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!

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).

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!

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.

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?

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.

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...

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

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

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

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).