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rijobe
28th Aug 2011, 10:56
Was wondering if someone could help me with this. In a Bob Tait practice exams I keep getting a similar question which I keep getting wrong and I am getting very frustrated

as I recall the question gives the information of an echo aircraft with a weight of 2655kg (main tanks full) and a moment of 666 then askes what is the max fuel that can be added to the auxiliary tanks. The way I have approached it is to work out the CG which comes to 2508, and as that is within the forward limit range (2400-2560), and it is adding weight, I believe that it can only be solved by using the loading sheet/graph. I am sure I am doing this correctly by plotting the weight, and moment, then adding another point for a moment derived from a hypothetical weight (100kg) and aux tank arm, then joining two points and marking the weight where the line crosses the forward limit of the envelope. Am I missing something? or am I just getting the wrong answer due to inaccuracies in plotting of the graph?

john_tullamarine
28th Aug 2011, 11:14
Bit out of touch with the Echo. Perhaps you could post your graphics for our consideration ?

I trust that you are plotting on a weight x moment (IU) rather than weight x CG graph ?

Might help if you give a link to the current Echo manual data as well.

If you want impractical "accuracy" you could set it up as the solution to a couple of simultaneous equations .. but I suspect that is not required.

rijobe
28th Aug 2011, 12:30
here is a link to the PDF that has the Echo info. Page 23-24 of PDF has the weight and balance data and graph. Unable to post any graphics as no scanner. http://www.aft.com.au/exams/bobtait/perf_supp.pdf

I plotted a weight of 2655kg and moment index of 666. Then plotted another point by adding 100kg of aux fuel. this gave a point of 2755kg and an index moment of 694 (100kg x 2800mm arm divided by 10000 giving 28 then added to the 666). I seem to get a weight of 2905kg before if passes out of the envelope, so a total weight of 250kg of fuel. the max fuel for aux tanks is 218kg (80gl) so I put the answer down as 215kg which was I thought the closest

The Bob Tait book states adding weight in the forward limit range can only be done with the chart and not mathematically due to the limit moving as the weight changes

jas24zzk
29th Aug 2011, 00:36
Total fuel avail in the aux's is 86 gallons, which comes out at 234kg, still below the 250 you plotted as available.

Slippery_Pete
29th Aug 2011, 02:06
Haven't done Echo for over 10 years now, but I get 169kg using information from the PDF link that you provided.

What is the actual answer?

Trust CASA to assess our country's commercial pilots with a fake aircraft that doesn't exist. Pretty much epitomises CASA.

rijobe
29th Aug 2011, 03:42
where did you get the 86 gallon figure from Jas. pretty sure it is 80?

not sure of the actual answer Pete. The thing with the bob Tait practice exams is that they are like the CASA ones and dont tell you the correct answer. They just tell you got question x wrong and the syllabus reference relevant to it (syllabus item 2.8.6.3 calculating the maximum fuel load). Very frustrating as if you know the correct answer you can usually work out where you went wrong. Anyway I am prety sure there is no "trick" I missed, just not accurate enough in my plotting of points as I got 54kg variance from Pete

wishiwasupthere
29th Aug 2011, 04:05
Categories - Bob Tait's Aviation Theory School Forums (http://www.bobtait.com.au/forum)

Try here and ask the man himself!

jas24zzk
29th Aug 2011, 04:29
Its in the pdf you provided, in the fuel table.

80 Usuable
86 total incl unusable.

CPL Page 4


edit, then again, reading the data under it, unusable is in the BEW....like others been a few years since i stared at echo

john_tullamarine
29th Aug 2011, 04:58
That CG envelope is not very suitable for plotting and the result will be a bit average no matter how careful you might try to be. However, if that's all you have, just use a sharp pencil and take a little care.

Then plotted another point by adding 100kg of aux fuel

That's just asking for trouble.

This involves extrapolation which magnifies plotting errors - especially when you are using poor quality charts and trying to get an intersection of two lines of similar slope. Always plot beyond what you need so that you are interpolating - that way the effects of plotting errors are minimised. I would have plotted a point outside the envelope so that the point to be read off is between the two plotted positions. For example you could have used, say, full aux fuel to get the plotted point. Much better strategy and one I would have thought Bob would have recommended to his students.

The Bob Tait book states adding weight in the forward limit range can only be done with the chart and not mathematically due to the limit moving as the weight changes

For the upper forward limit, that's a practical and pragmatic approach although the errors are significant for lines of similar slope. The calculation approach involves simultaneous equations which, while not being at all difficult, is not really relevant to the pilot's work.

One trap there, and not generally understood, is that the upper forward CG line is a straight line on the CG envelope but a quadratic curve, not a straight line, on the moment envelope.

Total fuel avail in the aux's is 86 gallons

Quite so .. but not relevant to the calculation.

Usable is 80 USG. Unusable is included in the empty weight and (although some of it may be usable - ie burnable - in reality) is not factored into weight and balance or routine operational calculations.

Best go back and re-read up on CAO 100.7 and CAO 20.16.

where did you get the 86 gallon figure from Jas. pretty sure it is 80?

For the calculation, you're correct.

rijobe
29th Aug 2011, 07:06
that sounds like very good advice about interpolating, of course a shorter line will have far less angle accuracy than a longer one. will do that from now on - thanks John :)

might try and see if Bob will answer me if I ask on his forum for Bob specific question next time - thaks wish :)

I have to say though I am pulling my hair out on all of those dodgy blurry lined charts in the VFR day booklet, okay to get a fairly rough figure but not so good for the pin point accuracy they seem to ask for :ugh:

Slippery_Pete
29th Aug 2011, 07:22
John is right on two counts:

1. the CG envelope is not suitable - the graph is distorted, and too small
2. the extrapolation you did by adding 100kg is too small. You are joining two dots very close to each other, so the tiniest of errors will be greatly magnified.

If you want to do the graphical extrapolation more accurately, just add fuel up to the MTOW in the auxes (I know this is more than they can hold). At least then the two dots you are joining together are much further apart and will give a slighty more accurate answer.

I don't really see why Bob Tait doesn't give four multi-choice answers like CASA do. That's why you buy practice exams. He might think he's doing you a favour by making sure you understand what you are doing, but that why you buy text books. Practice exams should be exactly that... a practice at exactly what you are going to get.

In the CASA exams, you would simply pick an answer and try it... if it's right, then it's right! If not, then just work out whether you need a bigger or smaller answer and try again.

Mathematically, 169kg is the correct answer.

john_tullamarine
29th Aug 2011, 07:57
okay to get a fairly rough figure but not so good for the pin point accuracy they seem to ask for

I don't think that pinpoint accuracy is the target. Why don't you find someone with a scanner and post some of your exercises ? It may be that we can suss out something not quite suitable in your technique ?

the CG envelope is not suitable - the graph is distorted, and too small

Not so much distorted and small. The use of the OEM datum results in these dreadful envelopes. This is why most of the trimsheets you might see use a trim datum in, or near, to the envelope. The result is a much more useable, boxy envelope.

By my reckoning, about 169.5, call it 170 kg .. but this can't be obtained from the chart. If you prefer to be pedantic in conservatism, one might round down to 169 kg.

rijobe
30th Aug 2011, 07:47
thanks for the help guys. did my exam today and got 82%, not really happy with that result but a pass is a pass I guess. law next week, and as my brain hurts from thinking about all this stuff too hard, I will have to come back to it. :)