MZFW
 

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?

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.

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.

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 ...

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.

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.

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

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

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.

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.

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

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?

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.

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.

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.

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.

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.

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

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


You cannot exceed the MZFW legally.

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