Just trying to get it all straight in my head… I had a fwd limit question where it gave me the weight and moment and told me the main tanks are full:.. AUX are emptyand then asked me how much fuel can be added to the Aux tanks. so I just use the envelope chart to find where, after adding some to the AUX tanks, it crosses the envelope…
I don’t need to remove the fuel from the main to check the ZFW because if I’m at the fwd limit at gross weight, removing all the fuel from the main just means I’ll move back in the envelope? Unlike the aft limit with fuel, I have to check the ZFW as removing fuel from the mains means I’ll move back out of the envelope?

Maybe post the specific question which is causing you the grief ? There are some traps you need to watch out for - generalisations often can result in burnt fingers ...

Think I’m thinking too much about it now…. But Some
Other questions I’ve done I have to take it back to ZFW to check k to make sure that it’s in balance… then add weight… I’ll see if I can find an example… this I’m sure was an aft limit question though…

Thanks, Sophie. Ends up pretty straightforward.

(a) I wouldn't worry about the ZFW for this one. All you could do if the load is out at ZFW is note it. I don't think the question was at all worried about the ZFW. In practice, of course, you certainly would check all the limits.

(b) for the question, you can approach it in several ways -

(i) plot a suitable aft fuel delta on the envelope and try your best to figure out the answer where the lines cross. Note that the forward limit line is not a straight line but often is plotted as such. Because the slopes of the forward limit and the aft fuel lines are very similar, this is a really average approach but is what we would routinely do with this sort of loading system on the line.

(ii) because you are given several possible answers, it might be easier just to run those and see where you end up. (d) you can bin, straight away - why ? When you do this you find that none of the answers is correct. I presume that this is due to the question drafter's figuring the answer by plotting as in (i)

It might be useful to run (ii) for the exercise to see how you go. You will have a basic calculator in the exam so you can run the options (a) through (c) to see what you get. You will need to calculate both the loaded CG and the forward limit CG using the algebraic equation for the (approximated) straight line limit. Bob gives you a version of this equation in his book. When you do that you come up with the result that none of the answers is correct. A bit more playing and you get an answer of 169.5 kg which is closest to (c). I haven't bothered to plot the question - you should do this and see what you come up with - I suspect that will account for the delta.

I had a quick check in PPRuNe, as these sorts of questions arise from time to time. This one actually came up and the folk tossed it around a bit. In another thread, there is some useful explanation on this and that which may be of use. I don't suggest that you go overboard in the exams with this stuff. The examiner is only looking to see that you get an answer reasonably consistent with the options. Just make sure that you avoid careless mistakes as they are what kill a candidate's chances of a pass.

If you are interested, the PPRuNe thread links are

www.pprune.org/pacific-general-aviation-...e-echo-question.html

and

www.pprune.org/pacific-general-aviation-...cpl-performance.html

(iii) if you are really keen, you could solve the simultaneous equations (one linear, one - strictly - quadratic) and get the answer in the blink of an eye on the PC. Don't go there as it is brain strain for no real value to you.

Thanks John…. After reading your reply.. the clogs are going round, and I’m thinking… hope it’s right… that when I take it back to ZFW the question is asking me something like.. how much cargo can be loaded… of course then I need to check that ZFW is within the envelope… or something like how much ballast fuel is
Needed… ?

Example of what I’m on about

The question's intent is just to test you on the fuel consideration.

In the real world you would have run the ZFW calculation and got it right, then added main fuel and figured out what you could add to give you maximum fuel for the trip.

Don't lose too much sleep over it. But, of course, you are quite right.

One of the difficulties doing this on a bulletin board setup is that we don't have the advantage of running the question face to face on a sheet of paper where all the ins and outs becomes clearer much more quickly. Just a limitation we have to put up with, I guess.