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Parallel Runways

Old 22nd May 2024, 08:08
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Parallel Runways

Good day to you all

I have a question regarding departures from parallel runways. Let's say we have 10 departures to various destinations, and the airport has two parallel runways, both of which are currently in use for departures. How does ATC decide which flights depart from L and which from R? Does it depend on ATC factors such as SID or wake turbulence, or is it simply a case of alternating L,R,L,R etc?

Many thanks.
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Old 22nd May 2024, 11:23
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An important factor will be whether the runways are independent or not, that is to say, whether what happens on one runway influences what can happen on the other and vice-versa, or can operations on each happen as if they are at different airports. This depends on how far apart they are. If they are not independent, the controller(s) will consider them as a single runway for wake turbulence separation purposes (can't recall the distance for independent ops and I think it differs between departures and other modes of operation and between different states). What happens once the aircraft are airborne also needs to be considered because (if they are IFT and under ATC) they need to be separated. On the ground, shortest taxy distance and minimising conflicts between taxi routes will be the main considerations (and, if it's relevant, minimising the number of runway crossings). Once in the air, the departures controller will be keen that the routes of those departures do not cross each other too much, so departures that are going to turn left going off the left-hand runway and those turning right going off the right-hand runway will generally be best. If it's so busy that two runways ae used for departures, the tracks followed by aircraft will probably be defined by SIDs and the controller(s) will have rules about gaps between aircraft following each SID route and which SIDs you can launch off each runway at the same time, etc. Adding in the different speeds that different aircraft will fly immediately after departure will just make the mx more fun!
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Old 22nd May 2024, 17:25
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Quite often they'll use one runway for take off and the other for landing.
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Old 22nd May 2024, 17:32
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Originally Posted by rudestuff
Quite often they'll use one runway for take off and the other for landing.
True, but even airports that normally operate in that mode may also have provision for simultaneous departures from their parallel runways.

For example, at Heathrow it's called Tactically Enhanced Departures (TED) and can be used at times when there is greater demand for departure capacity than for arrivals.
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Old 22nd May 2024, 19:18
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TEDs was a trial under the operational freedoms initiative, but was not continued after that.

Departures used to be unrestricted (i.e. ATC discretion) on westerlies, but in the late 2000s, HAL stopped that and there was, effectively, a designated departure runway on westerlies. Only flights fulfilling limited criteria (Royal Flight etc) could be exempt.
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Old 23rd May 2024, 03:44
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Departures from SFO on runways 01L and R simultaneously are the norm. ATC clearance allocates a heading to fly once airborne until Departure radar picks you up and issues further clearance. They also land on 28L and R simultaneously, usually with the instruction not to overtake the guy on the other approach. Both these sets of runways are very close to each other.
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Old 24th May 2024, 12:49
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Grateful for all your replies, many thanks!
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Old 3rd Jun 2024, 09:37
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The spread of use on the two runways can be dictated by many means.
Factors such as the aircraft parking position, SID, available taxiways, noise abatement all determine the selection of the runway for departure.
Sometimes a Departure Manager (Software) selects the runway for each flight based on the least overall delay for all flights.
In some airports or at certain times in these airports departures are given closest to stand to minimize the taxi time.
Sometimes especially in peak departure periods, the departure runway is allocated as per the SID (i.e. all departures to the North/West depart from one runway and all departures to the South/East use the other parallel runway)
With Noise abatement there may be a requirement to spread the departures equally over the available runways or to prioritize 1 runway that is not over a residential area etc.
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