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Maximum zero fuel weight & %MAC

Why do we have a Maximum Zero Fuel weight in our aeroplane POH (AC11)? And what happens if this weight is exceeded but maximum weight is not?

i.e. as an example if 3x 95Kg pax and full fuel was at max weight, and then you added one Pax and took out 125l of fuel to still be at max weight, but then the fuel loading is less so I presume you exceed the MZFW.

I've been trying to get my head around this for a while and can only presume that it prevents more than a certain weight from being carried in the cabin areas?

Secondly we have tables of CofG listed in %MAC. Does anyone have a link to an article which describes how these CG calcuations can be performed using %MAC rather than Kg/cm (lb/in) etc...

Thanks.

EGHS

My understanding is ; the more weight in the cabin and the less weight in the wings, then the more stress on the wing spar and wing roots. MZFW is a simple method of calaculating maximum permissible weight in the cabin. It could of course be simply stated as a figure in the POH, but then every aircraft has a different equipment fit and a different empty weight.

Egnm, United Kingdom

Yea makes sense I suppose. Anyway it is not really an issue because with 3 big blokes in the aeroplane, filled to tabs, we're still below MZFW. I suppose though that you could be above MZFW on short hops with lots of fuel onboard....Need to get my head around this! Cheers

EGHS

Flybymike is right. This is mainly an issue for planes with wet wings. It is the structural limit for the wing/fuselage joint. Adding fuel doesn't change much as it close to the center of lift.

MAC (mean aerodynamic chord) has been introduced to compare wings with complex shapes. Not all wings are like hershey-bars on PA28. MAC is the chord of a rectangular wing with the same surface, same center of pressure (lift) at a given AOA. %MAC is used to define CG in Airbusses and Boeings.

EGBE - Coventry, United Kingdom

It's the same rationale as having a maximum weight of non lifting surfaces in a glider (which will often carry water ballast in the wings)

It's supposed to be fun.
LFDW

My understanding is ; the more weight in the cabin and the less weight in the wings, then the more stress on the wing spar and wing roots. MZFW is a simple method of calaculating maximum permissible weight in the cabin. It could of course be simply stated as a figure in the POH, but then every aircraft has a different equipment fit and a different empty weight.

I'm no expert on this subject, but I can't see how the above can hold true. How could the addition of weight in the fuselage possibly affect the wing mountings, when the wing mountings are not supporting the weight of any fuel? Also, every Hawker 800 (for example) has an identical MZFW, and yes it is a limitation that is included in the Flight Manual, and it has zero to do with what additional equipment the owner of one Hawker may add to his aircraft.

I suspect that the answer has less to do with structural limitations and more to do with the practicality of writing the CofG limitation graphs for the landing configuration.

Don't fly too slow, and never fly fas...
at the moment I spend a lot of time in LFMN

To add, because I don't yet know how to edit...I mean the landing weight, not configuration. The worst case scenario does require the CofG to be calculated with Zero Fuel. If there was no MZFW the graphs would have to extend a long way. MZFW is rarely a limitation that one will stop you flying.

Don't fly too slow, and never fly fas...
at the moment I spend a lot of time in LFMN

Though there may be other reasons (C of G), the wing bending load into the wing fuselage joint (or across the single spar, in the case of the Hawker 800) is the main reason for stating MZFW. That is why wingtip fuel tanks are popular on some types, and indeed for many twin Cessnas, the tips are the mains, so you will always have fuel in them last.

During aircraft modification work we do, we often capitalize on this characteristic, whether a limitation or not, by adding wing mounted equipment, without increasing the gross weight of the aircraft. This had the effect of shifting payload away from the fuselage to the outboard, and reduces the loads on the wingspar/fuselage joint.

Home runway, in central Ontario, Canada

Raisbeck have a number of mod kits for the King Air, and one of them increases MAUW. One thing they do in this mod, at the cost of a small increase in empty weight, is add weights to the wingtips.

Darley Moor, Gamston (UK)

I'm no expert on this subject, but I can't see how the above can hold true. How could the addition of weight in the fuselage possibly affect the wing mountings, when the wing mountings are not supporting the weight of any fuel?

Don't think about it on the ground, think about a steep turn.

Darley Moor, Gamston (UK)
13 Posts
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