MZFW

[av8tor94]

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?

[john_tullamarine]

That's two of us who will be interested in the answer. Those of your colleagues who agreed with the suggestion .. what was their reasoning ?

... unless he is being cute and looking at the case where an STC program increases the stock OEM MZFW ? That's not uncommon for freighter conversions.

[Dufo]

Technically you can but you are operating outside approved envelope.
On 744F you also have variable MZFW option which goes above basic MZFM at the expense of MTOW.

[john_tullamarine]

Technically you can but you are operating outside approved envelope

What's the rationale there ? Either you are compliant with the TC .. or you are not.

A multi MZFW arrangement doesn't bother me too much ...

[Bluescan]

I'm also interested in knowing the reasoning.
Was this on a freighter or pax version?

Also, like DUFO said, the MZFW varies with MTOW on the 747F. See attachment.

[B2N2]

However a varying MZFW doesn’t allow exceeding it.
So it depends on how he exactky phrased the question.
‘Can you ever exceed THIS number?’
Yes, you can if you are approved for the variable MZFW.

But as a blanket statement you can’t exceed a limitation.

[Mikehotel152]

Could it have anything to do with the non-use of fuselage fuel tanks in order to carry a greater payload?

[Smooth Airperator]

Stab in the dark. Does fuel density have anything to do with this? Could be substantial for a 747 full of fuel

[Skyjob]

The MZFW is a structural limit. It cannot be exceeded.
On some aircraft types, like the 747, a sized MZFW exists for most operational situations and a tapered reduced MZFW under a set of conditions.
Usually you will see such an increase in MZFW can only occur if another weight variable has been eliminated from use, such as an optional weight (fuel cell) at the extreme end of the weight arm of the aircraft, furthest away from the CG in other words. Thus if such (holds/tanks) are used the MZFW reduces.
Reason for this can be varied, but are mostly to do with the aircraft starting to get to the envelope edge of its controllability.
In reference to fuel cells, care is to be taken when using the auxiliary fuel cells as a problem may arise in the supply to the centre tank, thus potentially ending up with dead weight in a part of the fuselage. This may be the reason for MZFW reduction when a certain amount of fuel is used there. Alternatively it may be due to the use of a second optional auxiliary tank which is installed occasionally.

[FlyingStone]

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?

What makes the MZFW so different from other limitations in the AFM? Why not exceed MTOW, MLW, Vmo, Mmo, service ceiling? These things obviously don't apply to examiners with superior experience and piloting skills.

It has nothing do to with performance, it is a purely aircraft certification thing. And as soon you stray away from the AFM limits, you are in flight testing zone. Good luck with getting any authority to allow flight testing in commercial air transport operations.

[fantom]

Or, to put it simply: what part of 'maximum' don't you understand?

[Officer Kite]

As an eager to learn ATPL student i just pulled out the CAP 696 for mass and balance to double check, for MZFM it says "Maximum Zero Fuel Mass (MZFM) is the maximum permissible mass of an aeroplane with no usable fuel."

How can you exceed the maximum permissible mass and it still be permissible?

[RAT 5]

How can you exceed the maximum permissible mass and it still be permissible?

Indeed; I'd always thought it was to do with, amongst other things, the bending moment at the wing root. Not wanting the wings to clap hands as you lift of I would have expected it to be a strictly adhered to limit.
ZFW is a fixed weight. It is static and can not change: I'm assuming the wings are full and thus no allowance for centre tank fuel etc. Let's KISS.

So back to av8tor94. Will you lease go back to the Line Checker and ask for an explanation and then enlighten us.

[Ex Cargo Clown]

I've fiddled with MZFW with the skippers permission on short-legs because it would be nowhere near MTOW. Plus we figured in the freight weight wasn't always accurate. If it stayed in MAC% it was OK. MZFW is still confusing to me, unless you fully fuel it and exceed MLW, there is no problem.

[vilas]

MTOW, MLW, MZFW, maximum structural taxi weight are structural limit weights. There is no way to legally exceed one at the expense of the other.

[Ex Cargo Clown]

Got to disagree. If you have a two hour 744f route, MZFW becomes pointless. MTOW or MLW will never be exceeded. I've used a sliding scale of MZFW vs MTOW, as long as it stays in MAC%. I've never seen an aircraft vanish through the tarmac over MZFW.

[mcdhu]

From the Airbus performance "Grips" Manual:

"Bending moments, which apply at the wing roots, are maximum when the quantity of fuel in the wings is minimum. During flight, the quantity of fuel located in the wings, Mwf, decreases. As a consequence, it is necessary to limit the weight when there is no fuel in the tanks. This limit value is called Maximum Zero Fuel Weight."

It's a limitation and should not therefore be exceeded.

[vilas]

Use whatever arithmetic manipulation but cannot exceed any of the limit weights. Your MTOW will be governed by the most limiting.

[FE Hoppy]

Got to disagree. If you have a two hour 744f route, MZFW becomes pointless. MTOW or MLW will never be exceeded. I've used a sliding scale of MZFW vs MTOW, as long as it stays in MAC%. I've never seen an aircraft vanish through the tarmac over MZFW.

Ex is good news for the rest of us. Clown still pertinent.


You cannot exceed the MZFW legally.

[Ex Cargo Clown]

Yes you can, I never suggested altering OEM figures. Just manipulating the figures. Happens every day. How do you think Assumed Teperature works?

[wiedehopf]

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.

[RAT 5]

Ex Cargo Clown:Age: 39

Not sure I wish to pursue this further, but............exceeding MZFW?

[B2N2]

https://media.licdn.com/media-proxy/ext?w=800&h=800&hash=e8Znf1XstUmUeUM1goB1S8jGaUo%3D&ora=1%2CaFBCTXdkRmpGL2lvQUFBPQ%2CxAVta9Er0Vinkhwfjw8177yE41y 87UNCVordEGXyD3u0qYrdf36_f5bbf7fyuQhFKX4clFAzefL5FGTmD5vvK9-6dIgjgsKxLY27dA4BYBI3iSdF_NQ8

[john_tullamarine]

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 ?

[FCeng84]

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.

[pattern_is_full]

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.

I believe that fuel in full wing tanks - since it is a virtually incompressible fluid - is assumed to stiffen and reinforce the wing structure. Effectively increasing their strength and reducing flex (especially if the tanks extend to the wing tip). Thus they can carry more weight safely, just as though a stronger wing spar (or a mid-wing support strut, a la Cessna) had been installed.

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.

[av8tor94]

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?

[Capn Bloggs]

Standby for incoming! :}

[tdracer]

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 ?


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. :eek:).
However as soon as something else goes wrong, the resultant wreckage makes it painfully obvious why those limitations are there :ugh:


Or, to put it simply: what part of 'maximum' don't you understand? Well put fantom...

[Jonty]

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.

[lurkio]

I am trying desperately to pretend the last post (#36) isn't there because sure as eggs is eggs someone will use it as a defence. God help them.

[vilas]

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.
The problem started with this answer. The answer should have been no MZFW cannot be exceeded for no other reason than it is a structural limit weight.

[Pugilistic Animus]

As long as we don't know more than Boeing or Airbus.

[RAT 5]

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.

[Owain Glyndwr]

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

[FE Hoppy]

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.