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## Ex 5.13 question 3

• Posts: 70

### MissSoph created the topic: Ex 5.13 question 3

Hey again ....
I’m working through these questions... but want to know why in this particular question... we have kept the present c of g as a decimal?
In my calculations.. I ended up with 2674.5098...
So I just used 2675... but the answer has it 2674.5...
( looking at the printout given to me with my book... an important note about accuracy in calculations....)
So how come now, in this instance, we have to use the decimal...?

• John.Heddles
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• ATPL/consulting aero engineer
• Posts: 473

### John.Heddles replied the topic: Ex 5.13 question 3

why in this particular question... we have kept the present c of g as a decimal? In my calculations.. I ended up with 2674.5098... So I just used 2675... but the answer has it 2674.5...

Two quite separate situations to address here, I suggest.

(a) in the real world, the CG is most unlikely to be accurate to more than 5-10 mm as loaded and, in many cases, will be worse than that. There is no point in taking calculations to more than a precision compatible with this sort of accuracy. In practice, that means a precision of 1 mm is reasonable and acceptable, so your 2675 is fine.

(b) in the CASA exam world, the examiner has the unenviable task of trying to discriminate between candidates who deserve a tick and those who really need to do a bit more work. An unenviable task to which he is welcome. One of the techniques he can use is to run calculations to a little more precision than warranted by real world accuracy and I have no problem with that approach. For the exam situation, it is probably sensible for candidates to run answers to the first decimal place. In practice, use the floating point precision of your calculator through the calculation sequence and then round off to the first decimal point in the answer. While, in the ideal world, the exams would have some sense of practicality, that is not their purpose so we just have to accept the reality that, to some extent, they are a bit artificial. Bob/Stewart may have some further thoughts on this matter.

Engineering specialist in aircraft performance and weight control.

• Posts: 2143

### bobtait replied the topic: Ex 5.13 question 3

John's advice is sound. I have always found it a little silly to start making such calculations to a decimal place of a mm. But the name of the game is to pass the CASA exam, so the examiner sets the pace. Personally, I would rather see these questions as a multiple choice format rather than a 'fill in the box' answer. I can't see that it would be difficult to create a multi-choice question that still requires the desired degree of accuracy by the candidate.

After all, why calculate the CofG position to a fraction of a mm when you estimate the fuel load by poking a stick in the fuel tank and guessing how much fuel you have. How far do we take it? Do we check that the aircraft is exactly level before we dip the tank? Do we distribute the cargo so that the total weight acts at the exact geometric centre of the compartment floor?

Do we determine a take-off distance by drawing lines on a chart with exceeding great care after we have guessed the wind velocity by looking at a piece of cloth on a pole located 500m away?

I'm afraid there will always be a big gap between the real world and theory exams. But, it remains true that the examiner calls the tune when it comes to allocating marks. If decimal point accuracy is required to get your mark, you'd be silly if you don't go along with it.

Bob