Why have a Max Zero Fuel Mass?

[OBK!]

Just wanted to know what the point of a maximum zero fuel mass was?

Cheers for any comments.:p

[AfricanSkies]

With no fuel in the wings, certain aircraft can only take so much weight on the gear. Once the wings are fuelled, they provide a balance to the central fuselage mass, in effect lifting it, with the gear being the pivot, and the total load on all the gear is less. Sounds strange but once the wings are fuelled, more payload may be loaded. MZFW is a structural limit.

[expedite_climb]

Never come across an aircraft that allows more payload once the fuels on board when zfw limited.....

For interest, in addition on the 757 if the wing tanks are full, and fuel in the centre tank must be added to the zfw - that can make it very limiting !

[Genghis the Engineer]

I know it mainly as a regulatory point to ensure that aircraft designers don't take the mickey for their own purposes and end up with an aircraft likely to be overloaded.

For passenger aircraft it's defined in parts 23 and 25 as the greater of MTOW minus 77kg (170lb) per seat, maximum oil, and minimum permitted equipment + 30 minutes fuel at MCP, or minimum flight crew, minimum equipment, and full fuel and oil.

For sports aircraft (at least here in the UK) it's defined as the greater of MTOW minus 86kg per seat, maximum oil and minimum permitted equipment + 1 hours fuel at MCP, or 86kg in the pilots seat, minimum equipment and full fuel and oil.


Some of the more high performance little aeroplanes that I deal with struggle to meet this, and when you think about it, you start to see the point. (Half the microlights in Germany have been grounded or made single seaters over the last couple of years because they've been taking the mickey (or whatever the German term for this is). When you consider that German manufacturers were building 2-seat aeroplanes with a 472kg MTOW and a ZFW around 310kg, you see a problem.

[ATPMBA]

My understanding of the max zero fuel weight is to prevent wing bending. If too much weight is concentrated in the fuselage then the wings will flex dramatically in flight. By placing fuel into the wings, the weight is distributed more evenly, there is not a concentration of mass in one area.

MZFW is a design limitation, to increase this weight designers would probably need stronger and heavier wing spars and attach points.

BIK_116.80

I agree with you, if I saw your post I wouldn't have put mine up.
I just missed yours by 2 minutes.

[OBK!]

Thanks for the responses!

I was just wondering though, if you fill the wing tanks will this not allow a great MaxZfw?

[Genghis the Engineer]

BIK_116.80

I think we're talking at cross-purposes slightly.

The regulatory point I made is, in my opinion still correct.

However, it is of-course a structural limit, and of-course it's about the ability of the wing structure (the root is not necessarily the weak point, that depends upon wing design - for example the weak point on a strutted wing such a a C172 is almost certainly the upper strut attachment area).

An aircraft will have an MTOW, which is a function of various design and regulatory requirements (including stall speed and undercarriage strength). The regulations will tell you what the highest MZFW may be as a function of that. They also define the minimum structural strength required at the worst case loading. Since the MZFW and MTOW define how heavy the fuselage can become, this defines how strong the wing structure has to be. The designer will come up with an MZFW value that meets the essential structural requirements, and which also meets the more arbritrary regulatory requirements.

When you get both right, the powers that be allow you to fly your aeroplane.

[Georgeablelovehowindia]

OBK! It's a Maximum ZERO Fuel Mass. You have to remember that you will be using that fuel in the wings and towards the end of the flight, there will be very little left, causing the greatest strain on the wing roots.

[FJJP]

Hit the nail on the head talking about wing bending relief. Any large metal structured aircraft airborne will always use the wing fuel closest to the fuselage first, leaving that closer to the wingtips as the last/reserve fuel. This is designed to counter the lift forces bending the wing upwards, which places a structural strain on the wing roots. The weight of the fuel closer to the wingtip counter-balances this upward bending.

The effect on the ground is that too much fuselage weight with no fuel in the wings places a strain (additional shearing structural load) on the gear and (bending shearing load) on the wing root structure.

[Genghis the Engineer]

The regulatory requirements I'm talking about are mostly the design code used for original type certification.

I don't have a copy of part 25 (airliners) to hand, but quoting from JAR-23, which I do (extracting from 23.25)...


(a) The maximum weight is the highest weight at which ....

(a)(2) Assuming a weight of 77kg (170lb) for each occupant of each seat for normal and commuter category aeroplanes and 86kg (190lb) (unless otherwise placarded) for utility and aerobatic category aeroplanes, not less than the weight with -

(i) Each seat occupied, oil at full tank capacity, and at-least enough fuel for one-half hour of operation at rated maximum continuous power; or

(ii) The required minimum crew with fuel and oil to full tank capacity.


(Omissions and spelling mistakes mine, not the JAAs).


So this is where, for regulatory purposes, the MZFW concept comes from, although structural requirements in part C have to be met for this MZFW.

[Bellerophon]

FJJP

I agree, in general terms, with what you said

Any large metal structured aircraft airborne will always use the wing fuel closest to the fuselage first

but always is generally a dangerous word to use.

There may be an over-riding reason why some types don't.

On my type, we use fuel from our reserve tip tanks early on, despite the loss of wing bending relief and adverse CG movement that this causes.

If we didn't, it would just boil off! ;)

[Dragon Knight]

In level flight, the total gross weight is supported by the air load on the wings, the air load being an upward acting load and the gross weight ( weight of fuselage, fuel and wing) being a downward acting load.

Since the upward acting loads on the wings (gross weight) are greater tha those acting downwards (wing and fuel), bending moments and upward acting loads are produced at the wing roots.

Thus the wing joint load remains constant provided the weight of the fuselage and its contents is kept constant.
In other words the fuel, when carried in the wings, off-loads the wing to the same amount as it on-loads it.

[Basil]

Yes, increasing mass in the fuselage increases wing root bending moment but increasing mass in the wing tanks does not - SO, when we FILL the wings and THEN start putting fuel into the centre tank WHY don't we have to reduce the ZFM by the mass of fuel loaded into the centre tank??
First correct answer on a postcard will WIN the editor's respect ;)

[Anchorman]

In response to the 757 ZFM calculation, surely the tables assume that the centre tank is full as part of the ZFM, so any filling of this tank starts at ZFM less empty tank weight, closing to ZFM as tank is pressed up?

My logic can't see how the tank can be an addition to ZFM, as this would put the aircraft over the ZFM, regardless of whether this tank is used before tank to engine.

Is the centre tank therefore filled to the MTOW if needed, as otherwise offoading would be needed to remain within limits?

Look forward to the real answer.

[mustafagander]

Could this refer to the case where fuel is being "tankered" in the CWT and hence will not be burnt off prior to wing fuel???

[Basil]

If the CWT fuel is being tankered and not burnt off then (on the types I've flown) it should be considered to be part of the ZFM.

[boxmover]

On the aircraft I fly CWT fuel is not inc in the ZFW. UNLESS you plan not to burn it first (ie the fuel is balast for the GofG).

I think the logic is that when the wings are full of fuel the spars can take a lot more load than at the end of the flight.

[Keith.Williams.]

This string is becoming rather worrying, in that some of the posts are in danger of thoroughly confusing a good many readers who are preparing to take their JAR ATPL M&B exam.

Although some of the regulations quoted by Genghis include fuel, the zero fuel mass of an aircraft does not (normally) include any usable fuel.

Zero Fuel Mass (or Weight for non JAA readers) is the dry operating mass (basic aircraft plus equipment lubricants, food water, crew and their baggage, and non usable fuel), plus the traffic load (Passengers cargo and any non-revenue loads).

Zero fuel mass does not (normally) include any usable fuel. But if an aircraft is tankering extra centre tank fuel with no intention to use it on the currently planned flight, then this fuel becomes non-revenue load and hence should be considered to be part of the zero fuel mass.

The maximum zero fuel mass (MZFM) is one of the limiting values that must be considered when calculating how an aircraft may be loaded. In most aircraft the vast majority of the items listed in the zero fuel mass are contained in the fuselage. So increasing zero fuel mass increases the bending stresses on the wing roots. As stated in some of the previous posts, the principal reason for imposing a MZFM limit is to ensure that the wing bending moments do not become excessive.

The regulations quoted by Genghis appear to be specifying a minimum acceptable value for the MZFM. In effect they appear to be saying that an aircraft structure must at least be strong enough to carry either a full load of passengers plus a nominal amount off fuel, or no passengers plus a full load of fuel.

Readers who are preparing for their JAR ATPL M&B exam should note that remaining within the MZFM provides no guarantee that the other limits (MSTOM, PLLTOM, MSLM, PLLM, or MRM) will not be exceeded. This fact is demonstarted by the data for the MRJT1 in the CAP 696. MZFM(51300 Kg) + Total fuel capacity (16092 Kg) = 67392 Kg. MSTOM = 62800 Kg

[Alex Whittingham]

I'm not sure you're right, Keith. My understanding is that aircraft that have belly fuel tanks as standard have the MZFM 'fudged' by the manufacturers to take account of this specifically to avoid distressing pilots with questions like 'Why do we have to take account of fuel in the Zero Fuel weight calculation?'. Some aircraft, particularly those that have been fitted in mid-life with extra fuselage tanks by somone other than the original manufacturer usually do have to take account of the centre line fuel in the ZFM calculation because the original certified limits won't have changed.

[Keith.Williams.]

You may well be right Alex.

But the main point I was making was that for readers who are preparing to take the JAR ATPL M&B exam, all this talk about ZFM and MZFM including fuel, is potentially dangerous. The CAP is quite clear about what must be included in the ZFM and MZFM. The definitions given for both of these terms specifically exclude all usable fuel. It is of course quite possible (indeed highly probable) that the real world differs from JARATPLLAND.

I have searched my copy of JAR 25 for ZFM and MZFM definitions using the terms quoted by Genghis, but have found none. This does not of course mean that they do not exist, but simply that I have not found them.

My suggestion that fuel being tanked could be considered as non-revenue load and hence part of the traffic load, is simply an attempt to reconcile the definitions in the CAP and the comments made by contributors to this string, who have stated that their companies include such fuel in the ZFM.

[Genghis the Engineer]

I think the nearest in part 25 (this is from FAR not JAR, but they don't differ much) is...


Sec. 25.25 Weight limits.

(a) Maximum weights. Maximum weights corresponding to the airplane
operating conditions (such as ramp, ground or water taxi, takeoff, en route,
and landing), environmental conditions (such as altitude and temperature),
and loading conditions (such as zero fuel weight, center of gravity position
and weight distribution) must be established so that they are not more than--
(1) The highest weight selected by the applicant for the particular
conditions; or
(2) The highest weight at which compliance with each applicable structural
loading and flight requirement is shown, except that for airplanes equipped
with standby power rocket engines the maximum weight must not be more than
the highest weight established in accordance with Appendix E of this part; or
(3) The highest weight at which compliance is shown with the certification
requirements of Part 36 of this chapter.
(b) Minimum weight. The minimum weight (the lowest weight at which
compliance with each applicable requirement of this part is shown) must be
established so that it is not less than--
(1) The lowest weight selected by the applicant;
(2) The design minimum weight (the lowest weight at which compliance with
each structural loading condition of this part is shown); or
(3) The lowest weight at which compliance with each applicable flight
requirement is shown.

and the structural bit, which is to do with gust loadings...

(5) The following reference gust velocities apply:
(i) At the airplane design speed VC: Positive and negative gusts with
reference gust velocities of 56.0 ft/sec EAS must be considered at sea level.
The reference gust velocity may be reduced linearly from 56.0 ft/sec EAS at
sea level to 44.0 ft/sec EAS at 15000 feet. The reference gust velocity may
be further reduced linearly from 44.0 ft/sec EAS at 15000 feet to 26.0 ft/sec
EAS at 50000 feet.
(ii) At the airplane design speed VD: The reference gust velocity must be
0.5 times the value obtained under Sec. 25.341(a)(5)(i).
(6) The flight profile alleviation factor, Fg, must be increased linearly
from the sea level value to a value of 1.0 at the maximum operating altitude
defined in Sec. 25.1527. At sea level, the flight profile alleviation factor
is determined by the following equation:

Fg = 0.5 (Fgz + Fgm)

Where:

Zmo
Fgz = 1 - ------------;
250000

Fgm = (square root of : ) R2 Tan(Pi R1/4);

Maximum Landing Weight
R1 = -------------------------;
Maximum Take-off Weight

Maximum Zero Fuel Weight
R2 = ---------------------------;
Maximum Take-off Weight



Which seems more open ended than part 23, although I do like the idea of trying that rocket bit.

[Keith.Williams.]

Thanks Genghis,

That is pretty close to what is stated in JAR 25.

But there is nothing here to indicate that ZFM or MZFM include any usable fuel. As stated in my previous post, I have searched JAR 25 and found nothing to indicate that usable fuel is part of the ZFM or MZFM. I have also searched the relevant section of the JAR OPS1 Manual and found nothing.

[Genghis the Engineer]

That was unfortunately the only reference to MZFM in all of FAR-25 (I have an electronically searchable copy, so I'm pretty certain about that), is there possibly anything in JAR-1 "definitions"?

[Keith.Williams.]

Genghis,

Sadly (do I really mean that ???) I do not have access to a full set of JAR manuals, so I could not conduct an exhaustive search even if I wished to (which I do not).

But the inclusion of the words "zero fuel" in "Zero Fuel Mass" and in "Maximum Zero Fuel Mass" suggests that fuel should not be included. The definitions in the CAP 696 loading manual clearly state that these terms include no usable fuel.

Some of the posts in this string indicate that at least some operators include tanked fuel as part of the ZFM. Presumably these operators have modified Load & Trim Sheets which enable tanked fuel to be accounted for separately from fuel intended for the next planned flight.

[Genghis the Engineer]

Hard to disagree with you Keith. However, if you are desperately in need of some bedtime reading, the JAA docs are all downloadable from www.jaa.nl (select JARs, then "part 1").

[HOMER SIMPSONS LOVECHILD]

Reminds me of the great hoo-ha at the Spotty M when going "JAA".All reference to "weight" was deleted and the word "Mass" inserted.They called it "M-Day". Pedantic trainers took great delight in criticising any use of the word "weight"from then on in.
They could not really predict exactly when the gravity constant (and therefore any difference)under which we as an airline operated would be changing .Shell suit express to Mars anyone?.;)

[Mark McG]

What about this scenario:-

Boeing AD stipulates 1000lbs of fuel to be carried in CWT to avoid leaving Fuel pumps uncovered.

Would you as crew pass me (load controller) the following figures for a Loadsheet:

Ramp Fuel with the 1000lbs of fuel for the CWT included in this figure

or

Ramp Fuel plus the 1000lbs of fuel for the CWT to be included on the loadsheet as a Service Weight Adjustment.

I have not used option 2 before(most holiday flights take full wings + some CWT fuel), but I have seen some Big Airways 757 Load Messages that have used option 2.

[DeltaT]

I was doing some searching around on the topic, and came across this old thread here (http://www.pprune.org/tech-log/85214-why-have-max-zero-fuel-mass.html) which has been closed.
No one seemed to mention this explanation I have found. It would seem this would explain why centre tank fuel is added to the operating weight of the plane (depending on aircraft type), because it is in inboard of the wing spars.

[john_tullamarine]

Not sure how the thread came to be locked and how the previous poster posted to a locked thread .. however ...

Re ZFW, wing fuel and can this be used to alter the MZFW, of course it can .. although not on an on the fly ad hoc basis.

Years ago, when Ansett converted the Electra passenger fleet to freighters, the MZFW limit was increased by the expedient of having a minimum wing fuel quantity limit. I don't fancy trying to track down the sequence this far removed but I presume it would have been part of the conversion STC. The bulk of the flying was short to medium range so the fuel limit rarely caused a problem operationally... wonderful aircraft when you needed to carry an alternate a long ways away ... actually, wonderful aircraft all round.

I suggest that, provided the fuel usage is per the AFM, the OEM has taken care of all these concerns including such matters as min fuel and pump cooling considerations. However, if centre section/fuselage tank(s) fuel usage is varied to carry more there than expected per the AFM, clearly it has the same effect as revenue payload in the fuselage and must be counted within the ZFW limitation considerations.

[DeltaT]

Hi, sorry, I think you might be missing the point I was trying to make, its not for revenue reasons, its because you are increasing the bending moment across the wing spar by increasing the weight inside the the wing spar attachment points. Therefore you want to decrease the allowable payload that is in the centre fuselage.

I believe the post was enabled by the mods who have opened up the thread.

[SMT Member]

As I was once told, here are the limiting factors for the maximum weights:

MTOW: Engine power (same airframe with two different installations, the one with the higher thrust rating will have a higher MTOW)
MLAW: Landing gear (as above, stronger gear will allow higher MLAW)
MZFW: Wing bending (payload is carried in the fuselage, lift is generated by the wings. To prevent the wings from bending too far, payload is limited)

[Metro man]

Watch this Youtube video to see why you want to avoid straining the wing roots.

https://www.youtube.com/watch?v=ybYeJVh1cew

Ignore the commentary, the sudden reduction in weight as the load was dumped caused the wings to flex at the roots and separate.

[john_tullamarine]

Some modifying thoughts ...

you are increasing the bending moment across the wing spar by increasing the weight inside the the wing spar attachment points

As, I think, Genghis made the point earlier, the wing root may not be the critical point along the wing. However, the general story is OK in that somewhere along the wing will be a structural consideration which limits MZFW. That can be modified, as with the L188s mentioned above but not without a quid pro quo somewhere along the way.

As I was once told, here are the limiting factors for the maximum weights:

A bit simplistic but a starting point for discussion, nonetheless.

Watch this Youtube video to see why you want to avoid straining the wing roots.

We need to keep in mind that there are, at least, two considerations at play with structural strength -

(a) instantaneous loading .. ie, put too much load on something and it will break

(b) fatigue loading .. much lower, small overloads, repeated over a lengthy period, progressively lead to cracking which reduces the capability of the structure to carry the loads presumed OK ref (a). If fatigue loads significantly exceed the design studies, eventually the cracking and section area reductions will lead to a failure at a much lower than expected load .. and much earlier than the OEM expected for the presumed service life.

For example, I own a heavy off-road caravan which Type is often subjected to a pretty hard life out bush. A number of examples have had stub axles fail at ridiculously low instantaneous loads due to significant fatigue cracking. Fix, in a manner similar to the aeroplane problem.. rework the design of the stub axle to improve the fatigue properties and ultimate load capability. That way, the owners can keep playing with a much reduced worry regarding the possibility of being embarrassed a long way from anywhere.

[Chris Scott]

Hi DeltaT:

Actually, you may have missed the point John was making!

As I read it, he was merely pointing out that, where there is one, the centre tank will always contain a small amount of unusable fuel, if only because the tank pumps cannot completely empty it and gravity feeding is not possible. (There may also be safety considerations.) Therefore the regulations include that mass of fuel in the the dry operating weight/mass. I guess it can be considered to be part of the structure of the tank. Any usable fuel added to that has to be treated in the same way as payload.

Must admit none of the types I flew was allowed to operate at a higher MZFW if the wing-tank fuel was raised above a certain value, but it makes sense for the reasons others have pointed out.

[Meikleour]

Chris: Many moons ago when I operated the B707-321C (ex PanAm) we had an increased max . Take-off Weight (+10,000lbs?) PROVIDED the centre fuel tank was loaded with at least 20,000lbs of fuel. Never did fully understand that but it introduced quite a few problems since we were operating freighter versions with high ZFWs. Maybe this was a c.g. issue?
With regard to the other discussions about wing root bending relief and max ZFW with centre tanks - the relief is taken care of by the fuel use schedules which always require the centre tank to be used from early on in the flight. NB I am not being specific here in case the pedants are out to get me!

[763 jock]

Never come across an aircraft that allows more payload once the fuels on board when zfw limited.....

For interest, in addition on the 757 if the wing tanks are full, and fuel in the centre tank must be added to the zfw - that can make it very limiting !
It's been a few years since I flew the 757. The only centre tank limit I can recall is a maximum of 907kgs if the wings were less than full. The only other occasion when centre tank fuel had to be included as part of the ZFW was when operating IAW with the MEL with one centre tank pump U/S.

One thing the 757 never suffered from was lack of fuel capacity or the ability to lift a huge payload!

[Owain Glyndwr]

Gentlemen,
At the risk of some repetition:
MZFW. is selected by the manufacturer. The choice may depend on several things other than wing bending moment.
MZFW is made up of an empty weight plus a maximum payload. The empty weight according to JAR 25.29 includes unusable fuel. Therefore if a manufacturer declares a minimum fuel to be retained in a tank at all times it is by definition unusable and becomes part of the empty weight. Note though that this does not necessarily mean an increase in MZFW.

The wing bending design load is more often than not the 2.5g manoeuvre load at MTOW/Fwd CG limit. Initial versions of a design (e.g the A320/100) may be designed by gust loads but as MTOW is increased during development the gust loads get overtaken by the manoeuvre case.
Wing bending is of course reduced by having fuel in the wing tanks, up to a limit when these tanks are full. Fuel in any centre tank does not contribute to wing bending relief - it effectively has the same effect as payload in increasing the mass that the wing must support at 2.5g.
The influence of centre tank fuel on the wing design static bending moment will depend on the relationship between MTOW and MZFW with full wing tanks. In effect maximum design bending will occur when MZFW (including any centre tank unusable fuel) plus wing fuel plus centre tank fuel equates to MTOW.
But as I said earlier, there may be other consideration setting MZFW - a large payload with an awkward load distribution could be a fuselage bending limit for example.

[ajd1]

763 jock is correct, at least as far as the 757 is concerned. And of course, on Boeings, the centre tank fuel is used first.

[Metro man]

Back when I flew Islanders, the tip tanks were fill first/use last.

Newer versions of the A320 have load alleviation software in the FBW system were spoilers deploy to reduce wing bending at high load factors.

[Chris Scott]

Quote from Metro man:
"Newer versions of the A320 have load alleviation software in the FBW system were spoilers deploy to reduce wing bending at high load factors."

Owain Glyndwr will recall that early A320s had a system called Load Alleviation Function (LAF), which judiciously and almost instantly deployed small deflections of a couple of pairs of spoilers and the ailerons to mitigate the effect of gusts. Rapid movement was achieved by the use of small hydraulic accumulators at the actuators of the relevant control surfaces. Serviceability was a mandatory requirement for high altitude flight above a certain ZFW.

After some years of operation, the proven integrity of the wing structure rendered the system more or less redundant, the restrictions were removed, and IIRC the LAF system was deleted from later serial numbers. I can't speak for the A320neo family.

BTW, in common with most big jets of my acquaintance, the outer wing tanks are supposed to be emptied last on the A320 for wing-bending relief. But it is not necessarily mandatory. Sometimes, at the end of a medium to long sector, when the outer-tank fuel temperature was below O degC, we used to empty them (into the inboard wing tanks) deliberately, in order to stop frost forming on the upper-wing surface after landing at airfields for a short turnround when the humidity was high. (Upper-surface frost is not acceptable for take-off.)

[Metro man]

Typically a sharklet equipped A320 will be 2 tons heavier than a non sharklet version at the same max alt, ie 65.4 tons at FL390 vs 63.4. Is this not down to load alleviation as I can't see the sharklets making that much difference ?

[Chris Scott]

Hi Metro man,

I think the MAX ALT is more likely to be limited by low-speed buffet margins than structural considerations, in which case the sharklets are probably the benefactor.

[megan]

Years ago, when Ansett converted the Electra passenger fleet to freighters, the MZFW limit was increased by the expedient of having a minimum wing fuel quantity limitA work around used by the RAAF on its Hercs many years ago also. Think it was when wing fatigue raised its head, but don't hold me to it, too long ago.

[RAT 5]

http://i1268.photobucket.com/albums/jj576/Snowfella/Dyxum/DSC06443_zpsrsyl0d67.jpg

What happens if you exceed MZFW.

[Owain Glyndwr]

@ Chris Scott, Metro-man

It seems then that the MTOW on the latest A320s has grown to the point where manoeuvre load alleviation is beneficial; augmented by the fact that sharklets will pull the wing centre of pressure outwards.
If, as seems likely, they have followed the A340 system principles, then the spoilers will not start to deflect until the aircraft gets to about 2g. That being so, and given that spoiler deflection is the last thing you want if you are looking for enhanced performance, the increased weight at max. altitude on the sharklet equipped aircraft is not going to be anything to do with load alleviation.
Without knowing anything about the engines fitted one cannot be sure exactly where the improvement comes from, but on the face of it a 3% increase in L/D from sharklets is not unreasonable.

[Chu Chu]

No one has said this directly (hopefully not because it's wrong, but I'll soon find out), but it seems like MZFW is simply a way of limiting the amount of weight in the fuselage. Because weight in the fuselage creates more bending moment at the wing roots than fuel in the wings, it would be possible to have an aircraft below MTOW and still overstress the wings. By limiting the total gross weight minus the fuel weight, MZFW ensures that doesn't happen.

Again I could be wrong, but I said that fuel in the wings contributes less to the bending moment because I assume fuel close inboard would increase that moment, even though fuel outboard would decrease it.

[DeltaT]

Correct, as per the attachment PDF in my post to re-energise this thread. Hence why if you have centre tank fuel (in the fuselage!) it now makes sense why this would be added to the operating weight of the aircraft because it has now added to that bending moment.

[Owain Glyndwr]

@ChuChu

No, you are quite right, MZFW is simply a means of limiting the weight in the fuselage.
I am not sure what you meant by fuel close inboard, but any fuel loaded outboard of the wing root fixings will contribute bending moment relief.
Discussion of the effect of fuel loaded into a centre tank is a bit of a red herring, since so far as wing bending moment is concerned there is no difference between a pound of fuel in a fuselage tank and a pound of payload carried in the fuselage. Besides which, there are a lot of airplanes out there that do not have centre tanks but they all nave MZFW limits.
As I wrote previously, the wing design bending moment is usually 2.5g at MTOW, but that should strictly be modified to read the combination of payload and fuel (no centre tank) that gives the highest wing loads.
So you could theoretically have an aircraft with a high MZfW and a low fuel tankage where at MTOW the wing bending moment was bigger than the same aircraft with the same MTOW but a lower payload and more fuel.
However, this would NOT result in overstressing the wings because if it is a permissible loading case then the design must be shown to be capable of withstanding the loads. This is true even if the TOW in this loading case is less than MTOW.
And again, MZFW is not necessarily set by wing bending

[Meikleour]

Owain Glyndwr (http://www.pprune.org/members/361951-owain-glyndwr):Serious query for you. Early model A330s came with a VARIABLE ZFW
schedule! That is MTOW of 215tonnes allowed MZFW of 167tonnes and a sliding scale to
MTOW of 209tonnes allowed a MZFW of 172tonnes. This seems counter-intuitive and the only explaination we were ever given from Airbus were that "they were selling fatigue life"!
Incidently, the sliding scale allowed a commersurate increase in MLW to match.
Your thoughts would be appreciated.

[Owain Glyndwr]

Meikleour: Good question, but I'll need time to think about it. I'll get back to you

PS, just to be sure I don't say something stupid, do you know what the associated fatigue lives were?

[Meikleour]

Owain Glyndwr (http://www.pprune.org/members/361951-owain-glyndwr): Sorry, we were never given specifics. I assume it might be a balance of the
lifetime maximum bending moments/cycles with the lower MTOWs reducing the deflections?

[Owain Glyndwr]

@Meikleour

I see a subtle difference between a "Variable ZFW schedule" and a straight choice of MZFW from a list of alternatives. The former gives a lot more operational flexibility if written "You can use a ZFW up to 172T provided you keep the TOW below 209T, or you can use a TOW up to 215T if you keep the ZFW below 167T, with suitable variations in between". The latter is a simple once and for all decision yielding MZFW. I am assuming you were offered the former.
After all this time it is difficult to recover the then existing numbers, but it seems to me that AI were offering operational flexibility at constant fatigue life.
Calculating fatigue life (actually damage tolerance is a more accurate statement) is a tortuous process involving calculation of manoeuvre and gust loads at various points in typical missions and them summing them, appropriately weighted for exposure time, to get the total damage. Since most of the time is spent in cruise, this condition will dominate the calculations. The usual calculation is a midcruise 1g load with a typical (say 10fps) gust.
Obviously the exact numbers will depend on the airline's route structure, but it is a fair assumption that if an airline opts for higher than standard ZFWs it is because it expects to carry higher than average payloads more often than average. One can however get a feel for the anticipated average situation by looking at the worst case.
In this particular instance they were offered 209T TOW with 172T ZFW which would give a mid cruise (half fuel) weight of 190.5T or 215T TOW with 167T ZFW giving a mid cruise weight of 191T. In other words they were offered flexibility of operation and additional carrying capacity when needed without sacrificing fatigue life.

Hope this helps!

[Meikleour]

Owain Glyndwr (http://www.pprune.org/members/361951-owain-glyndwr): Thanks for the explanation. Yes, it was the Variable ZFW schedule that was used on the first half of the fleet until the higher gross weight fixed schedule aircraft became made available by Airbus. The later models had a 233tonne MTOW and 175tonne MZFW.
Would the wing structure have been "beefed up" or not considering that the A330 shared the much heavier A340's wing but with only two pylons? Or, would this have been a paperwork exercise?

[Owain Glyndwr]

Meikleour
There was no substantial beefing up. The initial design weights were chosen to make the loads equivalent allowing for the extra bending relief from the outboard engines. Plus of course different local structure at the pylon/wing joint.

[Metro man]

There is a short range version of the A330 offered by Airbus which is certified for lower weights. It is marketed as a big A320/B737 for shorter routes where more seats are needed than the narrow bodies offer.

It can be upgraded to a normal A330 by paying Airbus for the paperwork. This enables them to offer the aircraft into a specific area of the market at a competitive price without reducing the price for airlines wishing to operate it on its intended medium to longer routes.

British Airways used to limit its B757s to lower weights to reduce landing fees charged by BAA. Their operation wasn't affected as they didn't need the aircrafts full capability on the routes they used it on.

[Chu Chu]

I was thinking that any fuel inboard of of the center of lift would try to rotate the wing down at the root and up at tip, thus increasing the bending moment at the root. But I'm probably missing something.

[mustangsally]

It is simple:
The maximum zero fuel weight (MZFW) is the maximum weight allowed before usable fuel and other specified usable agents (engine injection fluid, and other consumable propulsion agents) are loaded in defined sections of the aircraft as limited by strength and airworthiness requirements.
Maximum zero fuel weight - Wikipedia (https://en.wikipedia.org/wiki/Zero-fuel_weight)

https://en.wikipedia.org/wiki/Zero-fuel_weight

[DaveReidUK]

British Airways used to limit its B757s to lower weights to reduce landing fees charged by BAA. Their operation wasn't affected as they didn't need the aircrafts full capability on the routes they used it on.

Yes, that is/was very common across a lot of airlines and aircraft types, not just BA and their 757s. Also helped to reduce Eurocontrol user charges, too.

[RAT 5]

But that's MTOW not MZFW; is it not?

[Owain Glyndwr]

@ChuChu

The first action then would be wing twisting, not bending.
Two remarks:

Throughout the wing the fuel is more or less equally distributed fore and aft of the wingbox structural torsional axis, so fuel inertia loads don't contribute much torque.

At or near the root the wing thickness gives a very high torsional stiffness, so any twisting deflections there are minimal

[DeltaT]

I am seeing all these remarks zipping around and people keep reposting the definition of MZFW. The whole point of me adding to this thread and livening it up again was the attachment I found which explains centre tank fuel being used in reducing operational payload and what exactly is happening with wingroot/spar forces. Information that had not been alluded to before in the thread. Yes we all know ZFW doesn't include fuel, but that was probably invented before aircraft were designed that had centre tanks!
Chu Chu, yes correct, as per the attachment I put up in my post.

[Chu Chu]

Just to explain my thinking a little more rigorously (and consistent with DeltaT's attachment), it's true that any weight added to the wing, considered by itself, creates a downward bending moment about the wing root. But if you think about it, weight added to the fuselage, considered by itself, creates only a downward force on the fuselage and has no effect on wing bending.

But any weight added anywhere on the aircraft (assuming level flight, symmetrical loading, etc) requires an equal increase in lift. The increase in lift acts through the center of lift on each wing and (considered by itself) creates an increase in upward bending moment about the wing root.

If the the weight is added to the fuselage, the upward bending caused by the additional lift is the end of the story. But if it's added to the wing, the the downward force from the weight must be considered.

If the weight is added inboard of the center of lift, it acts on a shorter moment arm than the lift, and partially offsets the upward moment. The net result is an increase in bending moment about the wing root, but smaller than if the additional weight were in the fuselage. Weight added at the center of lift acts on the same moment arm, and exactly offsets the upward bending from the lift. And weight added outboard of the center of lift acts on a longer arm and more than offsets the upward moment from the added lift, resulting in a net decrease in upward bending moment.

[Chris Scott]

As an afterthought, is it fair to say that we've discussed wing bending on the basis of the simple case where the centre of lift always has the same moment-arm about the wing root? On swept-wing a/c it will change with speed, IIRC, but the structures men presumably allow for that. We've also talked about the merits of keeping the wing CG as far out as practicable for so-called wing-bending relief, which also and importantly reduces the stress on the wing root. Apart from design considerations such as the positioning of any engine mountings and the fuel tanks this can be achieved by the common practice of using fuel from the outer tanks last.

But a less-used, temporary method of wing-bending relief is to move the centre of lift towards the fuselage. On the Vickers/BAC VC10, for example, this was employed at high weights in the climb below F/L 240 by angling the ailerons, which were in the traditional position near the wing tips, slightly upwards. The system was known as aileron upset. With all its engines mounted on the fuselage, plus a centre-section fuel tank, the resulting deep wing root of the VC10 was a big design issue in terms of drag and weight. Consequently the VC10 and Super VC10 were less efficient in the cruise than their long-haul rivals, such as the B707-320B/C, which also enjoyed the twin advantages of a higher MZFW and lower APS weight.

[pilotmike]

AfricanSkies says:Once the wings are fuelled, they provide a balance to the central fuselage mass, in effect lifting it, with the gear being the pivot, and the total load on all the gear is less. Sounds strange...

Strange?!!!! Impossible more like! Unless the fuel has a negative mass. You learn it all here.:ugh:

[Taylor G]

Never come across an aircraft that allows more payload once the fuels on board when zfw limited.....

For interest, in addition on the 757 if the wing tanks are full, and fuel in the centre tank must be added to the zfw - that can make it very limiting !

I think the rule is to add centre tank fuel to the ZFW (or reduce the MZFW) if there is fuel in the centre tank with wing tanks less than full.

[Goldenrivett]

Airframe manufacturers seem to have a much better understanding of avoiding fatigue cracks in wing structures now.

I remember the Trident wing crack problems in 1977 https://www.flightglobal.com/FlightPDFArchive/1977/1977%20-%202406.PDF
Extra metal straps fitted under the wing and wing lift redistribution with less lift outboard (ailerons rigged upwards) and more lift inboard (flaps rigged slightly extended) in an attempt to extend the fatigue life.

Weight not carried in the wings will result in extra wing flexing / bending / stressing somewhere.

[green guard]

With no fuel in the wings, certain aircraft can only take so much weight on the gear. Once the wings are fuelled, they provide a balance to the central fuselage mass, in effect lifting it, with the gear being the pivot, and the total load on all the gear is less. Sounds strange but once the wings are fuelled, more payload may be loaded. MZFW is a structural limit.
Why make it simple when it can be complicated ?!!

MZFW is a SUM of maximum payload added to aircraft that has or has not any fuel.
Period.
How will you load the fuel is another subject,
that can limit Max Fuel if wings are not full, but will not limit the MZFW.

[Flion]

MZFW is a SUM of maximum payload added to aircraft that has or has not any fuel.

You mean traffic load?

However, it's not the formula that bothers me, rather the practical use of MZFM. A/C can be limited either on take off by MTOM or on landing by MLM, but what a MZFM limitation on take-off represents? Ensuring wings won't bend in scenario we lose all fuel in flight? I wonder 2 things:

How often MZFM limited MTOM happen in a real world? How often have you had to reduce your TOM due to MZFL limit?
Is it a limit load, or the safety factor is reduced? In other words, if such a scenario zero-fuel in flight happens, and we're at the MZFM, does the limit ensure the wing won't bend permanently, or they will bend but not fail completely?