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## CFPA Exam KDR - Determine ETI

• Posts: 4

### JasonDoritos created the topic: CFPA Exam KDR - Determine ETI

Hi,

I am struggling to understand my KDR when I thought I did it correctly in my exam. I got 67% on my exam on my 2nd attempt and feel unfortunate that I came across two ETI questions that I got both of them wrong.

I believe the question goes by TAS 180kt, Distance A to B 250nm, Flight Planned Track 330°M, GPWT wind 360M30KT, 100G fuel on board, 15G/h flow, Inter at Alpha, Tempo at Bravo, what is the time for the ETP?

GsOut = 180 - 26 = 154
GsHome = 180 + 26 = 206
No ETAS: Since it's below 10 degrees.

Distance ETP: (250 x 206) / (154 + 206) = 143nm
ETP to B: 250nm - 143nm = 107nm
Time ETP: 107nm@154 = 41.7mins or 42mins (as whole number)

Or perhaps the question asks for ETP to A:
Time ETP: 107nm@206 = 31.2mins or 31mins (as whole number)

Is this the correct way to do this type of question? I would ignore the irrelevant information that I put up there as well. Please let me know.

• John.Heddles
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### John.Heddles replied the topic: CFPA Exam KDR - Determine ETI

Engineering specialist in aircraft performance and weight control.

• Posts: 4

### JasonDoritos replied the topic: CFPA Exam KDR - Determine ETI

Hi John,

I have double checked my working and it looks similar to how Bob Tait does his. If I'm still wrong, can you please explain to me how I am supposed to do this type of question? I really appreciate your feedback. I will be doing my 3rd attempt on Weds, fingers cross I can pass.
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• John.Heddles
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• Posts: 867

### John.Heddles replied the topic: CFPA Exam KDR - Determine ETI

Jason,

First, are you aware of the CASA penalty for a third failure in a subject ? Three months on the sideline for some serious study before they will permit you to re-sit for a fourth time ..... and it gets worse from there if you don't knock the exam over.

Unless a time delay is of no concern or significance to you, I would seriously consider deferring your next attempt until you are confident that you are really up to speed for a sound pass. Crossing of fingers really doesn't play much of a role in this game.

Having given the obligatory lecture, I'm only too happy to help out but it will be a day or two before I can put some stuff together for you.

Engineering specialist in aircraft performance and weight control.

• Posts: 4

### JasonDoritos replied the topic: CFPA Exam KDR - Determine ETI

Hi John,

I can delay it until Friday. I am fully aware of the consequence of not passing the 3rd attempt.

As much as I have said fingers cross, it's merely the chart questions that I happen to always get 2/4 of them right. As much effort I put into it, due to how accurate CASA wants it, I'm not sure how else to explain the exam besides being lucky to get it right.

Aside from that, I really appreciate your help with the ETI question I'm not familiar with.

• Posts: 4

### JasonDoritos replied the topic: CFPA Exam KDR - Determine ETI

Hi John,

I think I understand what I got wrong with Time ETP.

GsOut = 180 - 26 = 154
GsHome = 180 + 26 = 206

Distance ETP: (250 x 206) / (154 + 206) = 143nm

A to ETP: 143nm@154 = 55.7mins
ETP to A: 143nm@206 = 41.6mins

ETP to B: 250nm - 143nm = 107nm, 107nm@154 = 41.7mins
B to ETP: 250nm - 143nm = 107nm, 107nm@206 = 31.2mins

• John.Heddles
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### John.Heddles replied the topic: CFPA Exam KDR - Determine ETI

I think that you are tending to approach the problem with a scatter gun attitude. Rather, try to figure out what the question is asking and then answer just that.

Let's have a run through what you have done, first.

What is the time for the ETP?

Unless the question specifically suggests something else, if you are asked for the distance or time to the CP (ETP if you prefer) then you are being asked for the distance or time from the departure point to the CP.

No ETAS: Since it's below 10 degrees.

The ETAS thing causes folks a lot of confusion and heartache. It is the component of TAS resolved along the TR/GS vector line, no more, no less. The arithmetic (trigonometry) is ETAS = TAS x cos(drift angle). Now, you don't need to know any of this at all to work the CR whizz wheel but it is useful for understanding this ETAS thing and the 10 degrees nonsense.

What is useful to know is three things -

(a) the correct solution is to figure out ETAS EVERY TIME. That gives you the correct solution, EVERY TIME.

(b) the cosine value is asymptotic (gets closer and closer to) 1.0 as the angle approaches zero. So as the drift angle approaches zero, the value of TAS x cos(drift angle) gets closer and closer to TAS. This becomes very noticeable once you get to a drift angle less than, say, 15 degrees and, in the final 5 degrees, especially, the difference is not worth worrying too much about.

I guess Ray Lahr came up with the 10 degrees idea thing when he was designing the CR back in the 50s. What he was trying to say was that you get to a point where there is not too much difference between ETAS and TAS as the drift gets smaller and smaller. Somewhere along the way, some silly person came up with the idea that it might be a good idea to forget about ETAS once you get down to these small angles (ie just use TAS) and that is not an unreasonable position to take. However, it has absolutely no value to anyone..

(c) Because you have to set the outer C/D scales up for another multiplication (TAS x sin(drift) = crosswind) that automatically sets up the scales for the ETAS calculation. It is just a simple matter of flicking your eyes around to the black part of the scale to read off the ETAS value .... so, why wouldn't you do that and save the effort of remembering the 10 degrees thing ?

ETP to B: 250nm - 143nm = 107nm
Time ETP: 107nm@154 = 41.7mins or 42mins (as whole number)

Why work this out ? It's not necessary. Wasted time in the exam.

Or perhaps the question asks for ETP to A:
Time ETP: 107nm@206 = 31.2mins or 31mins (as whole number)

Same here. If you really MUST do this, perhaps use the correct distance, ie 143 nm.

ETP to A: 143nm@206 = 41.6mins

Again, not necessary and time wasting.

ETP to B: 250nm - 143nm = 107nm, 107nm@154 = 41.7mins
B to ETP: 250nm - 143nm = 107nm, 107nm@206 = 31.2mins

This is all good stuff but not required and wastes time in the exam. Keep very clearly in mind that, especially if you are looking at CPL/ATPL, speed and accuracy become absolutely critical goals for the candidate.

You cannot afford to waste any time or make any silly mistakes, lest you get less than the magic goal of 70%.

Engineering specialist in aircraft performance and weight control.