time based separation

[golfbananajam]

Hi ATCers

Just reading an article in the March 2014 edition of E&T (Engineering and Technology publication) about the introduction of time based separation on approach to LHR rather than distance based separation which is expected to halve delays (their words not mine) and I'd be interested in your thoughts please

regards (and be gentle)

[le Pingouin]

Halving delays sounds very optimistic. Are you asking why it would reduce delays or about the halving?

[chevvron]

Is this for mixed mode ops?

[ZOOKER]

Have a look at the NATS website, there is more about TBS and a short video there.

[Gonzo]

The one sentence version is that we're taking the time achieved by distance based wake turbulence separation in low headwinds conditions (for example 4nm=90s) and then taking that time and applying it across all headwind conditions, so that in strong headwinds separation between Heavy/Heavy might actually be 3.7nm.

The result is that the landing rate is maintained is strong headwinds.

As Zooker says, lots on the NATS | A global leader in air traffic control and airport performance (http://www.nats.aero) website.

Chevvron, for LHR it's obviously aimed at a dedicated landing runway, but it's also being developed to ensure it can be used at busy single runway airports, where you can compress the 'departure gap' in headwinds.

[Del Prado]

....... it's also being developed to ensure it can be used at busy single runway airports, where you can compress the 'departure gap' in headwinds.


What have they been doing for the last 40 years at Gatwick* then?



*other busy single runway airports are available.

[DaveReidUK]

What have they been doing for the last 40 years at Gatwick* then?Waiting for TBS, like everyone else. :O

[golfbananajam]

Zooker, Gonzo thanks for the useful explanation (and I will follow the link to watch the vid, just can't from this machine).

[vespasia]

Waiting for TBS, like everyone else.

Actually, we've been using time based separation at Gatwick (and presumably many other airports) for at least 20 years, it's just that we express it as a distance because that's what the approach guys and girls see on a radar screen. Hence departure gaps vary between 7nm (slight tailwind), 6nm (calm), 5nm (reasonable headwind), or even 4nm (stonking headwind) with .5nm variations to tweak the gap to achieve the same time between landers...so time based separation is actually nothing new though I'm sure an expert somewhere will claim to have invented this magnificent innovation though....:ugh:

[DaveReidUK]

so time based separation is actually nothing new

I thought the whole point of TBS was to allow a reduction in the physical spacing of arriving aircraft in strong headwind conditions, to mitigate at least partly the effect of said headwind on the arrival rate? Obviously mainly relevant to segregated operations.

Maybe it will come in handy at Gatwick when you get your new runway. :O

[twentypoint4]

so time based separation is actually nothing new though I'm sure an expert somewhere will claim to have invented this magnificent innovation though....:ugh:

This "expert's" pat on the back will come from convincing the CAA to let us go below the current wake turbulence seperation minima! That's the main difference between what you've been doing at Gatwick all this time and what Heathrow might soon be allowed to do.

[HEATHROW DIRECTOR]

I just wonder how radar controllers will apply TBS? All I've seen are lines moving down the centreline, which presumably indicate times, which I assume controllers have to aim aircraft at.

[confused atco]

time based separation is actually nothing new

Wait around long enough.....

That been said Time Based Separation as now envisaged is part of the SES and SESAR.

Aircraft will be given a time to cross a given point +/- a parameter.

Long term it is believed to be part of the solution to some of Europe's capacity issues.
Once all aircraft are suitably equipped :rolleyes:.
The roll out of ADS-B above FL285 is another part of the grand plan.

Biggest difficulty for ATCO's will be during mixed mode operations.

[Gonzo]

Jeez, calm down.

The TBS concept effectively compresses the wake turbulence separation.

However, the spacing/separation tool is accurate down to 0.1nm so will allow Gatwick, or Heathrow, or any other airport if they deployed it, to more accurately refine the departure gaps in single runway ops. We all change the gaps in SRO according to the headwind and runway occupancy, but as you say vespasia to 0.5nm, not to 0.1nm increments.

[DaveReidUK]

Once all aircraft are suitably equippedWhat additional equipment is required on board aircraft to allow ATC to move to time-based separation?

Just curious.

[Gonzo]

DR,

I think confused ATCO is talking about aircraft being equipped to fly 4D trajectories with required time of arrival, not TBS final approach separation/spacing.

Anyway, if any NATS people want more info, PM me, for others, I know the NATS Press Office has a lot of material on TBS.

[bekolblockage]

Interesting, and on the face of it, reasonable solution to the problem. With our 65:35 Heavy:Medium traffic mix, we lose about 1 arrival per hour for every 5 kt h/w increase over 10 kts.

But is the logic flawed?

Certainly on strong headwind days, the aircraft are passing over a point on the ground at longer time intervals.
But the aircraft and the wake vortex they produce are both moving relative to the mass of moving air. Surely the wake vortex generated is not staying stationary over a point on the ground? If that was the case then yes, this proposal would be reasonable.
But to the following aircraft, won't the wake still be reached in the same time, for a given spacing, regardless of the wind?
In simple terms, the airborne aircraft and the vortices don't "know" there is such a thing as wind.
Sounds like you have the Regulator convinced anyway. :ok:

[llondel]

I was wondering whether the separation could be closer with a steady crosswind, because the vortex would be blown to the side and so the following aircraft wouldn't intersect it if they were all crabbing/slipping down the runway centreline.

Does the headwind version work because the following aircraft goes over the top of the vortex from the preceding aircraft?

[Gonzo]

Bekolblockage,

Wake degrades at a faster rate the higher the wind speed. There was a LIDAR research campaign as part of SESAR


llondel,

There is a partner to TBS in terms of concept, called CROPS, or Crosswind Operations, which works on that very idea. Both departures and arrivals can benefit from the effect you describe.

[DaveReidUK]

But to the following aircraft, won't the wake still be reached in the same time, for a given spacing, regardless of the wind?I'd have thought that the distance between the preceding aircraft and the wake vortex it generates would be a constant, regardless of headwind strength.

You seem to agree:

In simple terms, the airborne aircraft and the vortices don't "know" there is such a thing as wind.In that case, for a given spacing, the following aircraft would take longer to reach the preceding aircraft's vortex in a stronger headwind.

[phiggsbroadband]

I think you need to study the trajectory of the wake vortexes; They tend to drop 100ft quite quickly, then level off and not change their height thereafter.
So if anything, a headwind would blow the turbulence horizontally nearer the following aircraft, during the initial drop.


The 'straight' part of the vortex trail will however be parallel to the 3deg glide slope, and will be blown backwards away from the actual glideslope by the headwind.
However because of the low angle of the glideslope (where tan 3 only = 0.05), the amount that the turbulence moves away for a 90 second spacing and a 10kt headwind is only 78ft.... maybe better than nothing though.

[DaveReidUK]

So if anything, a headwind would blow the turbulence horizontally nearer the following aircraft, during the initial drop.Why "nearer" ?

If the headwind is blowing the vortex-containing air across the ground at, say, 30kts then the following aircraft is also moving across the ground 30kts slower than it would be in still air (for a given TAS).

If you take the moving air mass as your frame of reference, then nothing in the sky has changed. The only difference is in the rate at which the whole system (air mass, aircraft, vortex) is moving across the ground.

[bekolblockage]

Thanks Gonzo.
No problem with that if the LIDAR data shows earlier disipation.

for a given spacing, the following aircraft would take longer to reach the preceding aircraft's vortex in a stronger headwind

Dave, I'm worried I'm being a bit thick here but my point is for a given TAS down final, the time to reach the same point in the air will be the same, won't it???
The vortex is travelling towards the following aircraft (relative to the ground) at whatever the headwind is (notwithstanding its disipation as Gonzo points out). Right :confused:

So if NATS propose to reduce to say 3.5NM between 2 Heavies in a 30 kt headwind to maintain a time over the threshold of around 95 seconds (equivalent to 4NM in still air), I would expect the following aircraft to hit the wake vortex in about 83 seconds (3.5NM in still air).

Please tell me I'm wrong. (But I'll keep arguing :})

[DaveReidUK]

Please tell me I'm wrong.No, you're right, I was mixing up my frames of reference. :\

I'd been dividing the aircraft-to-wake distance by GS instead of TAS. What I should have said was

for a given spacing, the following aircraft would take the same time to reach the preceding aircraft's vortex regardless of any headwindSo I would agree that a reduction in spacing reduces the aircraft-to-wake-encounter time, all other things being equal.

[bekolblockage]

Phew. Thanks Dave.

Certainly, I think the idea has merit if its because of the earlier dissipation. No issue with that. We are working with our Met provider as well to try to analyse that behaviour.

But in their TBS video, NATS seem to be implying that they will keep "protecting" aircraft from wake turbulence because they will keep the time between arrivals over the threshold at not less than what the distance standard equates to in still air.
That is plainly incorrect.

In my experience in many aspects of aviation, this "ground centric" approach to things often causes misconception. Its often because we have trouble visualizing the medium the aircraft are flying in. If it were two boats coming up the Solent running at full tilt, its easy to see what is happening.

[phiggsbroadband]

Hi DaveReid... you quote...


If you take the moving air mass as your frame of reference, then nothing in the sky has changed. The only difference is in the rate at which the whole system (air mass, aircraft, vortex) is moving across the ground.


That would be true if all aircraft were flying en-route, but here the aircraft are constrained to the actual 3 degree glide-slope, (electronically generated line in the sky.) which is fixed w.r.t. the ground. i.e. each aircraft has a different bit of fresh air to fly down the slope.


At 10 kts windspeed the vortex moves backwards 1500ft, but only 78 ft down from the previous planes vortex trail.


Calcs.. 10kts=1000ft/min separation =1.5min therefore 1500ft horizontaly. wrt 3deg slope 1500 x tan 3 = 78ft verticaly.

[bekolblockage]

I think it might be clutching at straws now to factor in the GS angle, but let me throw you a curve ball, just for the sake of discussion, by asking what you consider the logical extension of that idea is as the headwind increases.
Is it true to say that the effective descent angle through the air continues to reduce as the the headwind increases?
Indeed taking it to the logical extreme, if the headwind were the same as the TAS, say 150 kts, the aircraft would have to fly straight and level to maintain the G/S. No? Albeit making no forward progress.

[phiggsbroadband]

Hi Bekol...
if the headwind were the same as the TAS, say 150 kts, the aircraft would have to fly straight and level to maintain the G/S. No? Albeit making no forward progress.


Quite true, and it happens at much lower airspeed with gliders and model aircraft.... In the above example a 4nm spacing would result in zero aircraft landing per hour, they would all be fixed in space over the glide slope.


Seems strange that this topic has become one of wake turbulence separation... Surely the reason to have TBS, Time Based Separation, is to ensure a constant stream of aircraft and passengers into the airport, at a constant rate, regardless of winds.

[bekolblockage]

Phiggs

Errr, well I think thats what their main objective is with TBS- to minimize the impact of distance-based WT standards by converting to an equivalent time.

The issue for me was the reference point for that time. It should be the air, not a point on the ground. In which case, the headwind is inconsequential.
(Apart from the apparently higher dissipation rate, as pointed out by Gonzo. )

[anotherthing]

Be interested to see this working in real life and not in the simulator which is not exactly very good at replication.

The problem I see is not the spacing on final approach, but the vectoring required when positioning aircraft in strong winds from north and south of the landing runway at the same time. I'd be surprised if you didn't have a lot more losses of separation at the point of turning onto final.

To get the aircraft the correct time based distance behind each other on final will require very tight spacing on the final turn in.

Still to be convinced I'm afraid. Looks good on paper but I know a few very experienced Heathrow radar controllers who are quite wary of the mechanics behind the procedure

[Tinpot2]

If my understanding is correct, TBS relies on aircraft maintaining their assigned speed, at Heathrow to 4Nm, which currently 35 - 40% fail to do.

[DaveReidUK]

TBS relies on aircraft maintaining their assigned speedSo no different from providing any other form of longitudinal separation, then.

[055166k]

The scenario to consider is that a landing aircraft might blow a tyre. The resulting go-arounds could produce a complex multiplicity of separation losses almost immediately. It will be interesting to see exactly how far the regulatory bodies will go in reducing radar separation minima without requiring much more accurate and precise data refresh for the controllers and their tools. Don't our ATC colleagues across the pond already have such equipment?

[Gonzo]

anotherthing and 055166k,

TBS only compresses wake pairs, not radar minimum or runway spacing constrained pairs.

[bekolblockage]

I must say I'm impressed with the moving timing markers down final Gonzo.
Are they following the sequence according to AMAN to get the different spacing for HH and HM pairs? What happens if the INTs change the order from that in AMAN?

Did your own software guys do that on the system or the manufacturer? What system are u running for TC?

[llondel]

Gonzo:
There is a partner to TBS in terms of concept, called CROPS, or Crosswind Operations, which works on that very idea.

Presumably if you get it wrong, the following aircraft comes a CROPper?

Nice to know I can still handle some physics though :8

[Gonzo]

bekol,

Not sure that info is out in the public domain, sorry.

[On the beach]

Bekol,

It's a bit old now, but this may answer a couple of your queries. Usual fees apply! :E

http://www.eurocontrol.int/eec/gallery/content/public/document/eec/report/2006/021_CDG_Real-time_Simulation_Time-based_Separations.pdf

[DaveReidUK]

And some more pretty pictures of the TBS Demonstrator here

http://www.eurocontrol.fr/projects/edep/documents/8.3/TBS_UserGuide.pdf

including a reference to three alternative algorithms that the simulation was able to use to compute the TTP vectors.

[Gonzo]

I wouldn't read too much into that DR, it's a java based TBS demo to run on eDEP, rather than an actual tool.

The SESAR concept has matured significantly since 2009.