Why have a Max Zero Fuel Mass?
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.
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.
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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.
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.
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"?
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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.
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.
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").
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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?.
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?.
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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.
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.
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MZFW
I was doing some searching around on the topic, and came across this old thread here 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.
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.
Moderator
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.
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.
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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.
I believe the post was enabled by the mods who have opened up the thread.
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)
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)
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.
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.
Last edited by Metro man; 4th Jan 2017 at 10:18.
Moderator
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.
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.
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.
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.
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!
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!
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One thing the 757 never suffered from was lack of fuel capacity or the ability to lift a huge payload!
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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.
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.
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.
Newer versions of the A320 have load alleviation software in the FBW system were spoilers deploy to reduce wing bending at high load factors.
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.)
"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.)