requirement does not exist.

Hope that makes sense!

Gonzo,
That's an extremely helpful explanation of TBS - many thanks!
Is TBS yet operational at LGW, and are there any further complexities involved with TBS on a mixed mode runway?
Best regards

[Atcboss]

Hi,

Basic idea in our region is to reduce the speed 210 on first contact (about 25nm to go / or downwind abeam the airport), then 190 on base, 180 to 10 dme and finally 160 to 4 dme.

Cheers

[Emma Royds]

A wise mentor indeed. And the guys/gals at Swanwick can actually do spacing to an accuracy of 1/4 mile. That’s why they’re the best in the business ����


Will distance based separation soon become irrelevant, with the (planned) widespread use of time based separation?

[Packer27L]

Will distance based separation soon become irrelevant, with the (planned) widespread use of time based separation?
TBS linked to some sort of departure manager would be the daddy - imagine providing spacing to suit the departure order (ie heavy and medium haul narrowbodies departures get bigger gaps, short haul gets slightly smaller gaps). Maximum runway utilisation. But at what £ ?

[HEATHROW DIRECTOR]

Packer27L What you describe is already achieved by air controllers at busy airports so I don't see the benefit of more gadgets. I'd far rather know that the air men have their eyes out the windows than having to spend more time on electronic gizmos.

[Del Prado]

HD, think of busy single runway airports where the departure order could be fed into Time Based Separation software.
Presently, departure gaps are about 6 Miles, sometimes less in a strong wing and sometimes more after heavy arrivals. What if the TBS indicators were tailor made for each departure? You could have 90 seconds for a 737, 110 for a heavy, you could even have the aircraft weight included in the equation so each departure gap was individually calculated for each departure.
All the radar director would see was an indicator precisely calculated for the next departure and all the air controller needs to know is the next gap is custom made for their next departure.

you could increase the hourly movement rate at busy, single runway airfields by 10-20%.
And no electronic gizmos to distract anyone.

[Packer27L]

Thanks Del Prado, that’s what I was getting at, but you put it more eloquently!

[Not Long Now]

Del Prado, not sure you could get another 10-20% out of Gatwick. Essentially, when busy, the gaps are already tailor made to get 2 away or whatever the plan is that FIN and DEPS have come up with.

[Del Prado]

I disagree.

A gap is usually 6 to get one away.
If you have a 777 following a 777 with a 747 to depart in between the gap required is 6.
if you have a 738 followed by a 319 with a 738 to go in between the gap is still 6.
Defaulting to the lowest common denominator spacing (which is based on the worst case traffic mix) does not lead to an efficient system.

There are savings to be had by having tailor made gaps calculated to suit each aircraft type rather than default to 6 miles, which is enough to get a heavy away between two heavies.

Also 737s could do 170kts to 5 and A319/320 could do 160 to 5, increasing stable approaches and reducing go arounds.

Wouldn’t it be better to have a systematic spacing delivery where landing clearance is consistently given at, say 200’ than varying between 50’ and 500’ ?