Wake turbulence separation
 

Good day all,

I have a doubt. There are basically two procedures
- Radar separation
- Time based separation

Let's assume we're departing from an airport where separation is based on radar.
Are we supposed to respect the time constraint as well??

Ex: cleared for T/O by tower (I can see on the ND that the preceding ac is at least 5Nm ahead). However, the time constraint (2 or 3min) is not elapsed yet.

Can we take off??

(I presume that is a NO, but I can't find any explicit document)

If you are under radar separation, you don’t need to comply with the time constraint.
You can find more information in the DOC 4444 “ Air Traffic Management”.

At JFK you can opt out of the radar separation and insist on the 2 minutes for a medium after a heavy.
But it needs to be announced ahead of taxi and you probably get put in line behind another medium where it's not an issue.

If you announce it as you are told to line up or take off you will find the controllers annoyed if there is any kind of traffic going on.

edit: oh and at JFK they are using a new system too as far as i know as part of FAA recat(egorization) effort
so if it's a 767 and not a 747 you might get 4 miles instead of 5 and so on.

IMHO "Cleared for takeoff" means just that. It's an ATC thing. Deciding to takeoff is in your hands. If you're happy then go, if not then pause. However, if on a single runway ops, and traffic at 2nm finals, one would trust that ATC 'know what they are doing and have taken everything into account'. If you don't go when it is safe then voices will be raised.
I've seen this happen, light behind heavy, that ATC said, "XYZ, in your own time cleared for takeoff." The light captain went ape-poo telling ATC he needed 2 mins etc. etc." ATC repeated, "in your one time........."

Interesting one about radar separation v timing. That's new to me. I thought radar spacing, on finals, was based on timing, hence everyone is at the same speed. How does that work on departure?

Thank you for your input guys!
Cheers

From Doc 4444, the separation is exactly the same for arrival and departure. :)

feather44, I think you will find that the DXB controllers (and many others - but not all) will only use time for departure as that is all they are allowed to use.

So you have either 2min, 3min or 4min depending on the combination of aircraft weight category and if the second departing aircraft is an intersection departure (greater than 150m from the first departure point = add 1min)

Also you will probably find that any decent controller will be uttering those magic "cleared for take off" words when you have about 45secs remaining for the time to run as it is measured wheels up to wheels up. They want you to be rotating at 2mins+1 sec or 3mins + 1 sec. Nothing more infuriating than a pilot who thinks he knows about wake turbulence and will not roll until the 2mins has elapsed. :=

If you are not happy with the ICAO standards then take it up with them :}

A very interesting question indeed.

I'm looking for one specific answer :
Is it the responsibility of the ATC to enforce wake turbulence separation ?
If so, I would be very worried because several ATCs cleared me to land or takeoff behind medium jets, well before the required time, while I was flying a 800kg aircraft.

Or is it the responsibility of the captain to ensure that the separation is satisfactory ?
If he deems it safe, can he reduce the separation ? (e.g, strong crosswind)

Actually I don't know, that's why I'm asking.

Forget about dxb, they are pros, no doubt.

My question was simply (besides airmanship) do you have to respect both constraints....that' is it!

You don’t have to respect both constraints. If you are under radar control, ATC is doing the separation for you. Forget about the time. The time constraint is more conservative and would be problematic in some very busy airports.
During the approach it’s the same: ATC is responsible of the minimum separation. Of course you should adjust your deceleration smartly accordingly if you see you are getting too close to the traffic ahead of you. If you don’t ATC will ask you minimum speed or give you a speed constraint as required to keep the minimum separation.
Standard minimum separation between medium behind a medium is 3nm and can be reduced down to 2.5nm in some airports.

Cleared for visual
 

One of my pet peeves with ATC was when they vectored me 3 miles behind traffic and 30 knots faster than traffic. Then they said "do you have traffic at 12:00 and 3 miles. Once I said I saw the traffic they said "cleared for a visual approach" with out telling me I was much faster than the traffic.

ah welcome to Beijing..

I have had a number of wake incidents whilst under radar control. Filed them, nothing happens. One of the times I filed caused a nightmare at the company because they grounded the plane until it had been inspected. So I won't bother filing anything again - its a job threatening waste of time.

That depends! If you started timing when the previous aircraft began takeoff roll... :E