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Effective TAS

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lucasgozzard created the topic: Effective TAS

Hi Bob,

I'm confused as to when I should be using Effective TAS in PNR or ETP calcs? Some students are claiming they didn't use it which is why they got the question incorrect in the exam?

Are you able to clarify?

Thanks.

Lucas.
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  • John.Heddles
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John.Heddles replied the topic: Effective TAS

Perhaps a bit of background, first, rather than just trying to answer your specific question.

"Effective TAS" confuses lots of students, mainly because the usual explanations avoid talking about basic trigonometry (or, more particularly, the geometric story of the trigonmetry) and, in so doing, make understanding more difficult.

First, if you don't have a suitable manual to hand, try this hyperlink and save the Jepp manual - www.jeppesen.com/download/misc/crinstructions.pdf

As you probably are aware, there are two main (there are others but not in general use) approaches to solving the wind triangle viz., the Jeppesen and Dalton styles. Not much in it as to which is the preferred option - the two use different sections of the wind triangle of velocities, with the Jeppesen using some trigonometry to solve the problem while the Dalton uses the basic wind triangle. The Jeppesen approach does have the advantage of being able to work in a smaller instrument which can be carried in one's shirt pocket.

The Jeppesen approach figures groundspeed as a simple addition/subtraction of TAS and W/C which is not quite mathematically correct unless you run a simple trig sum along the way. In practical terms, the error is pretty insignificant for very small drift angles but becomes noticeable above, say, 5-6 degrees, and must be corrected as the drift angle increases (say, beyond around 10 degrees) if silly answers are to be avoided. "Effective TAS" is the TAS after this trig correction has been applied ... I guess they had to call the result something and "effective TAS" is as good as any other term .. certainly, it suggests what the corrected answer is trying to provide.

So, what the Jeppesen triangle is calculating, is

G/S = effective TAS ± W/C

which, for very small drift angles, can be simplified to

G/S = TAS ± W/C

as the correction is very small and can be ignored for practical work.

Now, turning to your question -

(a) effective TAS has nothing specifically to do with PNR/ETP, per se. Rather it is a part of the wind triangle calculation if you are using the Jeppesen part of the triangle of velocities.

(b) strictly, effective TAS should be used for all wind triangle calculations if you are using the Jeppesen computer. However, because the error remains very small for small drift angles, it is usual to ignore the correction until the drift angle increases beyond 5-10 degrees.

(c) so, in summary, use effective TAS whenever the drift angle is significant (ie above 5-10 degrees) and you must always use it above somewhere around 8-10 degrees if you don't want to get strange answers.. Some folk will be a bit pendantic and quote this drift angle or that .. that is not the point .. the problem is that there is an error which increases as the drift angle increases and, at some point, you must correct for the error to avoid strange answers. The cutoff point is somewhere in the vicinity of 5-10 degrees .. you pick the spot you like. Having said that, it really is easier with the Jeppesen simply to use effective TAS all the time and then there is no need to think too much about it ... you will rapidly get a feel for where the error becomes worth accounting for.

(d) note that effective TAS has no relevance to the Dalton computer wind solution as the Dalton uses the stock standard triangle of velocities which can be solved geometrically using the wind slide.

Engineering specialist in aircraft performance and weight control.
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lucasgozzard replied the topic: Effective TAS

Gday John,

Thank you for your great reply. I am using the ASA E6B big flight computer. It has an effective TAS scale.

So the way to calculate GS with large drift angles is first calculate what the correction angle is then find that amount of degrees on the effective TAS scale and see what effective TAS it aligns with and that's the TAS you use to calculate the correct GS? Is this correct?

I will do this in the exam then if the crosswind is strong and the angles turn out to be more than 10 degrees.

Thanks John.
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John.Heddles replied the topic: Effective TAS

I am using the ASA E6B big flight computer. It has an effective TAS scale.

(Edit following some further net research)

I had an initial look on the net and found an ASA E6B computer which looks to be a stock standard Dalton (or E6B) device. This will have no application involving effective TAS, I suggest.

Subsequently, I found a device referred to as an ASA E6B circular computer. This is pretty much similar to the Jeppesen device and operates in the same sort of manner.

Point of confusion - the original term "E6B" apparently related to the military designation of Dalton's original device. Thus, to refer to the Jeppesen-style device as an E6B is a good recipe for considerable confusion. However, these days there are so many different designations for what is essentially the same device that confusion is just part of the game, I guess.

So the way to calculate GS with large drift angles is first calculate what the correction angle is then find that amount of degrees on the effective TAS scale and see what effective TAS it aligns with and that's the TAS you use to calculate the correct GS? Is this correct?

Suggest you have a read of the Jeppesen manual (see the hyperlink in the previous post - p30 and subsequent) and follow the manual's technique carefully. That's a lot easier than trying to explain the technique without the graphics to illustrate the points involved.

I will do this in the exam then if the crosswind is strong and the angles turn out to be more than 10 degrees.

I wouldn't fuss too much about 10 degrees ... but, certainly, at and beyond that sort of drift angle you must be applying the correct trigonometry.

Engineering specialist in aircraft performance and weight control.
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