ATC IssuesA place where pilots may enter the 'lions den' that is Air Traffic Control in complete safety and find out the answers to all those obscure topics which you always wanted to know the answer to but were afraid to ask.
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?
I think the more relevant part of Doc 4444 may be:
18.104.22.168 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. 22.214.171.124 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).
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.
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).
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.
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.
A bit like sticking someone on the ILS at 0600 who then says 'but we can't land until 0630'... 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...
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.
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.