Hi Bob,

I just purchased your online CPL Perf exams and sat the 1st one.

I got the question below incorrect and I would like you to revise the answer as according to mathematics you are in fact 1.27mm out of the forward limit if you use 180kg. so therefor 100kg has to be correct which places you 12.3mm inside the limit. a few members of my class also received my answer so some clarification would be great.

Thanks in Advanced!
Zac

Refer to page 18 of the work booklet.

An Echo aeroplane is loaded with the main tanks full and the auxiliary tanks empty.

If the gross weight and moment index are 2655 kg and 666 index units respectively, the maximum fuel that may now be added to the auxiliary tanks is -
Select one:
a) 215 kg
b) 180 kg
c) 295 kg
d) 100 kg

As I don't have the original documents for reference, better if Bob responds to the specific question.

However, as a sideline note -

You refer to CGs calculated to be 1.27mm outside and 12.3mm inside a limit. Please, I suggest, don't use precision in numbers way beyond realistic accuracy. In practice, you are limited by the accuracy of the starting empty weight data, actual weight errors in the associated calculations and, similarly, errors in the measured or assumed loading arms .. and this presumes that your calculations don't contain any further arithmetic errors.

In practice, you MIGHT get an answer accurate to, say, 5-10 kg and 5-10 mm if you are VERY careful and the original aircraft data have been provided with GREAT care. Often, calculated answers are somewhat worse than that. For the exams, fine, work, say, to the nearer kilo and mm but be aware that that is only an exercise and does not reflect reality. Quoting CGs with a precision to the thickness of a cigarette paper .. no.

I Understand where your coming from and In real world scenario 180kg would be the go, as yes, the echo is in fact a completely unrealistic aircraft, but in a CASA exam 1.27 may be the difference between a pass and fail.

So assuming you are basically saying don't be so pedantic over something so minuscule, I feel that in an exam sometimes it helps to be so precise. Thus, why I am asking for clarification on the actual C.G as even if it is 0.001 mm out, thats still exceeding a limit as unrealistic as it may be, therefor rendering an answer incorrect.

But yes I agree with you on the front that "quoting a CGs with a precision to the thickness of a cigarette paper... no" is in a working environment, in fact, near folly.

Cheers.

the echo is in fact a completely unrealistic aircraft

Not at all unrealistic .. why do you assert so ?

but in a CASA exam 1.27 may be the difference between a pass and fail.

I doubt that would be the case. What do you say on this, Bob ?

don't be so pedantic

Not what I am saying at all. Pendantry relates to something else altogether. ANY time you are using numbers, you MUST consider precision and accuracy .. the two go together and cannot be separated.

I feel that in an exam sometimes it helps to be so precise

Then, why didn't you give your number precise to 5 decimals ? .. or 10 decimals ? .. or, perhaps, 295 decimals ? Where do you propose to draw the line ? Ah, I see, at a point where the cited precision sensibly relates to the achievable accuracy, perhaps ?

even if it is 0.001 mm out, thats still exceeding a limit

'Fraid not. Suggest you have a delve into the regs and ACs and get a feel for what accuracy (and, by association, precision) goes into the Design Standards and POH. For the exams, suggest you run the calcs to the next decimal (ie 0.1 mm) but quote answers and the like to the nearer mm.

You can justify quoting US stuff to the nearer 0.1 inch (2-3 mm) but going more precise than the nearer mm is a tad silly and would get you laughed out of a certification discussion.

If we have any legal experts on the forum, perhaps they can opine what the judge might rule, but that is a different consideration altogether.

John,

you make some valid points, and I see where your going.

thanks for the input, will wait for Bob as he wrote the question.

Ta.

Actually from the feed-back I've had the wording of the question is often "The maximum weight of fuel that can now be placed in the auxiliary tanks is closest to -

In that case, you would pick the answer that is closest to your answer. However, recently some of the exam questions are not multi choice, but present you with a computer field for you to type in your answer. In that case, you would have to hope that the examiner would have the good sense to allow a reasonable margin for full marks.

I agree that it defeats the purpose of pilot education if we are expected to get involved in such nonsense. After all, plotting the centre of gravity position on the envelope is approved by CASA as a method of establishing whether or not the aeroplane is safe to fly. Even the distribution of weight on a given compartment floor will affect the moment generated by that weight. (I have sometimes amused myself in a little Citabria by making the nose pitch up and down just by leaning backwards and forwards in the seat!)

To save any heart ache, I'll amend the correct answer to 170kg. Thanks for your feed-back, it is appreciated.
(I'm still looking for a dip-stick that is calibrated to tenths of litres).

Thanks for the Clarification Bob.

Much appreciated and enjoying your exams.

Cheers

I'm still looking for a dip-stick that is calibrated to tenths of litres

Please do permit me to tell a related tale at my own expense and confusion .... but which carries a metaphorical message for all the new-to-the-system pilots working up to their various licences.

Many, many years ago .. as a young chap, I purchased an ex-CASA Cox and Stevens jackpad weighing kit. Now this kit contained a very nice long tape measure ... which I didn't look at all that closely at the time.

Not long after, I used this kit to weigh an aircraft .. can't recall what aircraft but it would have been something of DC3 size or bigger. For whatever reason ... now lost in the depths of memory ... I had to take some length measurements and, as you guessed, I used my brand new, second hand tape measure from the kit.

All fine until I got back to the office and ran the sums .. there was an error somewhere as the answers weren't doing the right things ... but, try as I could .. checked and re-checked the sums .. just couldn't figure it out.

Later that afternoon, over coffee, I was having a play with the tape when a puzzled look came upon my visage ... this rotten tape was no good .. it only went to 10 inches in a foot .. and then 10 twelfths in an inch. I had been ripped off.

Have you guessed the problem ? Indeed, after some head scratching, I pulled out another tape and laid them side by side. The kit tape was scaled in feet, tenths of a foot, and hundredths of a foot.

Message ? "Assume" is often interpreted as a process to make an "ass" of "u" and "me". Keep it in mind .. it might just save your life one day when you are out flying. Have a read of the Gimli glider report ... good example in which the pilots, having made their share of the mistakes, nonetheless managed to save the day ...