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CPL Performance - CASA Rounding Rules

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mindsneak created the topic: CPL Performance - CASA Rounding Rules

Hi Everyone,

I have been going through the text book via home study. I sat my first CPL exam (Performance) back in January but unfortunately only scored 61%. My KDR included balance and weight calculations and I think that I got them wrong partly because I was a little confused about rounding off etc so I just followed the book. However, I noticed last week on Facebook (see attached) that these rounding rules have now been completely clarified by CASA I assume?

I decided this morning that I would re-work Q. 5 from P.174 (please see attached PDF) according to the rounding rules posted to Facebook. The answer in the book says that the answer should be 87 kg for Q. 5 P. 174, but according to those rounding rules, my answer comes to 88 kg.

I am just hoping if someone could please clarify if my answer of 88 kg would actually be the correct answer if for example the exact same question was presented in the actual exam? Can I correctly assume that these rounding rules would now also apply to absolutely any other question presented in the exam as well (for e.g. % of MAC, PNR/ETP calculations etc) ?

The Dream - #RFDS
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bobtait replied the topic: CPL Performance - CASA Rounding Rules

I don't see how anyone could argue with that degree of accuracy. I'm sure even CASA would not expect a centre of gravity position calculated to a millionth of a centimeter. However, the weight being calculated is the minimum weight that must be added. So, I suppose you could argue that, if you require 88.2kg, then 88 kg would not be enough and you should call it 89 kg. I have been told that the exam usually tells you to round up or down. Also, if you check the errata for Performance on our web page, you will find a download showing ETP and PNR calculations.

(I don't know where you will find a dip stick calibrated in millionths of a kg).
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mindsneak replied the topic: CPL Performance - CASA Rounding Rules

Thank you Bob. I wasn't aware about the new errata download covering all of this. I will have a look at it so I can hopefully get this performance exam nailed once and for all on my next attempt. Anyways, thank you for all the help and clarification and for pointing me in the right direction.

The Dream - #RFDS
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bobtait replied the topic: CPL Performance - CASA Rounding Rules

All the best with your study. Let us know how you go.

Bob
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  • John.Heddles
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John.Heddles replied the topic: CPL Performance - CASA Rounding Rules

This sort of thread gives me great concern as an engineer and, in this case, a very experienced practising weights engineer - my WCA (AV1), as a sideline, was the first issued when the present system came into vogue mid-1976. When it comes to weighing, I've weighed well over 1000 aircraft from the smallest up to Boeings and similar.

One needs to understand, very clearly, that the accuracy of the empty weight data with which you start your sums … is NOT accurate to the nth degree. The data always includes a variety of errors which, on occasion, can be relatively significant. For instance -

(a) scales - the underlying scales requirements are in CAO 100.7 ( www.legislation.gov.au/Details/F2018C00475 ) at 4.2. Never mind that most of the older scales actually don't go close to meeting this standard. No scale is dead accurate - all have inherent errors in their design and construction. Furthermore, the calibrations are, at best, a bit dodgy and contrived and, if in use the calibration conditions aren't approximated reasonably, the reading one gets can be somewhere between fanciful and outright conjectural.

(b) distance measurements - weighing with platform scales (usual for lighties) involves measuring reaction arms and jig points to establish the datum plane. This ALWAYS involves approximations and errors. Things are a bit better for jackpad weighings as the jack points are established accurately by the OEM. However, there is no free lunch anywhere and, in this situation, jackpad weighings increase the error potential in the weighing numbers. So you still end up with only an approximate result ...

End result is that the stuff you start with for your calculations is accurate, at the absolute best for a lightie, probably to no more than 5kg and 5mm. If the weighing is done without lots and lots of care and attention, the errors easily can be considerably worse.

So what ?

What benefit/value/validity can there be in running your calculations to precisions of a squillion decimal places ? Utterly pointless and silly. My suggestion is that the calculations should be done to one decimal more precise than the purported accuracy desired in the answer. In this way you avoid needless roundoff error accumulation. If you are running the sums with an electronic calculator, by all means run the accumulator to whatever floating point significance the device might have - no point not doing so.

The examiner is between a rock and a hard place. He is charged with doing his best to establish (by means of part sampling of syllabus competence) that the candidate has learnt a few things and can do the usual sums. One of the ways he might go about this is to discriminate to a slightly higher precision than justified by engineering reality in his available answers - I have no problem at all with that approach. However, there is absolutely no value to be had in looking at precision to a million decimal places and, in reality, this leads to negative training and quite misconceived ideas at the candidate level.

I'm a bit up to my ears at the moment but I will run the problem the OP has raised when I get some spare time and come back with a more reasonable solution precision for your consideration.

As always, Bob approaches this stuff with a good, level head and his counsel should be listened to attentively.

Engineering specialist in aircraft performance and weight control.
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bobtait replied the topic: CPL Performance - CASA Rounding Rules

My concern, as always, is the student who is aiming for a CPL. Since the introduction of the 'type-in' answers in exams, intelligent motivated students are dismayed to find a KDR listing items such as P charts etc. The charts apply generous safety factors to allow for the fact that the wind for a take-off is often the pilot's assessment by observation of the wind sock (hardly a scientific instrument). The wind for a landing is often based on a TAF (somebody's educated guess). CASA wont disclose what margins are applied to their answer, but they seems to be pretty tight. How do we know whether their answers is 'perfect' to the meter or kilo anyway?

When it comes to fuel or weight calculations, what is the point of calculating minimum fuel to a number of decimal places when you poke a stick in the fuel tank to asses the fuel quantity that is the basis of the calculation! Calculate the ISA temperature to a fraction of a degree when the actual temperature is read from an analog scale on a thermometer, then multiply the ISA DEVIATION by 120 - which is an approximation!

I have largely rewritten the Performance book and included a calculation policy that has been submitted to CASA. They have indicated that they are happy with it.

I couldn't agree more with John. It breaks my heart to see a capable student denied a pass on the basis of a few decimal points. There are so many important safety items that we could be concentrating on.

Bob
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