ZFW Limited TOGW

[Haroon]

Hi

The trim sheets I've seen start with:

Box A = Performance Regulated TOGW

Box B = Max LDG Weight + Trip Fuel = Landing Limited TOGW

Box C = Max ZFW Limit + Takeoff Fuel = Zero Fuel or Fuel Limited TOGW

Max TOGW for the flight = Lowest of A, B or C

If you are above the box A limit, the structure limit is busted at takeoff.

If you are above the box B limit, the structure limit is busted at landing.

But if you are above the box C limit, I dont see any adverse effect, provided you are within box A and B limits.

TOW will be increased by payload or fuel.

Limit uptill which the payload can be increased is the Max ZFW limit.

Limit uptill which the fuel can be increased is box A or B (and of course tank capacity).

So I dont understand how does box C limit the Max TOGW?

Regards

[BOAC]

It is not technically a 'limit' although you simply cannot physically take off at a higher weight than MZFW plus loaded fuel. The whole matrix is designed to enable a calculation of the 'underload' available to dispatch depending on the limiting factor and only the Take-off and landing columns are relevant to limits. Box 'C' will not change with either trip fuel, LDG wt or RTOW, only with loaded fuel.

Remember that Wet weight is DOW+Fuel, so as you increase fuel so wet weight goes up, but max allowable payload will initially remain the same. Only when the 'ZFW limited weight' overtakes the other two weights as you pile on the fuel (or trip fuel reduces) do we actually hit a real limit, be it RTOW or LDG.

Its just the way it has been done for years! Clear as mud? If it is still confusing, just accept that Col C is not really 'limiting'.:). Take a few load-sheet blanks and run some different scenarios.

[Capt Claret]

But if you are above the box C limit, I dont see any adverse effect, provided you are within box A and B limits.

If you exceed the "Box C" TOGW, you've exceeded the MZFW, which is just as important a limitation as the MTOGW or MLGW.

[BOAC]

If you exceed the "Box C" TOGW, you've exceeded the MZFW, which is just as important a limitation as the MTOGW or MLGW. - no. Read post 1 again.

[Haroon]

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.

[BOAC]

You are still 'seeing' the Col C figure as a limit - it is not. It merely enables you to see the available extra payload when the figure is less than A and B. You CANNOT 'bust' Col C at any time - you CAN, of course, bust MZFW. Col C consists of an unchanging MZFW figure PLUS fuel loaded so cannot ever be 'busted' and payload does not enter the equation. Payload only appears when you have selected the 'lowest of the three'. Adding payload will not change MZFW! Adding fuel merely increases the figure. Similar to Cols A and B in that respect - they cannot be 'busted' since they are both based on unchanging Max values (apart from Tanks and Trip fuel). Look at the whole matrix, as I said, as a Dispatcher's/Load Agents piece. Both their and your, the pilot's job, is to monitor and ensure that no actual 'limits' are exceeded, while ensuring enough fuel and maxxing the payload when necessary..

[9.G]

Haroon, BOAC is right there's no such limitation as per C. Jeppesen flight plan format contains this calculation to determine the allowable load. Many guys mistake it for a limitation and actually believe that no more fuel can be uplifted. You can add more fuel disregarding the MTOW based on MZFW provided you don't bust the usual RTOW, structural and MLW. It's a nice to know figure nothing more. :ok:

[Capt Claret]

Oops. Shouldn't post "tired & emotional" late at night! :ouch::\

[Haroon]

Hi, Thanks for your feedback and time.

When I see the trim sheets and even on flight plans the term "ZFW LMT TOGW" I wonder why it is called a limit?

I dont know if this term "ZFW LMT TOGW" is used universally or only in certain parts of the world, but getting a hint from what BOAC said:

Its just the way it has been done for years!

I assume that this term is probably used in many parts of the world.

If its not a limit in any sense then why call it a limit? I guess an expert on this subject knows it better.

I am clear on what its used for and have no issues with the trim calculations.

However I am just trying to find some explanation that might explain why it is called a limit.

The closest I could get was the following scenario:

Data
-----

Max TOW = 340
Max LDG = 250
Max ZFW = 237

Trip Fuel = 20 tons
Takeoff Fuel = 30 tons

Dry Opearating Weight = 200
Opearating Weight = 230


Calculations
-------------

Box A = 340

Box B = 270 (250+20)

Box C = 267 (237+30)

According to the terminology used in the trim sheets and flight plan (as discussed above):

Max TOW Limit = 267 (Box C)

Allowed Traffic Load = Box C (267) - Operating Weight (230) = 37 tons

Assuming an actual tracffic load of 30 tons (i.e. an underload of 7 tons)

The TOW = Operating Weight (230) + Traffic Load (30) = 260 tons

So:

TOW Box C Limit = 267

TOW = 260

If we want to increase the traffic load we can increase by up to 7 tons, considering Box C as a limit.

If we increase the traffic load by 8 tons then our TOW will be 268.

This means we have crossed our Box C Limit of 267.

This in turn means that we have crossed our Max ZFW Limit of 237 by 1 ton. (i.e. DOW (200) + Traffic Load (38) = 238)


However this is just my assumption that may be because of this it is called "ZFW LMT TOGW".

Regards :)

[9.G]

Fairly complicated way to calculate the ACL, methinks. You'll get the same result if you do MZFW (237) -DOW (200)= 37 t. That's all you get as a payload. Using your example let's say you've got 37 t actual payload. The question here is can you uplift more fuel and so how much?

[BOAC]

I reckon the 'expert' who introduced this term has long since 'left the stand'. As I said in post #2, FORGET this word 'limit'. Let's look at your figures. Firstly let's increase our payload by 8T. What is the Col C figure now? What is the 'controlling' ie lowest perf figure? There is no clue yet that we have exceeded our MZFW from this column. The figure does NOT change with payload increase. The 'shock' discovery that you have exceeded MZFW will come from elsewhere as 9.G says, and when you choose Col C as 'the lowest' and then add the actual figures below it the 'secret' will become clear in that matrix. Now let's INCREASE our tanks fuel to 38T. What is the Col C figure? What is now the 'controlling' ie lowest perf figure (ie now actually 'limiting').

Much easier to ignore the word 'Limit'.http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/smile.gif

[Haroon]

Hi, thanks for your prompt feedback.

You'll get the same result if you do MZFW (237) -DOW (200)= 37 t. That's all you get as a payload

But if you've got a large amount of fuel then it is possible that your upper limit will be less than the MZFW limit of (237). Thats why Box C helps in determining the allowed traffic load.

Using your example let's say you've got 37 t actual payload. The question here is can you uplift more fuel and so how much?

Yes you can uplift 3 more tons of fuel. In this case your ZFW will be 237 and your upper limit will be Box B of 270. Therefore max fuel can be 33 tons.

However fuel is not an issue as I already mentioned that Box C will change and go up with increase in fuel.

Firstly let's increase our payload by 8T. What is the Col C figure now? What is the 'controlling' ie lowest perf figure? There is no clue yet that we have exceeded our MZFW from this column.

There is a clue. As I mentioned earlier, when you increase the payload by 8T your TOW will become 268, exceeding the Box C by 1 ton. This is the clue that you have exceeded the Max ZFW Limit.

The figure does NOT change with payload increase

Thats what helps in getting the clue

Once you get the clue then you can verify it by claculating in any way and find that MZFW limit has been exceeded. Only this point makes me assume that may be this was the reason that the expert who left the stand introduced the term "limit".

Much easier to ignore the word 'Limit'

on my end, no problem :)

[BOAC]

I'm glad the 'amnesia' is getting progressively easier:)

[Haroon]

Hi, thanks for your feedback

How can you be below limits A and B if you are above C limit ? :rolleyes:

You can if you increase your payload. Considering the example in the last post. As suggested by BOAC "let's increase our payload by 8T".

The TOW in this case becomes 268. It is above Box C (267) and below A (340) and B (270).

However my statement

I dont see any adverse effect

was wrong as I could'nt visualize the significance at that time, which was probably the reason I started off this post, till BOAC suggested to run some scenarios in blank load sheets.

Regards :)

Note: Just in case someone is wondering what I am replying to, the above quote was from a post which was removed after I posted the answer, that's why you cant see the post

[BOAC]

We need to think about when the first bit of this matrix is completed - normally well before any actual load figures arrive with you. Often it is on the outbound leg where a decision has been made about return fuel load and RTOW calculated. Should you then decide on turn-round that you don't like the weather at base and wish to add another 3T to the fuel, you have effectively raised Col C by 3T. The matter of whether ZFW exceeds MZFW is normally determined without reference to this part of the loadsheet. When you look at a DCS generated loadsheet (the ones I have used, anyway), this 'Col C' figure does not appear at all.In addition, none of the 'manual' loadsheets I have ever used show TOW in the matrix so, as we have said, the primary purpose of it is for looking at 'underload' (and, effectively, planned underload). I admit, however, if I had a negative underload I would certainly have a good look at it!!

I apologise to Capt Claret for the 'castigation' I rendered - he was actually quite right in reality. It is just that it is an unusual way to determine MZFW exceedence and since the Col C figure goes up 1T for every extra tonne of fuel up to max tanks a little impractical and confusing as Haroon has found.:)

In practice having established our limiting TOW via Cols A and B we work from the left column where actual weights are totted up and check a/c indices, limitations, load, weight and fuel figures are correct with no exceedences.