Why have a Max Zero Fuel Mass?
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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.
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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 !
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 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.
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.
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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.
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.
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.
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.
Last edited by Genghis the Engineer; 22nd Mar 2003 at 12:53.
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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.
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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.
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.
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.
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.
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!
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!
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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.
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.
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??
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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.
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.
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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.
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.
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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
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
Last edited by Keith.Williams.; 24th Mar 2003 at 18:30.
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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.
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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.
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.