Gaps between Departures

[VHF FLYER]

Hope you don't mind a humble pax asking what might seem a silly question.

Why do the departure gaps vary so much at busy airports?
I understand the need to avoid wake turbulence and gaps between big planes and small planes are always greater but that's about as far as my knowledge goes.

Take LHR and MAN as two examples that I have watched over the years (and sat at the holding points of both for what seems like years). Both have dedicated departure runways so no need to leave gaps for landing aircraft.

Observing the runway at LHR, an aircraft seems to roll as soon as the previous a/c has become airborne. This is probably as good as it gets, and ever can get.

MAN on the other hand, despite being less busy, seems to generate huge queues of a/c at the holding points at times. Look at the runway ops there and a/c seem to wait for ages after the previous departure. Both examples make allowances for different a/c size etc.

Could anyone explain why this happens to an often late passenger?
Thanks in advance.

[Gonzo]

In general, if a/c are follwing the same departure route, or a different route that has less than 45 degrees difference, then the separation is two mintues. If the succeeding departure routes diverge by more than 45 degrees, we can use one minute, or 'wheels up' separation.

Next time you're at LHR, have a look. If one a/c rolls immediately behind another, see which way they turn at two miles out.

[VHF FLYER]

Gonzo

Thanks, very interesting.

VHF

[Gonzo]

I don't know much about Manch's situation, but at a guess they have more routes that require two minute gaps.

[opnot]

vhf flyer
Unfortunately at Manch we do not have as many SIDS off 24R /24Lwhich diverge by 45 degrees or more as Heathrow does. The Hon, Monty, Nokin and to a certain degree WAL Sids fly the same route until 6nm after departure, therefore if you have a jet following a prop on these routes you have to apply a departure interval of at least 4 mins.We try to get the departures in the correct order to reduce these delays, however this is not always possible. Also you can introduce Pol and Desig Sids which diverge by 45 degrees to use time between the above departures. By the way, 06lL moves more traffic on departure than than 24R/L due to the way the SIDS are set up

[PMS]

in a radar environment with twr and not busy we will cancel the SID and use diverging radar headings so that we know we can achieve 3nm separation very quickly. this way the ADC can fire the acft sooner maybe a minute or so. but if in a procedural situation then the minimum will be 2 mins or more andespecially if a slower turbo prop in front of jet.

[HEATHROW DIRECTOR]

PMS.. presumably you're not in the UK? At our major airports we're hogtied with noise abatement which means the SIDs also have a minimum noise route incorporated and aircraft have to stay with that until several minutes after take off (is it until passing 4000 or 6000 ft??? I can't remember). If all this was ditched UK controllers could "fan" departures and shift more traffic.

[Inverted81]

Standard prescribed departure seps can be found on the CAA Publications pages..
go look for Manual of Air Traffic Services Part 1
Section 1 Chapter 3: 8.6

Obviously some units will have their own procedures written in their respective Part 2....
81

http://www.caa.co.uk/docs/33/CAP493PART1.PDF

heres the link.. :)