wake separation with crosswind

[misway]

Hi all

I'm trying to find guidelines on the reduction of separation required, due to a crosswind, between departing heavy and medium cat aircraft for take off. As far as I'm concerned 2 minutes is required, but last night out of Johannesburg we were asked if we were ready for an immediate take off less than a minute after a heavy departed. I said no and queried the ATC's separation - he said less was needed with the crosswind there was. (Rwy 03L, wind 360/13, crosswind component = 7kts.)

Many thanks.

[rudderrudderrat]

Hi misway,

There is an ongoing investigation called "CREDOS" Crosswind Reduced Departure Operations.

http://www.eurocontrol.int/corporate/gallery/content/public/event_docs/2009_11_09wakevortex/Global_Wake_CREDOS_v1.pdf

I haven't seen anything published in our manuals yet, so we can't accept the reduced separation either.

[Chris Scott]

7 kts crosswind-component doesn't sound enough to me. We used to work on the assumption that wing-tip vortices moved outwards at about 5 kts, so 7 kts is close to the worst-possible case.

Chris

[Todders]

Couldn't agree more in my head the port wing tip vortex would be held nicely where you wouldn't want it. Good call saying no it's you that has to ride it out after all.

[Slasher]

From WHERE does the 2 mins seperation for TO start?

I thought it starts at the begining of the offending aircraft's TO roll.

Another guy says its at rotation (I agree for DC10 MD11 757)

An ATC idiot in China times it from the verbal issue of the offending aircraft's TO clearance.

:confused:

A ICAO ref would be very apreciated

[rudderrudderrat]

Hi Slasher,

The 2 mins "separation" is just that. i.e. you should start your take off no earlier than 2 mins after the preceding heavy started his take off roll. By the time you rotate, you'll be 2 mins behind the preceding's rotate.

[Chris Scott]

Quote from my previous post:
7 kts crosswind-component doesn't sound enough...

In case anyone infers that I was suggesting a stronger crosswind might enable separation to be reduced: I was not.

By the way, does the 3-minute rule - when the following aircraft is starting its run from an intersection - still exist?

Chris

[towerboy]

I can not comment on the reduction in wake turbulence separation standards to (not) answer the original question.
In Aus we apply airborne to airborne.
My understanding is that wake turbulence corresponds with lift.
The moment an aircraft becomes airborne...ie lift is greater than gravity...then wake, or significant interference of streamline airflow occurs...resulting in turbulence!
It is interesting to note, on departure, firing a (M) behind a (H), or a (L) behind a (M), 5 miles or 2 minutes is required...AND with a slower following aircraft, it is more efficient to use a distance standard, or approximateley 1.5 minutes!
I would also like to point out the inequities in departure vs arrival wake turbulence standards...but that is for another day.
My 2 cents worth...please feel free to correct.

[towerboy]

Further to my last, and apologies for not reading the final question. The 3 minute rule does, and should exist.
The theory being that if a following lighter aircraft departs from the same intersection as a preceeding heavier aircraft, than it will climb before and above the wake.
If the lighter aircraft departs from a magical distance beyond that of the preceeding heavier aircraft commencing its take off role, than it will be closer to the wake envelope of the preceeding, hence 3 minutes.
I do not want to tell people to suck eggs, just putting forward my understanding of the rules.
Gotta go and have a beer and watch the footy. Cheers!

[Crash and Burn]

Hi Chris,

The relationship between wake turbulence and the timing between departures is dependant on the aircraft types involved in the scenario. In the United Kingdom at least, there is a 3 or 4 minute wake turbulent departure published in CAP 493 - Manual of Air Traffic Services (Part 1).

http://www.caa.co.uk/docs/33/CAP493Part1.pdf

I start my stopwatch as soon as the preceding departure rotates but from speaking to a number of my colleagues, there are only a handful of people that do this; the rest rely on ATC or some internal clock!

Eg: B747 departs full-length followed by an A320 departing from the same point with an optimistic take-off roll of 25 seconds. In the UK, CAP493 states a requirement of 2 mins wake turbulence therefore I would not start my take-off roll until 1:35 seconds after the B747 rotates, therefore I should have at least 2 mins.

On a couple of occasions I have been instructed to “roll now” with insufficient time remaining between the preceding departure and the next arrival were the wake turbulence spacing would have been unachievable. As for crosswinds and its effect on wake turbulent separation times, I am of the opinion that this would make life for our ATC friends a little too complicated for a number of reasons (including: SIDs, Missed approaches, Emergency turns etc) and in the absence of an approved procedure I just stick to the timings as listed in CAP 493.

One last thing to think about is the actual wind, when I used to operate out of Cardiff and on more than one occasion too, I have noticed the wind socks pointing at each other and nope my eyes are working just fine! I have noticed this at a few airfields as well but it is rather rare… I must be bored to notice these things!

Regards
C & B

[Chris Scott]

Thanks for the link to CAP493, Crash and Burn,

This thread was started with a Medium following a Heavy, but I see that the A380 has added further complication to the rules.

And, for the benefit of anyone who thinks that discretion might be used in a strong crosswind, this is what CAP493 (Section 1, Ch. 3, Page 12) says in relation to the UK:

"NOTE: ATC shall apply the minima as prescribed..., irrespective of any pilot request for reduced wake turbulence separation. ATC does not have the discretion to reduce wake separation minima."

Chris

[Pugilistic Animus]

http://www.faa.gov/air_traffic/publications/ATpubs/AIM/Chap7/aim0703.html#7-3-9


:)

[Slasher]

Thanks all for the refs.

This all works well for all engines operating. If you accept a reduced wake sep, consider what will happen if you lose one at V1 or more where cleanup is strait ahead till 800 - 1500 feet.

Its been done and its NOT nice! :ouch:

[decurion]

Check out Wakenet - WakeNet (http://www.wakenet.eu/) . You can find all lot of information on recent research of wake vortices and crosswind.