Wake turbulence

[BBK]

Quick question for my ATC colleagues

I always thought that a heavy departing after a heavy needed 2 minutes separation. I looked up the Ops Manual and it doesn't refer to the above situation - only light and medium types following behind. Any thoughts?

regards

BBK

[Gonzo]

In the UK there is no wake turbulence separation requirement for a H departing behind another H.

[1Charlie]

Depends where you are. In Australia trail separation is 4nm but no timed departure sep.

In NZ it's two mins between heavies off the runway.

[FlightPathOBN]

From PANS ATM 4444

http://operationsbasednavigation.com/wordpress/wp-content/uploads/2012/07/SC-23.jpg

Departures can be 1 min if diverging on 45 degree paths, or 2 mins if the leading ac is faster (or speed controls in place)

[Spitoon]

I think the more relevant part of Doc 4444 may be:
5.8.3.1 A minimum separation of 2 minutes shall be applied between a LIGHT or MEDIUM aircraft taking off behind a HEAVY aircraft or a LIGHT aircraft taking off behind a MEDIUM aircraft when the aircraft are using:
a) the same runway;
b) parallel runways separated by less than 760 m (2500 ft);
c) crossing runways if the projected flight path of the second aircraft will cross the projected flight path of the first aircraft at the same altitude or less than 300 m (1000 ft) below;
d) parallel runways separated by 760 m (2500 ft) or more, if the projected flight path of the second aircraft will cross the projected flight path of the first aircraft at the same altitude or less than 300 m (1000 ft) below.
Note.— See Figures 5-39 and 5-40.
5.8.3.2 A separation minimum of 3 minutes shall be applied between a LIGHT or MEDIUM aircraft when taking off behind a HEAVY aircraft or a LIGHT aircraft when taking off behind a MEDIUM aircraft from:
a) an intermediate part of the same runway; or
b) an intermediate part of a parallel runway separated by less than 760 m (2 500 ft).

[Denti]

And what about SUPERs? Nothing in those documents about that ugly thing.

[FlightPathOBN]

Spitoon,

That section is also correct. We have to remember that all of these standards are a minimum, and do not stand by themselves.

There have been many issues with timing standard, with the ac running up on the leader, and violating min separation.

So the short of it is, both apply.

Denti,
In regards to the supers, there are references and addenda to the document to account for these, currently I know of A380, B747-8, and B787 all have 10nm min sep.

[Spitoon]

FlightPath, you are correct that both bits of Doc 4444 apply but I think the OP was asking about WT separation for pairs of departing aircraft using the same runway.

The bit you mention about 'Departures can be 1 min if diverging on 45 degree paths, or 2 mins if the leading ac is faster (or speed controls in place)' is an IFR separation and WT separation minima will have to be applied if that is greater, i.e., if it's a single runway operation, a MEDIUM following a HEAVY will need 2 minutes separation (for WT purposes) regardless of the tracks the aircraft will be following.

[blissbak]

My manual reports 2 minutes Heavy behind Heavy, it provides SUPER separations too, 8NM radar the widest one, light following The Ugly

[Gonzo]

Yes, and IFR separation can be reduced by using visual or radar means.

Denti, in the UK we effectively add 1 minute on to the departure separation for aircraft behind an A380.

A380 followed by H = 2 minutes
A380 followed by anything else is 3 minutes.

[FlightPathOBN]

if it's a single runway operation, a MEDIUM following a HEAVY will need 2 minutes separation (for WT purposes) regardless of the tracks the aircraft will be following.

No, the MINIMUM is 2 mins,
AND Wake separation MINIMUM still apply.

B763/B738, vicinity Melbourne Australia, 2010 (LOS HF) (http://www.skybrary.aero/index.php/B763/B738,_vicinity_Melbourne_Australia,_2010_(LOS_HF))

[Spitoon]

FlightPath, I'm a little puzzled about the point you are trying to make - and I'm really not sure that it is going to help the OP (or me) to try and bottom it out.

It is quite true that separation standards are minima and can be increased if a pilot request it or a controller believes it appropriate. So, yes, a 2 minute separation requirement means a minimum of 2 minutes.

However, the separation requirement may originate from a need to establish IFR separation or WT separation. The two are done for different purposes and, generally, it is the more stringent of the two that the controller must achieve. It is important not to confuse the two.

In the absence of a need to establish WT separation, and on a nice day (and assuming that Doc 4444 provisions are applied by the State), ATC could launch two IFR flights within seconds rather than minutes by utilising 'reduced separation in the vicinity of the aerodrome' as Gonzo mentions. When I was operational I did this routinely even with large aircraft. If, however, there was a need for WT separation, then even if the controller can get IFR separation in seconds, the WT separation becomes the limiting factor.

Interestingly, in the UK the term 'spacing' was used with respect to WT requirements in order to distinguish it from IFR 'separation' until a couple of years ago (at which time it aligned with ICAO).

[BBK]

I posted the original question. Thanks for all the answers. I was asking as I was operating out of LHR and departed immediately, in a heavy, behind another heavy. When I say immediate I mean we cleared to take off as the preceding aircraft got airborne! That's not a criticism by the way as I appreciate the excellent service we get from London controllers.

Anyway, we had a mild wake turbulence encounter just as we got airborne. Nothing dramatic but we filed an ASR as per our SOP and that got me looking in our Ops Manual and had me wondering what exactly the rules are regarding separation for wake turbulence as opposed to traffic separation. I suspect the company manual has lifted the rules directly form the relevant ICAO/CAA document for these matters. I suppose the lesson is beware of wake turbulence at all times.

regards

BBK

[Gonzo]

BBK, operating a Heavy out of LHR you should be aware that you can go 'wheels up', as we call it, behind any other Heavy....if you're all lined up ready to go, this can be as little as 45 seconds.

If you don't want this, then please advise TWR when in the holding area that you require 2 minutes (or whatever you need) behind a certain type....providing that you tell us before you enter the runway, we can work around it.

The worst thing for us is to have someone on the runway cleared for take off and then tell us that they need another minute due to wake turbulence.

[Talkdownman]

A bit like sticking someone on the ILS at 0600 who then says 'but we can't land until 0630'... :rolleyes: A waste of precious runway time.
Needing 'an extra minute' on the runway delays everybody by that extra minute. There's always someone else who will use it if you can't...

[FlightPathOBN]

Thanks for the explanation.

The original post did not mention any specific airport..and only 2 min separation...so I was trying to point out that there is a myriad of standards, not just 2 mins.

As pointed out, at LHR, DP ac do an immediate turn. This is a scenario gets you to a 45 second departure timing.

Every operation has rules for its specific combination of operating procedures, that have waded through the regulatory quagmire to get to what works for them.

I am curious, back to the original question, how would an operator know, unless a regular, to expect wheels up in 45 seconds behind a heavy?

[HEATHROW DIRECTOR]

<<As pointed out, at LHR, DP ac do an immediate turn. >>

How about those that go virtually straight ahead?

[FlightPathOBN]

I look at this from a wake turbulence perspective. As you are aware, that is what we do, measure wake turbulence for Departures and Arrivals.

There are many variables that effect the departure spacing from an ATC perspective, and many variables from the pilots perspective...

If you and the leader go straight ahead, there is no way you can use 45 seconds, that would take you into WT separation. You can mitigate this, by rotating earlier, but there is little means for ATC to require that from an aircraft.

Even with the turn, the following aircraft must watch the rotation point of the leader, if you rotate afterward, you stand a very good chance of a WV encounter before the turn. If there is a marine, inversion layer, or a headwind, even greater chance.

[Gonzo]

FlightPathOBN,

Now you're confusing me....

Wake turbulence separation either applies or it does not apply. Currently the direction of turn does not affect whether WTS is applied or not. In the future that may well change with Crosswind ops.

If WTS does not apply, then in the UK the departure separations will be stipulated in MATS Part 1, and amplified by MATS Part 2 which will include means such as the use of the ATM or where specific cases of the use of reduced separation in the vicinity of the aerodrome may apply.

For the UK, operators find the wake turbulence requirements in AIC Pink 72/2010.

[FlightPathOBN]

Gonzo,

During the course of daily ops, one would expect different minima to apply.
There must be a matrix of 'if-then' scenarios.

On DEP, if there is a crosswind, radar sep may govern, if there is a tailwind, time spacing may govern, if there is a headwind, WT sep will govern.

On approach, ROT governs, then it may be radar sep, time spacing, or WT sep. depending on the conditions.

So, under what specific conditions are you saying that WT applies?

[Spitoon]

I have a feeling I'm going to regret this but I'll try and offer a simple explanation.....

Let's do the really simple thing first - ATC separates aircraft according to very specific rules. If two aircraft are in a situation where they should be separated under these rules, the controller's job is to make sure that the aircraft are at least separated by the specified separation.

WT separations are specified for pairs of aircraft and are based on the WT category (which is braodly based on the weight) of each of the aircraft. Depending on the situation, the minimum separation may be specified as a time or a distance. If a particular pair of aircraft are in a situation that has a WT separation specified, then ATC must ensure that the aircraft are at least as far apart as the specified separation.

Using the example that the OP gave, there is no WT separation required between two HEAVY aircraft taking off from the same runway under the ICAO (and UK) rules - so ATC doesn't have to worry about WT separation when issuing the take-off clearance. However, there are many variations - mostly minor - on the ICAO WT separation requirements applied around the world.

IFR separations are applied between aircraft that are flying under IFR. These separations may require that there is a certain minimum time between take-offs, particularly if the aircraft are following the same route.

A controller will look at each particular situation and work out whether either WT or IFR separation is required. If both are required, the controller will apply the more stringent (i.e. bigger) separation. As an example, radar separation (which is an IFR separation) in a particular area aproaching an airport may be 3NM but if there is a B747 being followed by a C150, the WT separation minimum is 6NM. The controller must then apply the more stringent WT separation 6NM.

At present WT separation either applies or it doesn't, depending on the mode of operation and pairs of aircraft, that is to say, the criteria for whether or not to apply WT separation (and, indeed, the separation criteria themselves) is not affected by other factors such as weather conditions. This will probably change in some regions before too long as research provides evidence that it is safe to do so.

There are differences in the way that WT separations are applied to aircraft that are flying visually or under VFR - and these differences vary from one State to another - but to consider them now is probably unwise when the basic rules appear to be at issue.

So, to sum up, controllers have a set of rules (separations) that apply in defined circumstances. If the circumstances fit two of these rules, then the controller will apply the more stringent of the two. If the rules don't apply to a particular situation the controller will happily sit there doing nothing and let the aircraft carry on doing what they want.

It's that simple - but if you start conflating different separation standards you will quickly confuse everybody!

[reportyourlevel]

Good reply Spitoon. I like to make a point of referring to IFR separation and WT spacing, which I find helps to keep the two apart (bad pun, I know).

As an example, radar separation (which is an IFR separation) in a particular area aproaching an airport may be 3NM but if there is a B747 being followed by a C150, the WT separation minimum is 6NM. The controller must then apply the more stringent WT separation 6NM.

I make it 7NM, but admittedly we don't get many 747s around these parts.

[Spitoon]

I make it 7NM, but admittedly we don't get many 747s around these parts.You're probably right - I haven't spoken to an aircraft in anger for years and it's all getting a bit blurred in my memory!

[FlightPathOBN]

Spitoon,

Thanks for the time and explanation. I know there is a myriad of conditions that drive the DP and AR sep/spacing.
Here is what I am not quite understanding. Everything that I have, such as doc 4444, states that there is WT separation between heavies.
http://operationsbasednavigation.com/wordpress/wp-content/uploads/2012/07/SC-23.jpg

Again, excuse me for being dense on this, and I understand there are conditions that can reduce time based sep, such as immediate turns...but this table and the associated WT section, sure make it seem as though there is min WT spacing, even between heavies...

Really, I do want to understand this....
What am I missing here?

[Spitoon]

No prob, FlightPath. Sorry for the slightly cynical reply earlier but I was thinking that there might be a bit of winding up going on.

You're right that there is a matrix of 'what ifs' that a controller uses - but that's the just day to day work. What the controller is routinely doing is making sure that any separations or other rules that should be applied, are in place - a lot of the work effectively is proving that everything is as it should be. In high-density airport traffic environments, where every second or mile that two aircraft could have been closer simply adds up to greater delays for later flights, other things being equal, the controller is trying to pack the aircraft as close together down the approach or get them off the ground as quickly as possible whilst staying within the rules.

Some of the rules are absolute and inviolate, others have a variety 'options' that can be applied in particular circumstances. In some cases there are additional sets of rules that can be used which modify some of the others in the right circumstances.

One example is 'reduced separation in the vicinity of an aerodrome' - in very simple terms, if the controller (or the pilots of the aircraft involved) can see the aircraft and make sure they won't collide, many of the IFR separations can be reduced. The controller can use this procedure to separate the aircraft for as long as the right conditions pertain but after that he/she must have another type of separation in place. You mentioned one such separation where departing aircraft turn onto tracks split by 45 deg or more - but if it's a nice day and the controller can see the aircraft until they are well on their way and, say, seen by the radar controller 5 miles apart (i.e. the normal radar separation minimum for that environment), all is well and at least one of the IFR separations has been in place at all times. In some cases, a small radar in front of the tower controller can be used to 'watch' the aircraft if the weather is not good.

WT separation is one of the rules that cannot be varied (OK, in some countries, there are a few oddities but generally the rules MUST be followed. However, the risk caused by WT to another aircraft differs in different phases of flight and the separations reflect these differences, both in the distance that might actually exist between the aircraft and the way that they are stated (some in distance, others in time). You are quite correct that the extract from Doc 4444 that you have posted relates to aircraft flying close to an airport, but not actually on the runway, In fact the separations that you have posted are most usually applied between aircraft on the final approach path (usually established on an ILS or turning to intercept the LOC). They do also apply to departing aircraft as the text mentions but in most situations some other separation - or just the routes of the aircraft - mean that there is a greater distance between the aircraft anyway.

There are a lot of minor variations in WT rules around the world - most of these are local refinements of the ICAO rules based on empirical experience. A common example is the B757, which generates particularly strong wingtip vortices for its weight (the usual determining factor) on approach. A number of States have put the type into a higher category that its weight would normally require or have specified an extra mile or two of space before the next aircraft follows it.

Finally, you may come across other time-based separations for departing aircraft - usually set out on tables and referenced to the speed that the aircraft climb at. These are IFR separations and, in my experience, are used where aircraft will be following the same route on a SID for the early part of the flight and the times in the table are designed to ensure that, at the end of the SID, some other form of IFR separation will exist - it's the same basic principle in play that I mentioned earlier, there has got to be one separation in place at all times.

[Talkdownman]

Dunno, but at Heathrow on Deps there used to be 4 sides to each piece of jigsaw. In my day the parameters were:

Initial route separation
Speed group separation
WT spacing
CTOT

The Air Deps person would fiddle about with the 'pieces' to achieve the optimum departure rate then execute the plan with full-length or intersection departures.

If the initial route sep was 2 mins that was designed to achieve 5nm delivery to the ACC. Usually in reality 2 digital minutes worked ie. 91 seconds...
The WT 2 mins had to be the full 120 seconds and the WT 3 mins 180 seconds.
The CTOT was -5+10 and the speed group separation (could be fudged) from a chart...

Gonzo will probably tell me it's all different now with Big Brother watching...

[blissbak]

In my opinion, on departure, 4 miles is less than 2 minutes and maybe they can do that in 45'' , the result is of course "tight" :E

[FlightPathOBN]

Spitoon, all,

Really, thanks for the time and the reply....
I am sincerely asking about this, because I realize there is the criteria, that was written be PhD's in Atmospheric Science, using calculations that have little foundation in the real world, or with real aircraft.
In my experience with RNP procedure design, is that ATC is NEVER involved or consulted, hence I have quite a few RNP procedures that have been dictated and approved by regulators, that are permanently NOTAM'd out because the design is not something ATC would ever authorize...my favorite example is idle descent, oversold as saving fuel and track miles to the governments, but completely unmanageable in a blended queue.
Procedure designs where I forced regulators to include ATC input, look completely different in many ways, and make operational sense for the aircraft and ATC.

I have watched the approach/departure sequencing for LHR on Web Trak, so I have a good idea of the operations, at least for what is shown with ADSB equipped ac...

So, real time WT measurements, combined with real time ops, can be a very powerful tool to optimize operations, and provide a toolset for ATC to 'see' the wake turbulence real-time. Wake turbulence is a very complex animal, and I am hoping that once the data gets out there, it will radically influence ops, and in a very good way.
I am asking all of these questions, as I am currently working on the departure system, which is certainly different than approach configurations in many ways.
I will hazard a crude animation of what the approach system does, taking measurements every 3 seconds at 3 critical approach phase locations. (the results shown diagrammatically are center vortex, actual, from a 737-8)
The animation is at the bottom of the page... Wake Array (http://operationsbasednavigation.com/wake-vortex/)

Again, I certainly appreciate any and all input...

[Gonzo]

At LHR:

Wake vortex on departure only applies in the following cases:

Leader Follower
J (A380) H/M/S/L
H M/S/L
M/S L

From the same take off point, it's 2 minutes (apart from J followed by M/S/L when it's 3 minutes), add a minute if the follower departs from an intersection further down the runway.


As Talkdowman says, WTS is one part of the departure separation game.

Assuming WTS does not apply, various other separations may apply:

IFR separations (can be applied without radar, determined by SID route and speed group):


1 minute when routes diverge by 45 degrees(ish)
2 minutes when they don't.

Reduction in IFR separations:


By using radar, we can reduce the separation between any diverging aircraft (i.e. this cannot apply when successive aircraft follow the same SID) to 3nm when the following a/c paints on the radar following departure (we tend to use this on pairs of a/c that diverge less than 45 degrees).
By using visual separation, we can reduce the separation between departing aircraft, providing we transfer the aircraft to the departure sector when they are 3nm apart and increasing (we tend to use this on all diverging a/c if the weather is good enough, especially on those 1 minute IFR separation pairs that, if Mediums, we can get down to 45 seconds).