Why have a Max Zero Fuel Mass?
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 ?
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.
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.
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
Join Date: Jun 2000
Location: last time I looked I was still here.
Posts: 4,507
Likes: 0
Received 0 Likes
on
0 Posts
Join Date: Jun 2011
Location: West of Offa's dyke
Age: 88
Posts: 476
Likes: 0
Received 0 Likes
on
0 Posts
@ 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.
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.
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.
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.
Join Date: Mar 1999
Location: Someday I will find a place to stop
Posts: 1,021
Likes: 0
Received 18 Likes
on
6 Posts
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.
Join Date: Jun 2011
Location: West of Offa's dyke
Age: 88
Posts: 476
Likes: 0
Received 0 Likes
on
0 Posts
@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
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
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.
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.
Join Date: Jun 2011
Location: West of Offa's dyke
Age: 88
Posts: 476
Likes: 0
Received 0 Likes
on
0 Posts
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?
PS, just to be sure I don't say something stupid, do you know what the associated fatigue lives were?
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?
lifetime maximum bending moments/cycles with the lower MTOWs reducing the deflections?
Join Date: Jun 2011
Location: West of Offa's dyke
Age: 88
Posts: 476
Likes: 0
Received 0 Likes
on
0 Posts
@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!
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!
Last edited by Owain Glyndwr; 6th Jan 2017 at 15:26.
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?
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?
Join Date: Jun 2011
Location: West of Offa's dyke
Age: 88
Posts: 476
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
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.
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.
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.
Join Date: Aug 2006
Location: USA
Posts: 142
Likes: 0
Received 0 Likes
on
0 Posts
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
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
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.
Join Date: Jun 2011
Location: West of Offa's dyke
Age: 88
Posts: 476
Likes: 0
Received 0 Likes
on
0 Posts
@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
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