View Full Version : Why have a Max Zero Fuel Mass?


OBK!
22nd Mar 2003, 01:47
Just wanted to know what the point of a maximum zero fuel mass was?

Cheers for any comments.:p



AfricanSkies
22nd Mar 2003, 05:51
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
22nd Mar 2003, 07:53
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
22nd Mar 2003, 08:46
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.

G

ATPMBA
22nd Mar 2003, 11:10
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!
22nd Mar 2003, 11:16
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
22nd Mar 2003, 12:30
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.

G

Georgeablelovehowindia
22nd Mar 2003, 12:36
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
22nd Mar 2003, 16:51
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
22nd Mar 2003, 17:39
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.

G

Bellerophon
22nd Mar 2003, 18:56
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! ;)

Regards

Bellerophon

Dragon Knight
23rd Mar 2003, 09:15
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
23rd Mar 2003, 16:26
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
23rd Mar 2003, 17:26
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
24th Mar 2003, 02:18
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
24th Mar 2003, 09:05
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
24th Mar 2003, 14:41
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.
24th Mar 2003, 18:19
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
29th Mar 2003, 01:10
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.
29th Mar 2003, 02:21
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
29th Mar 2003, 05:41
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.

G

Keith.Williams.
29th Mar 2003, 16:36
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
29th Mar 2003, 17:14
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"?

G

Keith.Williams.
29th Mar 2003, 22:51
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
31st Mar 2003, 01:08
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").

G

HOMER SIMPSONS LOVECHILD
1st Apr 2003, 03:21
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
1st Apr 2003, 05:05
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.

Mark