Wake Turbulence Timing.

[A21]

Dear all,

I would like to know the official rule for the start of timing regarding wake turbulence separation on T/O, as some start timing at brake release and others at rotation of preceeding.(Any document)

Thanks a lot for your inputs.
Regards

[Will_McKenzie]

I remember being taught it was when the nose wheel first lifts, as that is when the wake turbulence commences and finishes when the nose wheel touches down.
MATS Part 1 Section 1, Chapter 3, Page 10

[Gonzo]

A21,

In the UK it's rotation to rotation.
In my experience, quite a lot of the rest of the world measures start of roll to start of roll.

[DFC]

ICAO DOC 4444 Chapter 5 - Separation.

In the case of one departure following the other it is specified as a minimum departure separation i.e. it applies from the moment of departure (becoming airbourne).

If for example one was to apply the separation to the start of the roll then it is possible to have the second aircraft (short roll) airbourne a matter of seconds behind the first (long roll) and straight into it's wake.

Wake turbulence disipates with time and as a very general rule after 2 minutes the most dangerous turbulence will have subsided. Therefore it makes sense to apply the separation at the point where it is possible for the second aircraft to encounter the wake of the first.

Have a look at the diagrams in DOC 4444 to see how it is applied on crossing tracks etc.

This is the most common separation standard where pilots exercise their right to demand increased separation.

[Crazy Voyager]

How do you tower ATCOs judge the seperation when it's based on rotation? Do you have a general idea how long it takes for a plane before it rotates (so you count to 1.40 and then start the next based on the anticipation it won't rotate until at lest 20 seconds later) or how do you judge it? Or do you wait 2 minutes from rotation until the next start roll?

[HEATHROW DIRECTOR]

Crazy Voyager... It's all down to training and experience. It doesn't take long to know how many seconds it takes a particular type of aircraft to get airborne so applying the separation accurately is straightforward. A fully laden heavy jet may take twice as long to lift off than a lightly loaded shuttle.

[DFC]

Which is why many years back an instruction had to be issued reminding everyone that when applying departure separation for wake turbulence purposes 2 minutes is 120 seconds i.e. don't round the airbourne times for wake.

[HEATHROW DIRECTOR]

It was extraordinary the number of pilots who would say "we're happy to go now" considerably less than 2 minutes after a heavy! Needless to say, ATC would not permit that.

[Desert Hunter]

Needless to say, ATC would not permit that.

They might! It's dependent on the civil aviation authority rules of the country. When I was controlling in the UK, I too was surprised at the number of pilots who would request a waiver because it was never granted. However, working in the UAE I am permitted to grant a waiver from wake turbulance separation provided certain criteria are met.

From the GCAA Civil Aviation Regulations part VIII:

1.15 Wake turbulence separation

1.15.1 A specific pilot request for a waiver from any wake turbulence separation may be granted provided –

1. The air traffic controller does not prompt, instigate or invite a
pilot to request a waiver from wake turbulence separation; and

2. When the other aircraft is an ICAO heavy category, or B757
aircraft, the air traffic controller reminds the pilot requesting the

waiver of the category or type of the other aircraft.

[windypops]

I know of pilots who ask to go before the 2 minutes (3 if it's an intersection), and their reasoning is that they can take into account the current met whereas the controller can't. One would say things like "because of the strong winds it will dissipate much quicker". There is probably some truth to it but I don't know enough about the science to do it myself. I prefer to default to safe and go with the 2 mins (or 3), using my own stopwatch.

I wonder what their insurers would say if they needed some work done due to encountering wake turbulence and not waiting the 2/3 minutes???!!!

[Denti]

And then there is an airport where ATCOs demand for any medium aircraft a 2 minute departure interval behind a 737-800 because they consider it a "heavy" type. Spanish ATC at its best...

[HEATHROW DIRECTOR]

ATCOs don't "demand" anything themselves - they simply follow laid down instructions. If a Spanish airport has instructions that say they have to employ 2 mins separation behind a particular type, the controllers don't have much choice. If you fly there regularly I suggest you discuss it with them..

[Spitoon]

ATCOs don't "demand" anything themselves

Judging from some of the comments on another thread I can't help feeling that Spanish controllers may be an exception that proves the rule!