MZFW
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excuse me, but i don't quite get what you mean when you say "manipulate the figures".
do you reduce the cargo weight with the stroke of a pen or do you change the earth gravity field?
Assumed Temperature works by reducing thrust because you don't need that much.
But (i hope) it's based of takeoff calculations, not on reducing cargo weight without actually reducing it.
do you reduce the cargo weight with the stroke of a pen or do you change the earth gravity field?
Assumed Temperature works by reducing thrust because you don't need that much.
But (i hope) it's based of takeoff calculations, not on reducing cargo weight without actually reducing it.
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Yes you can, I never suggested altering OEM figures. Just manipulating the figures
Interesting .. might we ask you to show us the detail of what you are doing and the rationale ?
Interesting .. might we ask you to show us the detail of what you are doing and the rationale ?
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Wing root loading a function of MZFW, not wing fuel
For a typical tube with wings airplane configuration, the wings generate the lift necessary to (1) lift themselves plus the fuel they contain (2) lift the fuselage to which they are attached. Obviously total wing lift must match / exceed total weight in order to get off the ground and stay aloft. The amount of load that the wing root must carry is related to the weight of the fuselage and its contents. The the wing root structure is not loaded by fuel in the wing, but is loaded by anything (fuel, cargo, SLF, etc.) located in the fuselage.
It has always been a curiosity to me that for some models, center tank fuel must be counted as cargo weight when the wing tanks are not full, but is not considered for MZFW when the wing tanks are full. From the physics this makes no sense to me - weight in the fuselage is still weight that must be carried by the wing root regardless. For older models with most of their tank capacity in the wings this is not such an issue. For new designs with thinner wings the percentage of fuel in the center tank is much higher.
When we get to flying wing designs, MZFW will be much less of a concern and may actually not be a consideration at all provided MTOW is not exceeded and the fuel on board supports the mission duration plus reserves.
It has always been a curiosity to me that for some models, center tank fuel must be counted as cargo weight when the wing tanks are not full, but is not considered for MZFW when the wing tanks are full. From the physics this makes no sense to me - weight in the fuselage is still weight that must be carried by the wing root regardless. For older models with most of their tank capacity in the wings this is not such an issue. For new designs with thinner wings the percentage of fuel in the center tank is much higher.
When we get to flying wing designs, MZFW will be much less of a concern and may actually not be a consideration at all provided MTOW is not exceeded and the fuel on board supports the mission duration plus reserves.
It has always been a curiosity to me that for some models, center tank fuel must be counted as cargo weight when the wing tanks are not full, but is not considered for MZFW when the wing tanks are full. From the physics this makes no sense to me - weight in the fuselage is still weight that must be carried by the wing root regardless.
That is half the reason for burning off center/fuselage tank fuel before using any wing fuel - the other half being the more obvious and intuitive effect of reducing the fuel weight lifted through the wing root.
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MZFW question
Examiner asked me a question about MZFW on my last line check. Can we exceed MZFW and still go if no other limiting weight is exceeded? (on B747-400 and -8 we are most often limited by MZFW). I answered no because the computerized load planning system won't allow it. He accepted that.
However, he did say that we can in fact exceed the MZFW and still depart. I'm still a bit confused by this as a result of many discussions with colleagues; some agree but most don't. Any performance engineers here care to comment?
However, he did say that we can in fact exceed the MZFW and still depart. I'm still a bit confused by this as a result of many discussions with colleagues; some agree but most don't. Any performance engineers here care to comment?
Having spent a good share of my career involved in certification type work, comments like Clown's scare the crap out of me. Between good engineering and the regulations, modern jet aircraft are designed with large safety margins. Those margins are there to insure various 'unknown unknowns' won't result in a catastrophic outcome.
OTOH, those margins mean that people can intentionally exceed established limits without consequence so long as nothing else goes wrong (I recall once hearing a 747F pilot from a now defunct operator brag that the 747 was already a "million pound airplane" - back when the 747F certified MTOW was under 800k lbs.
). However as soon as something else goes wrong, the resultant wreckage makes it painfully obvious why those limitations are there
Or, to put it simply: what part of 'maximum' don't you understand?
The only thing I can think of is that aircraft on the line are never at Zero Fuel. So it’s an academic limit as you will never see it.
For example if your payload exceeds the ZFM, you could still load it because the aircraft is not at Zero Fuel, there could be loads of fuel on it.
I don’t quite agree with this argument, yet it’s the only one I can think of.
For example if your payload exceeds the ZFM, you could still load it because the aircraft is not at Zero Fuel, there could be loads of fuel on it.
I don’t quite agree with this argument, yet it’s the only one I can think of.
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Examiner asked me a question about MZFW on my last line check. Can we exceed MZFW and still go if no other limiting weight is exceeded? (on B747-400 and -8 we are most often limited by MZFW). I answered no because the computerized load planning system won't allow it. He accepted that.
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Still waiting for there poster to bring the answer from their examiner. Until we hear that theorising we are, mostly all, in agreement it can not happen. Those reasons have been exhausted. I want to hear the examiner's thinking then we have something to get our teeth into.
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MZFW is a structural limit, nothing to do with performance or handling. The wing design load for many aircraft is a 2.5g manoevre at MTOW and fwd CG. The wing has to carry this load but the inertia loads associated with wing fuel give wing bending moment relief.
So long as the wing tanks remain full one can juggle between MZFW and centre tank fuel maintaining the same MTOW without affecting wing design loads.
If, still at MTOW, you try to push MZFW above the point where there is no centre tank fuel then you will need to take fuel out of the wing tanks, the bending moment relief will be reduced and the wing design loads potentially exceeded. To respect the certification conditions in such a case MTOW will have to be reduced. There is a snowball effect which means that the MTOW reduction is greater than the MZFW increase. In the graphic posted earlier for example an increase in MZFW of 11T was accompanied by a MTOW reduction of 27T. One can therefore exceed MZFW at the expense of MTOW IF THE MANUFACTURER HAS PUBLISHED THE EXCHANGE RATE otherwise you are guessing and illegal.
However, nobody has mentioned the fact that increasing MZFW above the manufacturer's value will increase the fuselage bending moment in the event of a hard landing. This is particularly relevant because the additional payload can be distributed all along the fuselage and give rise to large bending moments near the wing box. There is no way to relieve these inertia loads.
Statistically, hard landings are more likely than pulling 2.5g in the initial climb.
MZFW is there for a good reason and must be respected
So long as the wing tanks remain full one can juggle between MZFW and centre tank fuel maintaining the same MTOW without affecting wing design loads.
If, still at MTOW, you try to push MZFW above the point where there is no centre tank fuel then you will need to take fuel out of the wing tanks, the bending moment relief will be reduced and the wing design loads potentially exceeded. To respect the certification conditions in such a case MTOW will have to be reduced. There is a snowball effect which means that the MTOW reduction is greater than the MZFW increase. In the graphic posted earlier for example an increase in MZFW of 11T was accompanied by a MTOW reduction of 27T. One can therefore exceed MZFW at the expense of MTOW IF THE MANUFACTURER HAS PUBLISHED THE EXCHANGE RATE otherwise you are guessing and illegal.
However, nobody has mentioned the fact that increasing MZFW above the manufacturer's value will increase the fuselage bending moment in the event of a hard landing. This is particularly relevant because the additional payload can be distributed all along the fuselage and give rise to large bending moments near the wing box. There is no way to relieve these inertia loads.
Statistically, hard landings are more likely than pulling 2.5g in the initial climb.
MZFW is there for a good reason and must be respected
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In my old Tanker days on a large trijet, we had fuel tanks fitted in the cargo bays. Weight of fuel in those tanks was ZFW. If we were were operating in a pure AAR role MZFW was often the limiting factor. The MTOW was well above full tanks and performance was normally not an issue either depending on where we were based.
Using the same airframes for Air Transport we would put a couple of hundred squaddies and their kit on board which would be well below MZFW then fuel for the trip. So long as we didn't use the tanks in the cargo bays. This could, depending on the leg be MTOW limited.
Using the same airframes for Air Transport we would put a couple of hundred squaddies and their kit on board which would be well below MZFW then fuel for the trip. So long as we didn't use the tanks in the cargo bays. This could, depending on the leg be MTOW limited.
