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Wake Turbulence KDR Question

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MJake14 created the topic: Wake Turbulence KDR Question

Hi Guys,

I got an almost like for like question in my CASA aero exam that is attached and received a KDR relating to it.

The question was phrased along the lines that a heavy jet took off at 2500m you know you will be airborne at 1300m, what do you?

I recall narrowing it down to 2 options: no delay required as vortices only commence as the heavy aircraft rotates or wait 3 minutes.

If I had not done this practice question the other day I would have gone with the wait 3 minutes.

Is there any evidence to back up the practice question as I would like to go back to CASA and question the answer.

Thanks for your help.

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  • John.Heddles
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John.Heddles replied the topic: Wake Turbulence KDR Question

Probably worth a few comments here, methinks.

First up, the accepted theory of lift (circulation theory .. things long ago progressed past Bernoulli in real life, if not in pilot theory examinations) shows that the vortex phenomenon commences as soon as the aircraft starts its takeoff roll. If you want an easily understood and good readable story of circulation, have a look at Arvel Gentry's sailing articles (Gentry, who died not all that long ago, was a very knowledgeable aerodynamicist with Macdonnell Douglas, as well as a well-regarded sailing chap) .. some of these can be found at the following site www.gentrysailing.com .

There is a starting vortex shed across the wing span at the commencement of the takeoff roll and the usual wingtip vortices shed as the takeoff progresses. However, as the amount of lift being generated prior to rotation is minimal, we don't fuss too much about the potential effect of these on other aircraft during the jet's takeoff run. Hence the usual admonition that we start worrying about vortex interactions post jet rotation.

Second, the general observation is that the wingtip vortices (in flight) descend a bit and then remain more or less steady until viscosity kills them off with the wind moving both trails as one would think. Note, though, this isn't always the case ... sometimes the shed vortices can climb above the generating aircraft ... if you are interested in having a look at these sorts of effects, try a read of NASA Contractor Report 4767, which you can download via a simple google search.

So, how to approach this examination problem ? Conventional wisdom (which works most of the time) is that if the preceding aircraft rotates considerably further down the runway, then the following aircraft is going to be above the preceding generating aircraft's shed wingtip vortices.

Looking at some of the comments in the OP's post ...

The question was phrased along the lines that a heavy jet took off at 2500m you know you will be airborne at 1300m, what do you?

I'd takeoff without any initial worry. However, depending on the preceding jet Type and weight, I probably would be looking to request (presuming a controlled ATC environment) an early turn into wind after takeoff to put some distance between my aircraft and the preceding jet's presumed vortices locations.

I recall narrowing it down to 2 options: no delay required as vortices only commence as the heavy aircraft rotates or wait 3 minutes.

No delay required .... I'd go along with that. Vortices only commence ... Not quite correct but we understand what is meant. or wait 3 minutes. Why ? .. and, if you had a valid reason for waiting, the timing is going to be heavily dependent on the preceding Type and weather conditions.

As with any multi-guess question, check out EACH answer to discard the distractors and then worry about what's left in the bag.

Option (a) How were you planning to intersect the preceding aircraft's flight path ? It's faster, further down the way and MAY OR MAY NOT be climbing at a gradient less than yours. If the last point were a concern, an appropriate turn takes care of the worry. I'd give this option the big flick without any further thought.

Option (b) How were the relevant sections of the trailing vortices (ie after the preceding aircraft is airborne) going to come back to you on a calm day ? Give this option the flick as well.

Option (c) An interesting option. The first bit is fine .. you will be off the ground prior to the point at which the preceding aircraft was. However, the second bit isn't a foregone conclusion at all. Your light aircraft may well climb at a gradient very much less than the preceding jet's ,, it all depends on the two Types and their weights. Keep in mind that the all engine operating climb for a twin jet at low weights can be VERY impressive ... What you would do, in practice, is keep an eye on things and, if a concern arose, request an early into wind turn to put some distance between you and the jet's likely vortices positions. Overall, this option is acceptable, subject to the gradient consideration.

Option (d) I don't think so. How do you suppose that a calm day might make things more optimistic so far as the vortices are concerned ? Give this one the flick as well.

That leaves us with option (c) so I run with the examiner's choice for this one ....(subject to the unjustified presumption regarding climb gradients).

Engineering specialist in aircraft performance and weight control.
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MJake14 replied the topic: Wake Turbulence KDR Question

Thanks John,

That is very helpful insight. I will have a read of the NASA report.
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John.Heddles replied the topic: Wake Turbulence KDR Question

The NASA report will provide some interesting reading but you will derive better value from reading Gentry's articles. If they are not at the referenced link, there will be a subsidiary link there somewhere. His original website is defunct, it seems, and the present sites are attempts to keep his data available for general access.

His papers are some of the easiest explanations to understand I have read over the years. He knew his stuff inside out, backwards, forwards and every other which way.

Postscript - a better link for Gentry takes you straight to his papers of interest - arvelgentry.jimdo.com/articles/ . While all the papers should be of interest to the sailing folk (I sail little TS machines ..) the main one to have a look at is "Origins of Lift".

Engineering specialist in aircraft performance and weight control.
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Kaos replied the topic: Wake Turbulence KDR Question

We also had a similar question in the CASA exam - the correct answer (for reference) is wait as you risk encountering the turbulence.

Whilst you may out climb the jet initially (and you'd be well out the way if you turned off the track into a circuit etc) you still risk meeting the wake as the jet will climb much faster than you will, despite you have a better climb angle.

Also, regardless of whether or not you will out climb the vortex - legislation states if you are a light aircraft following a heavy you MUST wait the 2/3 minutes.See the below link for clarification. So even if you wont encounter the vortex, you legally have to wait and ATC cannot give you clearance

www.legislation.gov.au/Details/F2014C001...8f-aad7-db643e08e7bf
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