Compton SID

[goldox]

From where I live near Benson (and specifically from my back bedroom window) I am used to seeing the traffic from Heathrow on a Compton SID.
As a layman, I would have thought that this "Standard" departure means that all traffic would be approximately at the same height and position at or near CPT.

They would normally be (estimate) 8,000 - 10,000 feet, and following specific path across my eyeline, but I note quite a lot of variation, often hearing an aeroplane and not seeing it where I expect, but very much higher, or nearer to me than usual.

Is this because there is a certain amount of leeway in the procedure?

[Gonzo]

Once aircraft are above 4000ft, London tend to take aircraft off the SID on a more direct route (or to position aircraft to one side of an airway to allow for traffic coming the other way).

[goldox]

Thanx Gonzo, I didn't realise the departure was so flexible. I just assumed it was a fixed procedure to be followed rigidly.

[HEATHROW DIRECTOR]

Goldox... There's leeway in the vertical dimension too. On a very quiet evening, as a Tower controller at Heathrow, I have cleared departing aircraft straight up to their cruising level.... but it doesn't happen often, probably not at all nowadays.

[goldox]

Thanx for that HD. As a matter of interest what motivation is there to get to their cruising level more quicky? To clear them off your sector :), or fuel efficiency/costs/time for the airline etc?
Is there any advantage for them to reduce their planned flight time or do they just end up being held up at destination anyway if early?

[HEATHROW DIRECTOR]

Fuel is conserved appreciably the higher the aircraft goes.

As for holding if an aircraft arrives early, that's not usually how things work. ATC has nothing to do with scheduled times and basically deals with aircraft on a first come, first served basis although there are reasons why this system varies sometimes..... but it has nothing to do with airline schedules, just the need to obtain maximum runway utilisation.