G'day Bob,

Just had a question regarding forward limit accuracy for the cpl performance exam. I'm referring to question 6 pg 184 (there are a few others I’ve noticed)
ZFW 2400kg moment index 590 find the greatest fuel that may be added to the main tanks only.
I did the same thing as you explain in your answer, add 100kg of fuel to the main tanks so we now have a weight of 2500 and a moment index of 607.8. Then plot it on the load sheet. I got roughly the same answer of 90kg of fuel can be added.

However, in your answer you also say you can double check your answer by adding 90kg of fuel to the original weight and moment. So we now have 2490kg an arm of 2433.8 and a moment of 606. Calculate the forward limit (2490-2360)x0.27+2400 = 2435.1
Therefore if we add 90kg of fuel we are outside the forward limit. I did some further calculations and found it to be 87kg of fuel that can be added.

Now I know this is a bit nit picky, but I know from experience in previous CASA exams that this can sometimes be the thing that catches you out. So what I'm asking is, how accurate do I need to be? If you place the same question in a multiple choice answer for example. A. 100kg B. 90kg C. 80kg D. 70kg
Would the correct answer be C? Because if we add 90kg ( B ) of fuel its outside the limit? Or if its an enter in style question do I round up to 90 or try to find the most accurate 87?

Thanks for your help Bob!

First, I don't have the original document so I am going on your description only.

The secret to your dilemma lies in your observation "I got roughly the same answer of 90kg of fuel can be added".. Running the calculation graphically is fine in practice but the accuracy and precision are, necessarily, a tad rough. We address that by plotting/reading conservatively to ensure that the answer does, in fact, put you inside the envelope.

If, on the other hand, you run the sums, you will get a more precise answer than that obtained graphically .. that's the nature of arithmetic. Precise but, in practice, no more accurate as the starting empty weight values are not to that sort of accuracy in any case.

So, in effect, what you are comparing, is something akin to measuring a dimension with a school rule, on the one hand, and with a micrometer, on the other. The two approaches just won't/can't give you the same sort of answer precision albeit that the functional accuracy may well be similar, depending on the exercise details.

Checking the graphical answer via the calculation is fine and beaut but (assuming you check the working and you are comfortable that you have done the deed correctly) you should use that check to refine the value read off the graph.

We can do the graphical exercise to the same precision as the calculation check, but that involves solving simultaneous equations (one linear, one quadratic) and that is just not what the examiner wants to see. So, the best you can do is one of the following

(a) run the graphical solution and then fudge it a bit using the calculation .. all the while making sure that you finally end up inside the envelope

(b) start as (a) but then run the calculation iteratively to come up with a similar result.

Would the correct answer be C? Because if we add 90kg ( B ) of fuel its outside the limit?

If 90kg puts you outside the envelope, while 80kg puts you inside, the choice is a programmed decision ...

Rounding is ALWAYS done in the conservative direction.

John's points are all valid. We had this out with CASA some time ago and the examiners pointed out that the wording of the question was-

'the maximum weight that may now be added to the main tanks is closest to'.

With that wording, you would be justified in choosing an answer that is technically just outside the limit providing it was closest to your answer.

Now that's all very well, but lately, the performance examination has featured questions that provide a text field on the computer screen for you to enter a numerical answer rather than a multi choice. That raises the question of what margin of error is permitted. I have not been given an answer to that. All I can say is be as accurate as you can.

It's a different story in practice of course because there are margins built into the charts to allow for the limitations of plotting graphical solutions. The trouble is this is a theory exam, so accuracy takes on more significance.

Thank you both for your replies!

I guess its all down to CASA's interpretation, 'closest to' or 'maximum weight'. After sitting my performance exam this morning, it seems they have taken this in to consideration with the questions and answers. For example, one question I had required me to add the minimum number of 10kg packages to the rear compartment to bring the aircraft inside the forward limit. This was an 'enter the answer' style question. I answered the question in a similar way to the way you recommend in your book. Add 100kg to the rear and plot the two points. The answer was just inside the forward limit (about 2mm inside) so if you added an extra pack you would be well inside the forward limit, or if you added too few packs you would be well outside the forward limit.

There was a similar question for the rear limit, however, this was a multiple choice style question. The answers were either well outside, well inside, or the correct answer just inside by a similar margin (2mm or so).

One other question I had may be of some interest to you, I'm not sure if you mention it in your book. I was asked what I would circle on the flight notification form if I was carrying specific equipment on my flight. AIP ENR 1.10 - 13. I was fortunate enough to have had experience with this form from my flight training. This was a pretty simple question, However, I can only assume they could ask about more difficult sections of the form. Flight rules, types of flight, both easy enough to remember. Try to remember what each letter refers to in the equipment and capabilities section without reference to the AIP.

Thanks again for your help!

Thanks for your reply. I am aware that the recent Performance exams have included questions on the flight notification form. I have included a couple of extra pages advising students to revise the AIP ENR 1.10 section on submitting the flight plan form. You didn't say if you had passed the exam.

Bob

Definitely worth having a read over that section of the AIP, an easy mark if you do! For the first few of my cross country flights I filled out the flight notification form, then used it as a reference to complete the online form. A worthwhile exercise for anyone yet to complete the cpl performance exam.

Sorry, I did pass the exam, I am very happy with the result 90%. It's very satisfying to see the long hours of study pay off in the end! Definitely glad I've got your books to help me along through the CPL exams!

Moving on to Navigation now. Ive got the exam booked for the 9th, hopefully I can get through it! I didn't leave myself a lot of time for this one! I'm just wondering if I need to worry about the new weather forecasts coming out? Do you know if I will be required to extract wind from GPWT charts?

Cheers

The advice from CASA is that the new system will introduced on the 9th , so the exams should reflect that. But I don't see that having much effect on the Navigation exam. I think it's more likely that any question on Heading/Ground Speed calculations will simply give a wind in the text of the question. We have a download from BOM on our home page. Congratulations on an excellent mark - especially for Performance by home study.

Hi Bob,

I decided to study your Navigation book first then have a look over the new weather charts. I didn't have to extract wind from the GPWT charts, however, it is a good idea to know how to do so. The only question I had relating to the new weather charts was one asking what forecasts I would require if flying from A-B. Pretty simple question to answer, find both aerodromes on the AUS PCA and draw a line, You will require all forecasts relating to that flight.

I also had an interesting question on first light. I was asked to find first light in UTC for a particular location in Australia. A - 0900 B - 0700 C - 0800 D - 0930. Before getting too carried away with first light charts in the AIP. None of these answers could be correct. The location is UTC +0930, the answer would have to be in the vicinity of 2000-2300 UTC. The question definitely asked for "first light in UTC" I read it about 10 times to make sure. I calculated last light for the same question and ended up getting the correct answer. This made me think, if you get a question like this in an exam, and for whatever reason do not get it right, you could miss out on 2 marks. What should you do next? You are not allowed to take anything out of the exam room. Should you email CASA? If you get the answer right but the question was "wrong" should you email CASA to inform them of the question? Do you think they will take the mark away from you or make you re-sit? I'm happy sitting quiet, purely because I don't want to re-sit or have the mark taken away especially with a perfect mark for this exam. Just wondering what your thoughts are on this Bob?

Cheers,

Adam

Hi Adam,

If the question asked for first light in UTC, I would agree that none of those answers could be correct. If you assume Eastern Australia, 0700Z is 5pm and 0930Z is 7.30pm. No way that could be first light anywhere in Australia. All you can do is email CASA and explain it to them just as you have explained it to me. As long as you are not mistaken about the question specifying first light and not last light, there is no way it could be anything but an error. I would hope that CASA would welcome your feed-back. I think you might have to pay a fee though.

Hi Bob,

I'm wondering if you have any updates or advice regarding this. In my third performance attempt all four of the echo max weight possible questions I had to enter whole numbers. I got all four wrong, and I seem to be failing this exam simply due to accuracy on my charts and then further not knowing what margins are expected of me in these whole number type questions. This is the one thing catching me out.

I believe I grasp the concepts and formulas well enough. It's the level of ambiguity in the questions that leaves me to pick the wrong the answer. Any tips for how I can interpret answers that leave me on the borderline of exceeding or being within a limit; especially 'closest to' the questions. thank you