Thanks so much Bob, have a way better understanding of when to use the graph and when to use the flow chart now. just wanted to also ask about a question. On page 184 question 6 ask. An echo is loaded at ZFW with a total weight of 2400kg and at a total moment index off 590mm. Find the greatest amount of fuel that may now be loaded to the main tanks only. Now I'm aware because we are adding weight you need to use the graph, but i thought you use the graph when we are outside of the forward limit. When i plot 2400kg and 590mm on the graph we are inside the limit. do we still have to use the graph although we are in the envelope due to the fact that the main are before the COG and putting weight in the mains will push the COG further forward

Any help is always much appreciated.

An echo is loaded at ZFW with a total weight of 2400kg and at a total moment index off 590mm.

Please be very wary of confusing units lest you trip yourself up. The index of 590 relates to moment units of kg-mm, not CG units of mm. To determine the relevant CG in mm you would need to run the sum 590 x 10,000 / 2400 = 2458 mm. I understand that you are just being a little casual with the units here .. but, be aware that housekeeping is what trips up most folks with weight and balance stuff and a simple mistake is the last thing you need to contend with in the exam.

I thought you use the graph when we are outside of the forward limit.

As Bob, no doubt, has told you, you can use the graph for any calculations. Similarly, you can do any of the calculations analytically by using appropriate sums. However, for the upper forward limit, the sums involve the solution of simultaneous equations (one quadratic, the other linear) which is both boring and way outside what pilot folk need to be able to do. For the other limit regions, the sum gives a slightly more "precise" answer than does the graphical solution. I put precise in quotes as, in the real world, the starting empty data's accuracy doesn't warrant running calculations to such precision .. however, if that keeps the examiner happy, then it's the way to go for the exam.

For the upper forward limit, the graphical solution allows you to overlay a moment change line and the (varying) limit line. Whether you are outside or inside the envelope to start with makes no difference other than to technique. So, if you are outside the limit, you pick a convenient weight delta (change) which puts you inside the limit (this requires that you are loading into a spot up the back somewheres) and read off the weight delta which just gets you to the line. If you are inside the limit to start with, you pick a convenient weight delta to put you outside the limit (this requires that you are loading into a spot up the front somewheres) and read off the weight delta which just gets you to the line.

When i plot 2400kg and 590mm on the graph we are inside the limit

In this case, you are a bit inside the envelope and loading fuel into a tank which is going to pull your CG forward (that means that the slope of the fuel added line will be a bit steeper than the limit line). Therefore, plot the starting point (as you have done), pick a suitable weight delta to end up outside the limit (say, 250 kg .. always try to pick a big delta to minimise errors associated with plotting and drawing lines). This will put you outside the limit and you simply run back down the line to the intersection with the limit and figure the weight delta for that position.

Again, you can do it by sums .... but you really don't want to go down that path, I suggest.

So, what answer do you come up with, now ?

I have stumbled upon a question which asks for me to find how much fuel can be added to the Aux tanks, now when i plot it, it comes up on the aft end of the graph, the gross weight is 2550kg and the moment is 682...i have tried adding fuel, using the graph but now im just really stuck!

Because of the envelope datum position used for the Echo (up towards the front of the aircraft), the graph slopes back and is a tad rough both in precision and accuracy. You certainly can do the exercise on the graph, but you will get a pretty average sort of result.

How, then, to do it ?

The problem is just a simple ballast question, in this case using fuel as the ballast medium. Does that help a bit ?

Okay great, so first find the centre of gravity, then use the aux tanks as the arm to which I'm adding weight?

Thank you so much,

You've got the story in a nutshell.

Good Afternoon,
if is a forward limit question, can i use the forward limit formula and then use that in the flow chart,
for example if I have a weight of 2550kg and Index units of 530, and cargo has to be moved from the front to the back, can I use the forward limit formula to find the CoG limit, and then use the flow chart to find the weight to be shifted?

kind regards,
Esha

if I have a weight of 2550kg and Index units of 530, and cargo has to be moved from the front to the back, can I use the forward limit formula to find the CoG limit, and then use the flow chart to find the weight to be shifted?

As I don't have a copy of Bob's text, I don't know the details of the flow chart process. I'm sure someone will tie up the following to the text process for you in due course.

However, there exist several ways to tackle the problem (one of which, no doubt, is described in the flow chart process).

(a) in line operations, you would read off the forward limit in IU from the chart. More than accurate enough for purpose if you then move just a little more weight than the minimum absolutely required to ensure you are back to the limit line.

This then gives the moment (or IU) change required to bring the CG back into the envelope. It is then a simple exercise to move x kg from up front to down back to obtain the required moment change.

Caveat - always keep an eye on the aircraft load limits to ensure that you don't try to take more weight than exists from a compartment or put more weight than the maximum permissible in another. Often you will need to run the exercise multiple times to get a solution if the moment change required is large.

(b) if you want a bit more precision (as distinct from accuracy) you can run the calculation via CG figures as you have suggested. Once you have the two CG values, again it is a simple exercise to figure the weight to be moved to achieve the CG change

(c) although I don't recommend it, as an alternative to (b), you can use the same sort of approach with the IU chart. The equation for the forward upper limit is a simple quadratic and, once you have the two moment (IU) values, you proceed to figure the weight movement to achieve the requirement as in (a).