There is only one time when you cannot use the flow chart. That is when you are ADDING OR SUBTRACTING weight to reach the FORWARD LIMIT.

Since the forward limit itself varies with changing weight, you don't know how much weight to add until you know the forward limit and you don't know what the forward limit will be until you know how much weight you are going to add.

Catch 22. You must use the graph to solve these problems.

However, if you are SHIFTING WEIGHT, you can ALWAYS use the flow chart for either aft or forward limit problems. You can also always use the flow chart for ALL problems involving the aft limit.

It is only when you are changing weight to hit the forward limit when you must use the centre of gravity graph.

ohhhh okay, great!
Thank you so much! I thought that is what I had read in the book but then I wasn't 100% sure because I used the flow chart and my answer wasn't far off the answer in the book!
This is my lucky last exam and Im the most nervous for it! But I whole heartedly trust your books! They have served my very well for the previous six!

Kind Regards,
Esha

I have gone back and had a look at the question (Question 11 'final test' in the text book) which confused me, now I am just stuck on how you know when its a forward limit problem. Do I use the Centre of Gravity equation and check if it is beyond 2400mm ie 2442mm? and if it is, then I know the problem involves the forward limit?

Thank you!
Regards,
Esha

Hi guys, I'm having the same trouble working out when to use the graph or flow chart. Bob said to use the graph to see if it's a forward limit problem or an aft problem. Would it be correct in saying anything after 2680mm is an aft problem and anything before 2680mm is a forward limit problem.

Any help is much appreciated as it's my final exam and would love a pass on the first attempt 🙂

OK, of course the first thing you need to know is whether the problem is going to be at the forward or aft limit. The best way to do that is to look at the starting combination of weight an moment - that has to be given in the question. Then if you do a rough check on the envelope (even if it just with your finger tip) you will see where you are to start with. You may not be going to use the envelope to solve the problem, you are just getting a rough idea of where you are to start with.

If you're just inside the forward limit and you are going to add fuel to the main tanks or cargo to the nose locker you are going to have a forward limit problem. If you are just outside the forward limit and are asked to find the max cargo or ballast to add, once again you will be working to the forward limit. In that case, you can only use the flow chart if you are shifting weight.

Also, if you look at the arms of the compartments, you will see that there are only three places that are ahead of the forward limit - the nose locker (500mm), row one (2290mm) and the main fuel tanks (1780 mm). Since the most forward position of the forward limit is 2400mm, you don't need any mathematics to see that those compartments will always be ahead of the forward limit. Adding weight to those positions will always move the centre of gravity forward and subtracting weight from those positions will always move it aft.

Every other position where you can put weight is behind the aft limit. row 2 (3300mm), row 3 (4300mm), rear compartment (5000mm) and the auxiliary fuel tanks (3550mm) are all well behind the aft limit of 2680mm. Adding weight to those positions will always move the centre of gravity aft and subtracting weight from those positions will always move it forward.

You can breathe a sigh of relief if the problem turns out to be an aft limit problem, because in that case, you can always use the flow chart.

Hi Bob, thanks so much for the reply, I have just noticed that question 10 on page 236 the moment index is 2620mm and the weight is 650. Now the question ask how much more fuel we can add to the aux tanks, being behind the aft limit I thought we could use the flow chart rather than the graph but it seems that my understanding is wrong yet again. Excuse my ignorance but when you plot 2620mm and 650kg on the graph how do I know it's a forward or aft limit. Am i correct in saying if it's before the envelope line it's a forward limit and if its behind the envelope line it's an aft limit problem. After reading your previous comment I'm still a little confused how to tell on the graph if it's a forward or aft limit problem.

Thanks so much for taking the time out to to always provide us with help. We all appreciate it

Could you maybe draw an example on the envelope.

Thanks bob 🙂

[attachment=980]CofG movement.png[/attachment]

Thank you so much for your help, that graph has made things so much more clear,

thank you so much!

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.

[color=blue][i][b]An echo is loaded at ZFW with a total weight of 2400kg and at a total moment index off 590mm.[/b][/i][/color]

Please be [color=red][b]very[/b][/color] 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.

[color=blue][i][b]I thought you use the graph when we are outside of the forward limit.[/b][/i][/color]

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 [b]outside[/b] the limit, you pick a convenient weight delta (change) which puts you [b]inside[/b] 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 [b]inside[/b] the limit to start with, you pick a convenient weight delta to put you [b]outside[/b] 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.

[color=blue][i][b]When i plot 2400kg and 590mm on the graph we are inside the limit[/b][/i][/color]

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,