Wake Separation

[PhineasC]

How is wake separation defined? It could be a distance between aircraft or the time taken for them to pass the same point, for a constant speed these are equivalent. If time is the measure then as speed increases the distance between aircraft decreases, this would appear to take the relationship in the wrong direction.

If distance is the measure then the most efficient use of the airspace would be to have the aircraft fly at maximum speed, the distance between aircraft is the same if they are both flying at 120 knots or 240 knots.

I’ve been trying to come up with a couple of lines of algebra that shows that as speed increases the turbulence will also increase this would suggest that neither time nor distance is the correct measure

[Mechta]

I was under the impresion that the induced drag and the tip vortices are directly related, so the tip vortices increase as the speed decreases. Therefore if the aircraft are flying slower (all other things being equal), the separation needs to be greater.

[DaveReidUK]

How is wake separation defined? It could be a distance between aircraft or the time taken for them to pass the same point, for a constant speed these are equivalent.

It's usually expressed as a distance.

If distance is the measure then the most efficient use of the airspace would be to have the aircraft fly at maximum speed, the distance between aircraft is the same if they are both flying at 120 knots or 240 knots.

Well that's an extreme example, but you have a point.

With distance-based separation, a headwind on approach has the effect of increasing the time interval between successive aircraft passing the same point (since their groundspeed is reduced) and therefore affects the arrival rate.

[Gonzo]

PhineasC

In the UK we separate for Wake Turbulence by distance during the arrival phase, leading to the impact upon arrival rates that DaveReidUK has described, and by time for runway separation on departure.

Work is ongoing to switch to a time-based system for arrvivals, hence providing resilience to the arrival rate in headwind conditions.

[PhineasC]

To clutter already busy airwaves thank you for your answers.

From what has been said it is departures that are much of a capacity constraint as arrivals. It must be frustrating when captains are optimistic about pushback times or take more time to get to the runway than is necessary

[grounded27]

Just talking about wake separation the other day after hearing the shearing air (sort of like someone whispering a tearing in your ear) quite some time after an aircraft flew overhead. It is impossible to calculate, what we do know is that in still air the wake turbulence last's longer. Distance and a/c type are used for separation as a standard.

[DaveReidUK]

It is impossible to calculate, what we do know is that in still air the wake turbulence lasts longer.

Is it really impossible ?

Google CREDOS and CROPS.

[Gonzo]

Depends what is meant by calculate.

CREDOS/CROPS allow for reduced separations due to forecast behaviour of vortex based on vortex modelling and wind measurement.

TBS allows for reduced separation based on wind measurement which is used to modify current vortex separations which do not take account of differing ground speed.

You cannot calculate the behaviour of vortex any more than you can calculate the behaviour of a cloud. You can measure, record, feed that that into a model and therefore forecast and predict its behaviour.