Diverging SIDs on departure
 

Dear All,

I could use with you specialist help on this issue. According to ICAO Doc 4444, for two consecutive departures:

"One minute separation is required if aircraft are to fly on tracks diverging by at least 45 degrees immediately after take-off so that lateral separation is provided".

I am assuming the aircraft are of the same wake vortex category, so wake separation criteria do not apply.

Is it possible (or practised) to apply 1 minute separation by providing only vertical separation between consecutive departures (i.e. both aircraft fly on the same track but the preceding one is launched at a steeper angle to ensure 1000ft vertical separation from the follower)? Alternatively, are there any means to reduce the 45 degree criterium for diverging departures?

I ask this in the context of simultaneous departures on parallel runways. If divergence of 45 degrees is required, then am I right in saying that is is practically impossible to achieve double the departure capacity of a single runway from a two parallel runway system (assuming the required runway separation for independent departures is in place)?

My thinking is that the second runway constrains the ability to diverge a departure towards it (since there will be other departures from that second runway that may conflict with departures from the first runway).

The requirement of ICAO doc on simultaneous parallel ops that parallel departures must also diverge at 15 degrees away from the extended runway centre line I guess further limits the choice of departure track.

Looking forward to your thoughts and suggestions (and corrections of my logic)!

I'm answering from UK procedures here so their may be slight differences in ICAO.

If wake vortex is an issue you would have to provide the greater of the two. You could still be using the 1 min as your dep separation but providing the wake vortex on top i.e 2/3 min.

45degree is a lateral separation so rate of climb is not usable.

You can reduce the 1 min separation using reduced separation in the vicinity i.e your visual with both and will insure the separation.

On parallel you would try and depart north bound from the north runway and southbound from the south to alleviate this issue.

45 deg sep
 

I'd like to add that 45 degrees can be tricky...4444 don't say whether 1st or 2nd acft is turning. For example, if small Cessna took off runway heading , would you dare to let B737 (which makes 45 or more turn after departure) after 1 min?
Any experiencies on this issue? ATCO's....

Zoneman,

You would not use the 1min separation based on those aircraft. That separation is used for aircraft of similar performance, so a B737 following a B737. In the example you gave you would require more than 1min!!

The 45 degree turn is one type of separation, and not to be "mixed" or confused with others, such as visual, or altitude.

If you elect to use vertical, (or visual) separation to get the second departure away before the required time is up to validate the 45 degree turn, you are not (and never will be) using the turn separation. And in that situation, it would be best to have pre-coordinated the situation with the next (accepting) controller. That controller will have the authority to decide when he/she is happy with the space between the two a/c concerned. This would normally be achieved using radar.

That will all take a bit of faffing around. Probably take about a minute. Maybe a little more.

Probably better just using the second runway for landers :ok:

Thanks very much for your responses, it helps to build a picture for how things work in the ATC world (I am more involved on the airport side).

Any comment on my second point regarding the hipothesis that it is impossible to achieve double the departure capacity of a single runway from a two parallel runway system (assuming the required runway separation for independent departures is in place), due to the reduced range of possible tracks? (Deps from one runway conflicting with deps from the other)?