I created a mini trim sheet in microsoft excell and after playing around with some numbers came up with the following conclusion:
Box A and B are purely technical structural limits and the significance of exceeding these limits are well known.
Including Box C among A and B helps in calculating the allowed traffic load.
Significance of Box C Limit is related to the increase in TOW.
TOW will increase by either uplifitng more fuel or more traffic load.
1) Increase in TOW with more fuel:
If TOW increases with more fuel then the box C limit increases to a higher value and you are limited by box A or B (no more box C limited).
Therefore with uplifitng more fuel you cannot bust this limit as it will go up and up.
However too much uplifting of fuel can end up in leaving behind some payload. That means loosing revenue.
So in this sense box C is a takeoff limit but an operational/revenue related one and not a technical one.
That's why while uplifting last minute fuel we take Actual ZFW (as the bottom limit) and the lower of Box A or B (as the upper limit).
2) Increase in TOW with more traffic load:
If TOW increases with more traffic load then the box C limit will not change.
Therefore, with an increase of traffic load above the allowed limit, you can bust the box C limit as it does not change (wont go up like it did in the former case).
If it is busted with an increase in traffic load then the Max ZFW limit will be busted.
e.g. If allowed traffic load is 50 and actual traffic load is 40 then the max traffic load that can be increased is 10 in order to avoid busting the box C limit.
If actual traffic load is increased by 15 then box C limit will be busted and the Max ZFW limit will be exceeded by 5.
In this sense box C becomes a technical limit.
Last edited by Haroon; 20th October 2012 at 17:16.