IMC separations <-> VMC separations
 

Hey,
I have a question about the separations in different meteorological conditions. In some literature (for example capacity surveys for Airport Frankfurt oder Paris) they differ between imc and vmc separations. In the ICAO DOC4444 I just can find information about the "official" radar respectively wake vortex separations. I guess these are the separations that have to be maintaned in IMC.

But what is the different under IMC?

Many thanks!

For the UK, specifically for Heathrow, we have many different types of separations between pairs of departing aircraft.

• Wake turbulence, which can never be reduced.
• IMC Separations based on direction of turn and speed difference (which are time based, not radar based, so they could be applied if the tower had suffered a radar failure.
• 3nm radar separation for diverging aircraft, which requires radar(!)
• Visual separation (which is I guess what you refer to as VMC separation) which can be applied to reduce everything apart from wake turbulence.

So, a DVR followed by a MID departure from 27R in IMC is 2 minutes. The DVR turns inside the MID route. With radar I can reduce that to 3nm when the second aircraft appears on radar (which would probably be equal to about 60-70 seconds if both mediums), and then in VMC I can apply visual separation to reduce it even further to 'wheels up' which for mediums is about 45 seconds, provided I have another form of separation when I pass the aircraft to their departure sectors.

Gonzo,

do you have any specific vertical or horizontal limits for visual separation? I mean, how far and how high are you allowed to apply visual?

'In the vicinity of the aerodrome'.

No more specific than that.

Interesting, is that an EGLL-ism? I (ADI & APS) would allow an IFR aircraft on a visual approach and maintaining their own separation from th eone in front to get closer to get closer than the wake turbulence distance.

The CAP493 says:

When a flight is operating visually (i.e. IFR or SVFR operating under the reduced separation in the vicinity of aerodromes, VFR, or IFR making a visual approach) and is following or crossing behind another aircraft, the pilot is to be informed of the recommended wake turbulence separation minima as listed at paragraph 9.5.1.

which I think means that the controller has passed the responsibility to the pilot and it's his lookout. It doesn't say you have to maintain the WT spacing in this case, nor does it specify any action if it is eroded.

But that is not done on the same runway at Heathrow. Never would a pilot be told to maintain his own separation from one ahead on the same runway. ATC apply wake turbulence separation at all times.

At times when both runways are in use for landing, usually early morning, an aircraft approaching one runway may be passed traffic information on one approaching the other runway ahead. When the second pilot has the first aircraft in sight the approach can be continued with no ATC separation between the two. Unless rules have changed wake turbulence separation is not required between aircraft on adjacent runways at Heathrow.

Thanks a lot for your answers! It's hard to learn more about separations on the basis of scientific papers. It sometimes seems like the authors don't really know how the traffic is handled in real life...

@ Heathrow Director:
You mentioned that both runways of EGLL are sometimes used for landings. I thought the airport always operates in segregated mode for noise abatement reasons.
Does heathrow use both runways for landings regularly?

I suppose the reason one would say that WT cannot be reduced, as a EGLL controller, is that we rarely have any of those situations you refer to.

As I am sure you will agree that while allowing one pilot to conduct a visual approach is nice and easy as a APS controller, it does come with a certain degree of uncertainty with regards to the spacing of subsequent aircraft. With EGLL being more or less non-stop landings all day, the final spacing is crucial and cannot be left to the pilots to manage themselves - no offence :ok:

Similarly, do we very rarely, if ever, have small aircraft in the circuit which you would advise of the recommended spacing behind the preceding etc.

So practically, at EGLL, WT separation cannot be reduced.

Just my two cents :)

Both runways amy be used for landings under special conditions. Usually first thing in the morning when there is a delay to inbounds and few departures and occasioinally at other times when there are delays. That is on westerly operations. On easterly operations aircraft may land on the departure runway at the discretion of ATC but it doesn't happen often due to traffic.

Gonzo may comment (he is a current Heathrow controller).

As I understand it, use of both runways for landing between 0600 and 0700 happens regardless of whether Heathrow is on easterlies or westerlies.

That's correct.

The point about easterlies is that the departure runway maybe used for landing purely subject to traffic whereas on westerlies special procedures apply.

Which special procedures are you referring to specifically?

Just curious :ok:

OK, I'm with you now - you're talking about what happens after 0700, when segregated operations start.

Westerly landings on the departure runway (i.e. out-of-alternation) are allowed where severe inbound congestion occurs, or is anticipated to occur, involving delays of 20 minutes or more.

Easterly landings on the departure runway are purely at the discretion of ATC, are not subject to the above delay criteria, and are not considered as out-of-alternation since there is no alternation (yet) on easterlies.

Sorry, I should have clarified that I was discussing departure wake turbulence only.

Yes, if an inbound is conducting a visual approach then wake turbulence separation becomes the responsibility of the following aircraft.

Dave Reid, delay criteria also apply to the 0600 hour on westerlies.

<<if an inbound is conducting a visual approach then wake turbulence separation becomes the responsibility of the following aircraft.>>

Bet that doesn't happen often at Heathrow. I used to apply WT separation behind one on a visual and I think my colleagues did too.

IMC vs VMC
 

Talking about littérature on capacity in Paris, there is nothing linked to VMC or IMC but only on LVP , Pre-LVP activities that request additional séparations to protect the ILS signal ( There are only some définitions on Visibility conditions as the CTR is protected by a class A airspace ,figures are published, mainly for helicopters flying VFR by derogation.)

I guess part of that study referred to frankfurt where there was a distinct capacity difference between VMC and IMC operation back before the new runway opened. Since the runways where too close to each other to allow independent operation they had to separate aircraft approaching alternate runways still by the normal separation, however in VMC they could ask aircraft to maintain own separation, only condition was that they do not overtake each other.

From what i see that is no longer the case since the new northern landing only runway is independent of the other runway used for landing. Haven't been asked to maintain own separation since then at least.

To the OP, I have a question: do you want to find out what is the minima for switching between IMC and VMC or how to separate an A/C in IMC and one in VMC?

Actually I want to know how to separate two successive flights in the same metrological conditions.

Two successive arrivals are separated by radar minimums respectively wake vortex minimums. I read that there are reduced separations under VMC (visual separation?). Is this actually used in everyday operations?