LHR arrival separations
 

Dear All,

Inspired by a thread on the Tech Log forum (http://www.pprune.org/tech-log/50221...-til-4dme.html) regarding approach speeds into LHR, I wanted to ask controllers about some of the specifics of maintaining separations between aircraft on approach to LHR (I am guessing it will probably apply to other airports as well).

In the case where minimum interarrival separations of 2.5 NM are applied (assuming no need to apply wake vortex seps), are these separations applied prior to 4 DME (i.e. while aircraft are speed controlled at 160 kts)?

Since aircraft then decelerate after passing 4 DME, naturally the separations will deteriorate. Is this taken into account when setting separations before 4 DME? In other words, are slightly higher separations applied so that when the leading aircraft crosses the trheshold the following aircraft is not less than 2.5 NM behind?

Would be most grateful for your expert input.

NotaLOT.

I think they refer to this as "NATS-style spacing". :E

...at which stage 'reduced separation in the vicinity of an aerodrome' can be applied.

A whole raft of conditions applies before nats' 2.5nm spacing can be applied. The final arbiter is the Air Arrivals controller...who could well be sitting on the edge of his/her seat throughout...

To achieve 2.5nm on the localiser the separation on the base leg has to be extremely tight at about 2nm because half a mile is lost in the turn. If separation drops below the SMF parameters to 1.99nm the SMF will punch the number two director out of his/her seat and into the office, no tea, no biscuits. Is it really worth laying one's licence on the line for one's employer...

Maybe Gonzo will have something to say about 2.5nm 'separation'...

2.5nm is the radar minima. Normal practice is for 3nm spacing to 4DME to be applied, so that the compression that occurs is still legal in IMC (when the Tower cannot see you so cannot apply Reduced Separation in the Vicinity of the Aerodrome).

If we can see you out to 6.5nm, and there is a headwind, we can apply 2.5nm spacing to 4DME, the proviso that as the Tower are visual with the one crossing 4DME and slowing down, and the one at 6.5nm, the longitudinal separation can legally reduce below 2.5nm.

From what I understand, the spacing is based on a minimum measure crossing threshold...

Curious how you get less than 2.5nm sep, with wake turbulence issues?

FPOBN,

At LHR, spacing and wake turbulence separation is applied to 4DME.

Well, of course, if there is a more constraining separation requirement, such as wake turbulence or diagonal separation due to an aircraft on approach to the parallel runway, then that applies. We only go below 3nm on pairs that are not wake constrained.

So for example, A320 followed by A320, or 737 followed by 767, etc.

As I said, we only go for 2.5nm when the conditions exist that when the 2.5nm begins to compress, i.e. when the first aircraft reaches 4DME and slows down, the Tower can apply visual separation.

Thanks Gonzo,

That explains it perfectly. Does the same approach apply when wake vortex minima are used?

E.g. say you have a heavy with a medium following. The wake vortex sep. requirement is 5 NM.

Do you then add a 0.5 NM buffer on top to give a separation of 5.5 NM when both aircraft are on the approach prior to 4 DME, to take into account the compression when the heavy passes 4 DME and begins to slow down?

Much obliged for your help!

Thanks...forgot that you guys are allowed visual below MRS..

Here is the FAA version...(very confusing reversing CAT..)

http://operationsbasednavigation.com...2/12/SC-12.jpg

http://operationsbasednavigation.com...2/12/SC-13.jpg

NotaLOT,

No, we would provide 5nm to the point at which the Heavy reaches 4DME.

Once the Heavy crosses 4DME, then we can permit the separation to compress to 4.5nm before taking action.

I do like this principle of allowing the separation minimum to "compress"! Is this not really a matter of officially sanctioned cheating, for which lesser mortals would be duly castigated?

2 s

Hi Gonzo,

How did you get your regulator to agree to reduce ICAO wake vortex sep standards inside 4DME? Can't be RSVA as this doesn't apply to WV.

This would be of immediate benefit to any airport which is oversubscribed during prolonged peak arrival periods.

N.L.

Suspect the agreement was written on the back of a fag packet about 30 years ago, since lost, and wouldn't stand too much scrutiny if re visited today. I bet HD knows the real answer to this one though :)

It's not ATC which makes such regulations or even asks for them and a good few controllers I worked with did not like 2.5nm spacing. ATC received no specific training - 2.5nm was introduced when I worked and that's going back many years. Major airports are commercial organisations and the airlines demand space for more and more flights to land at them. One answer is to put the aircraft closer together. Who determines finally that this shall be done I do not know (Gonzo might) but one day I arrived at work and was told that 2.5nm may be used under certain conditions so it was a case of sit down and get on with it.

It is commercial pressure encroaching upon safety.

Leaving wake turbulence alone for a little while (it's an approved procedure) the 2.5nm separation is a minimum radar separation. To reiterate again the separation is allowed to compress only when the tower controller is visual with both the aircraft concerned. Separations maintained with Reduced Separation in the Vicinity of an Aerodrome, i.e. visually, are not subject to the same minima and hence this is not cheating.

Two aircraft less than 2.5nm apart on a clear day from a tower when both are established on precision approaches is perfectly comfortable. Issuing safe and legal landing clearances is the challenge and we owe a debt to the professionalism of the aircrews involved to expeditiously and safely vacate the runway, almost always with a great deal of situational awareness of what is occurring behind them. Of course, if it doesn't work, then a go around results and this too is a perfectly normal and safe procedure, albeit to the detriment of what the commercial pressure was trying to achieve. Oh well, in this case safety wins, and it's the professionalism of the controllers that does not allow the pressure to warp their assessment of any given situation.

In my opinion the 2.5nm rules as they are allow the maximum safe throughput of aircraft on one piece of tarmac in good visibility, still conveniently putting wake turbulence aside since this is a further limiting factor.

The separation that Gonzo referred to as being allowed to "compress" was in respect of wake turbulence, not what the UK terms "standard separation". Hence my observation.

2 s

well-written "rationale" @hangten.
And as far as I know, 2.5nm is not just NATS-style but an ICAO approved (prescribed - with several conditions) separation as well.

However the question of Medium behind Heavy is still to be clarified;
According to standard (time) wake turbulence sep. - 2 min are needed.
Radar (distance) sep. - 5nm.

At least that is what the ICAO has produced in their books...

So, is the 4.5nm separation a "local" standard, prescribed by the appropriate ATS authority?

Isn't it "Gangnam Style" separation? :E

I am certainly not following this rational on wake separation...Perhaps some views on the thought or approved process.

The ICAO wake tables are based on a minimum of MRS between many and similar aircraft wake cats.
While these distances are based on ATC and radar control, I dont follow how that can be extrapolated to visual separation, while you can 'see' the aircraft..you cannot 'see' the turbulence....

How far does one take 'similar' aircraft in this scenario...an A380 2nm behind an A380?

Aircraft still make a vortex, and the mechanics of vortex creation are not fully understood nor developed. As an example, a lightly loaded 737-800, on final may only need flaps 30, while a heavily loaded 737-800 will need flaps 40. These flap settings create a completely different type, strength, and advection rate of the vortex pair.

Crosswinds do not only affect advection rates, it also influences creation of the vortex

I would note that it is well documented that the wake turbulence is stronger and lasts longer on a clear day.


Adding this later..the FAA RECAT program does not allow for visual sep between any wake cats, even same...
"Familiarity with RECAT wake separation standards is particularly important during visual approach operations as pilots assume responsibility for avoiding wake turbulence when cleared to visually follow preceding traffic."

http://www.faa.gov/other_visit/aviat.../SAFO12007.pdf

"I am certainly not following this rational on wake separation...Perhaps some views on the thought or approved process."

There are two separate separation standards being discussed here - the 2.5nm (then Reduced Separation in the Vicinity of the Aerodrome) is completely separate to the wake vortex issue. Subject to certain conditions, the closest that UK radar controllers were permitted to get aircraft was 3nm - whether they were following, passing, crossing etc. Any closer than 3 miles was a loss of separation. At some point it was determined that, as discussed by others more knowledgeable above, 3 miles could be reduced to 2.5 miles for aircraft on final approach to LHR. As an aerodrome controller, you can easily get aircraft much closer than 2.5nm anyway - imagine two departures rapidly turning onto different tracks, or an inbound following something visually etc. The judgement falls to the aerodrome controller to visually assess if the separation is safe and appropriate, and as hangten says, if we can safely issue a landing clearance at a useful stage to the second aircraft.

Wake turbulence is a completely separate issue and overrides any other reduced separations.

The final approach spacing will be a combination of the above, plus any adjustments required either by the tower (eg availability of exits, braking action, departures to go in between arrivals), weather (eg tailwind, icing conditions requiring a/c engine anti-ice hence higher approach speeds) etc etc.

FPOBN,

There is either the requirement to separate due to wake, or not. Two Lower Medium category aircraft, such as B737s, do not require any wake turbulence separation, just as a pair of Cessna 150s do not require any wake turbulence separation.

Hence why we have a 2.5nm spacing procedure at LHR where, for non-wake pairs, providing Tower can apply visual separation when the first aircraft reaches 4DME, separation is 2.5nm delivered to 4DME, which can compress to even less than 2nm as the first aircraft touches down.

RECAT1A, where it mentions MRS, does not mean that 2.5nm is the wake turbulence spacing, just that this is the closest aircraft are permitted under radar separation on final approach. If aircraft pairs where "MRS" is specified are being separated visually, there is no radar separation requirement. Would you really expect an A380 to have to have 2.5nm wake turbulence spacing behind a E120? The table even explains that it could be 3nm, and implies that it could be greater....some areas in the UK MRS is 5nm. As this figure varies, it demonstrates that MRS is not a factor in wake separation.

Sorry, but in the FAA world that is exactly what it means.

IF the runway has a 2.5nm MRS, then the min wake is 2.5nm, IF the runway has a 3nm MRS, then the min wake is 3nm. I just went through this at KMEM and FedEx.
The FAA defines min separation as just that, a minimum, measured from threshold to the FAF, or in the case of RECAT, 10nm

Here are the details from FAA RECAT matrix. This is also what is built into ATPA.


and the example...


note minimum 2.5nm sep...
http://sondeiaviation.com/wordpress/...6025586941.jpg

If that's the case, I'm struggling to understand why your graphic has two completely different matrices for wake separation and MRS, with the RECAT value obviously being the higher of the two for the relevant leading/following pair.

Or are you simply using the term "wake" to mean the resulting in-trail spacing, rather than specifically that which is determined by vortex considerations ?

This is not 'my' graphic, but from the FAA RECAT program currently being used.

If you look at the diagram, there is an asterisk...at the lower left, the asterisk is explained. With the FAA, 2.5nm MRS requires a waiver.
With the FAA RECAT, there is no wake spacing allowed lower than the MRS value.

Note the aircraft diagram included...a B747 following the A306...2.5nm minimum wake turbulence separation.

FlightPathOBN, you certainly have a good argument regarding the "compress" of the 2.5nm on the very final phase of flight.
In ICAO chapter 8, where reduced separation minima based on ATS surveillance is described, the minimum between two acft within 10nm of the runway end is 2.5nm.
This is surveillance (radar) separation.

But I would agree with Gonzo, 2.5nm is not a wake turbulence separation, or at least not described as such. I think the paragraph v) of the chapter mentioned above clearly states that 2.5nm may be used (apllied) when distance-based wake turbulence separation minima do not apply...

I also have a question for Gonzo; Who is speaking to the acft until 4DME, or where is the Transfer of Control point (normal operations, not low visibility) between TWR and the unit doing the approach phase? Do you "negotiate" about the visual final part for the every single acft in sequence and landing?

I guess there are two issues here (at least)

First off, this IS the FAA RECAT program, Recategorization of wake turbulence separation... not MRS or other separation, but specifically wake turbulence

As you can see, they have detailed the wake and non-wake related operational issues, BUT for the wake turbulence minima...
the minimum, even between same aircraft is still MRS...

Second, as the mechanics of wake generation and transport are not generally understood, especially in the final phase of flight, with the specific danger zone being one wingspan in altitude, it is lemming talk to bring them that close at the threshold.

Wake encounters are reported by the same people who are held responsible for the encounter? Cant imagine why there are few encounter reports...

Sorry, perhaps I'm being obtuse, but I don't see that.

The diagram shows wake turbulence separation of 4nm applied when a A306 (RECAT Group C) is following a B747 (Group B).

But when the B747 is following the A306, no wake turbulence separation applies (RECAT rules same as ICAO), so the 2.5nm (minimum radar separation) applies.

In fact, looking at the wake element of the RECAT matrix, there are no combinations of leading/following category that result in a 2.5nm separation on wake turbulence grounds (nor are there in the ICAO matrix).

Why cite FAA standards?

UK CAA allows for wake turbulence separation minima to be applied between pairs of aircraft until 4 dme, provided the following aircraft is at same speed (or slower) than lead aircraft when the first passes 4 dme. Not to touchdown.

UK CAA continues to build on a huge database on wake turbulence encounters by encouraging a very healthy reporting system whereby both flight crews and ATC are requested to report on any incident.

This isn't anything new for the UK. Wake turbulence separation to 4 dme (not threshold) has been in place for decades. In my experience, the worst upsets inbound to heathrow have occurred in the initial and intermediate approach rather than final.

FPOBN, wake encounters are reported on by pilots. If there is a breach in WT separation, then the controller will also file a mandatory report.

No conspiracies here.

Maybe I am wrong (I am always open to debate) but...

I went through the documents;

http://www.faa.gov/other_visit/aviat.../SAFO12007.pdf

and

https://fdx.alpa.org/LinkClick.aspx?...%3D&tabid=4536

Regarding the FAA rules, you are right, Table 1 is showing the wake standards at the Threshold.

But I'm not sure about the statement:
I mean, if the airport is not allowed to use 2.5nm spacing, it does not mean that the wake turbulence separation (minima) changes... As far as I understand, the 2.5nm is always the very minimum wake (FAA), but to be "unambiguous" regarding the minimum separation in sequence for different airports, the MRS is stated. (And MRS is never below 2.5nm, so never less than minimum wake sep.)

You can also go through;
http://www.wakenet.eu/fileadmin/user...T_FAA_Lang.pdf
and you will find some blank cells in ICAO tables...

Anyway, the first impression when reading the presentations is, that wake sep. are still "overly conservative". ;)

I think the point made above about the 4nm cut off is critical. The wake separation applied in the UK only applies until the first aircraft is at 4nm. The first aircraft then reduces to threshold speed so some distance compression will occur; however as the second aircraft will also reduce speed from 4nm to the threshold the time separation between the two flights will remain constant or at least very similar.

AFAIK this arguement has always been accepted by the CAA.

If you applied vortex spacing to the threshold at Heathrow from today the movement rate would fall off a cliff.

Wake vortex reports are taken very seriously by both NATS and the CAA and I have never known anyone attempt to cover any up.

del prado,
The reason for citing FAA RECAT is that ICAO asked EUROCONTROL and the FAA to re-categorise aircraft and develop new wake turbulence separation standards.

What you are seeing is the EUROCONTROL/FAA RECAT Phase 1 program. IF this is adopted by ICAO, this will be coming to an airport near you.

UpperATC,

Under RECAT 1, there is one separation standard. There is not a different minimum sep per visual/radar conditions.
The only difference in the standard, is as illustrated in the table..IF the runway has the 2.5nm MRS sep, the min RECAT is 2.5...IF they have the standard 3nm MRS, then the min sep is 3nm...

http://sondeiaviation.com/wordpress/...6109299769.jpg

From the FedEx document you reference, the runways they use do not currently have the 2.5nm waiver, so the min wake sep of 3nm is illustrated...


http://sondeiaviation.com/wordpress/...6109432470.jpg




As you have noted, the RECAT standard for sep is threshold to 10nm. This was moved out from threshold to 5nm (typical FAF) to harmonize with TRACON.

*other freight carriers are also available ;)

I must say, it takes big stones to lecture a Heathrow controller (Gonzo) on final approach spacing, especially for someone who isn't a controller. Chapeau.

http://sondeiaviation.com/wordpress/...6118895729.jpg

Application of Wake Turbulence Separation at London Heathrow

adding this on reporting wake...as it is as clear as mud...

6 Wake Turbulence Encounter Reporting and Research

6.1 Wake Turbulence Encounter report forms are available to download from the CAA website as follows:
(a) Pilot Report Form: SRG1423: Wake Turbulence Report Form | Publications | About the CAA
(b) ATCO Report Form: http://www.caa.co.uk/srg1422

Full instructions for submission of the report forms are contained on the forms.

6.2 Pilots of aircraft believed to have created the wake turbulence will be informed by ATC and are requested to complete the appropriate sections of form SRG1423. The sections are identified on the form. (emphasis added)

http://www.skybrary.aero/bookshelf/books/1166.pdf

FPOBN, UK already has a difference to ICAO as I described above (wake separation to 4 dme not threshold). That's why I don't see the relevance of the info you've posted. If ICAO adopts RECAT I cannot see UK adopting it. The drop in movement rate at heathrow would be too great.
Do you believe the UK CAA would risk heathrow by adopting it when the present system has worked for many years?

Your quote under 6.2 in your last post was what I was getting at earlier when I said both flight crews are requested to file (sorry if that wasn't clear).

Work is ongoing to develop time based separation at heathrow, where the time interval between arrivals will be constant but the gap will vary according to the head/tail wind. How does that fit with RECAT?

<<6.2 Pilots of aircraft believed to have created the wake turbulence will be informed by ATC and are requested to complete the appropriate sections of form SRG1423. The sections are identified on the form. (emphasis added)>>

That's been the case for m any, many years. It means that if a pilot reports wake turbulence then the pilot of the aircraft causing it is asked to complete a form. I must have done it hundreds of times.

del prado,

The post, was directly from the document by Paul Johnston, Application of wake turbulence at London Heathrow, read the document link...

https://docs.google.com/viewer?url=h...ul_Johnson.pdf

that is not what is stated in the document...

I placed the red box over the text....

.. applied to 4DME FROM touchdown

Are you making a distinction here between "touchdown" and "threshold" ? Or is your highlighting of FROM intended to mean something else ?

FPOBN,

Yes, it is applied all the way down the approach UNTIL the point where you are 4nm (4DME) FROM touchdown.

It is not applied from the point to touchdown. The "from" in this context does not denote the starting point of the separation, merely the way of measuring the end point.

As you well know, nothing is certain when we talk about ICAO/harmonisation of ATC procedures. FAA have introduced RECAT1A (A for America!). Europe may introduce RECAT1E (E for Europe) which is subtly different.

However, I still don't understand the point you're trying to make.

Applying wake turbulence spacing to 4DME is a well-understood procedure in the UK on the part of the controllers and the regulator.

I am not aware of any safety issues.