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PNR/ETP use ETAS or GS?

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bradd created the topic: PNR/ETP use ETAS or GS?

Hi All,
Re: using GS as opposed to TAS figures.
I understand the logic as to why we need to use G.S as opposed to 2x TAS when the wind is not from dead-on track.

My query is, if a head / tail wind is not given in the exam question, will a wind velocity/forecast wind always be provided so I can work out the Ground speed's each way?
I have seen some other threads in this forum with the question only providing a crosswind component eg: 40 knot crosswind, and a TAS.
Is this type of question a possibility in an exam?
I have an old school rectangular type E6B flight computer and not sure how to figure something like this out when not provided a wind velocity..
Cheers,
Bradd.
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  • John.Heddles
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John.Heddles replied the topic: PNR/ETP use ETAS or GS?

I think that we might need a little bit more information to help to any great extent as it's not entirely clear to me just what you are endeavouring to achieve. However, some comments to start the discussion off -

I understand the logic as to why we need to use G.S as opposed to 2x TAS when the wind is not from dead-on track. Good

if a head / tail wind is not given in the exam question, will a wind velocity/forecast wind always be provided so I can work out the Ground speed's each way? Can't say for sure - perhaps Bob can hazard a guess ..

I have seen some other threads in this forum with the question only providing a crosswind component eg: 40 knot crosswind, and a TAS. That's fine but we would need some more information to do much. With TAS and crosswind you can figure the drift angle (and the effective TAS, if you like). Providing you have a bit more to go on we can solve the problem - otherwise there exist numerous solutions for the wind triangle. On the Jeppesen, you have all the trig data you need on the calculation side while, for the Dalton, you can check the angle directly from the basic wind triangle for each of the many triangles which have the same starting data.

I have an old school rectangular type E6B flight computer and not sure how to figure something like this out when not provided a wind velocity.. As indicated above, you can figure the drift angle simply from the basic wind triangle as that is a scaled picture of the navigation solution. However, we would need a little more to go on to figure out just where your question might be leading ?

Perhaps you might spell out a typical sort of question with which you are having difficulty ?

Engineering specialist in aircraft performance and weight control.
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bradd replied the topic: PNR/ETP use ETAS or GS?

Hi John, thanks for the info and reply.
It's been a long time since have used this computer and I think I have now got the hang of it. Maybe? haha.

Example Question that I ran across in an older thread:

Given a TAS of 150 KT and a crosswind component of 35 knots, find effective TAS (ETAS)?

Would I mark my wind dot 35kts up from the centre dot, and turn track 90 degrees and put my marked wind dot on the 150 TAS line?
eg: Set my Wind up 35 kts on East, rotate to my Track of say North, slide the marked dot down to the 150 line to read a GS/ETAS of 145ish knots?

Cheers,
Bradd.
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John.Heddles replied the topic: PNR/ETP use ETAS or GS?

Answer is 145.9 so you are pretty close to the mark.

Probably useful if I run up a couple of graphics for the benefit of those interested in the process.

Important point - you can figure effective TAS (ie the component of TAS resolved along the track vector) with the information given. However, this does NOT give you GS in any shape or form - you need some more information to figure that out.

GS only equals effective TAS if there is nil wind - ie then TAS = ETAS = GS

One of the problems is that most training emphasises the step 1 - step 2 - step 3 of how to run the whizz wheels but provides very little in the why do you do this and that. If you have an idea of the latter, then the former becomes a whole lot simpler and easier to nut out along the way in each problem.

On the question of ETAS - use it all the time with the Jeppesen solution. Sure, if the drift is small (say, less than 5 degrees) the difference between TAS and ETAS becomes trivially and vanishingly small - but why agonise about when you must and when you can be optional about running the trig resolution in the little wind triangle ?

If you prefer playing with a Dalton instrument, ETAS is totally irrelevant as it uses the main wind triangle and runs the calculations graphically. You only need to run the ETAS calculation if you are using the Jeppesen solution.

As to which computer you should use ? Doesn't matter until you get to ATPL. For faster aircraft you need to account for compressible flow rather than the simplified incompressible flow which is fine for aircraft which go low and slow.

The Jeppesen units incorporate the compressible equation in the fancy abaque on the calculation side of the wheel as well as the incompressible equation in the traditional cutout which is the same as on the Dalton.

Most of the Daltons one can purchase don't give you any way of running the compressible sums. Some do and if you use one of those then there is no advantage to the Jeppesen and, indeed, the Daltons, generally being made of alloy, are far more robust if you toss them on the glareshield in mid-summer. The Dalton approach to compressibility is either to include an F-table to effect CAS/EAS corrections (which you do mandraulically and is the traditional way to run the sums) or use another cutout which gives you the same result a bit easier for the user.

Makes it all pretty silly that you can take one style of computer with the compressible equation incorporated into it, or another with the F-table data engraved on it, into the exam but you can't take the same F-table data printed on paper into the same exam - go figure.

Engineering specialist in aircraft performance and weight control.
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bradd replied the topic: PNR/ETP use ETAS or GS?

Thanks a lot for that info John.

Yes, it would be very helpful if you could present a graphic example of finding that answer if you have the time.

Quite interesting to learn about the atpl standards as I will be getting into those subjects later in the year- thanks for the info.

Regards
Bradd
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bradd replied the topic: PNR/ETP use ETAS or GS?

G'day,

I sat the Performance exam last week and passed with 86%.

I was given a couple of different scenarios in different questions for calculating ETP or PNRs - one of the questions provided headwind or tailwind info, another question provided the forecast wind (degrees true) and magnetic variation.
For the one where W/V was provided, working out the ground speed on the back of my Dalton E6B did the trick to get my ground speeds out + home.
I think I messed up the 4 pointer question somehow as i only had 3 KDRs listed.. onto the ATPL subjects next!

Thanks again John for the background info re: Dalton wind triangle.

Bob, great stuff in the text book - there were really no surprises in the exam.
Cheers,
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bobtait replied the topic: PNR/ETP use ETAS or GS?

Of course the examiner must give you either a wind component along FPT or a wind velocity that can be used on a flight computer to obtain GS information. It would be pretty silly to ask you to provide your own...
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bradd replied the topic: PNR/ETP use ETAS or GS?

Thanks Bob, yes that's right..
For some reason I was a bit confused when i saw a previous question raised in the forums where only a Crosswind component was provided and it threw me off.

Thanks again for the quality content you provide - passed all my subjects first attempts with 80+ scored (except for HUF with a score of 70% - strange subject in parts)..
Cheers,
Bradd.
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