MZFM - why have one?

[Alex Whittingham]

Whilst explaining the loads on an aircraft to a class of ATPL students I found myself comparing MZFM and MLM. The loads on the main spar in both cases seem to be acting through pretty much the same points and a main stress point in each case seems to be the wing roots. The dynamic loads on landing are obviously considerably greater than the static loads for MZFM.

http://www.bristol.gs/download_files/loads.jpg

This makes me wonder, if you have to comply with landing loads at 10fps at MLM for a normal landing and 3G for an emergency landing for certification, why bother at all with the static loads at MZFM that seem on the face of it to be less limiting?

I suppose one answer could be that the continuous static loads at MZFM have a comparable effect on fatigue to the fewer short term dynamic loads on landing, is it that?

It also occurred to me that the lateral CP might be quite a bit further outboard than the main gear, leading to a greater moment on the wing root in flight than on the ground and making 'in flight' the limiting condition for MZFM. In this case the two limits would not be so easily compared.

A third possibility is that the main gear mounting points are the limiting structure for MLM and the wing roots for MZFM.

Are there any structural specialists out there that could give me a steer?

[fernando]

I saw a picture of an aircraft on the dessert with two blocks of concrete hanging underwing from the points where usually we have the engines.

This is not an answer to your question but it might help.

[inbalance]

And don´t forget the Fuel and think why there is a zero fuel weight..

[FE Hoppy]

Firstly
ITS A TRISTAR!!!!!!!

consider the two things together.

aircraft at max zero fuel landing at max landing weight.

max landing - max zero fuel = fuel in wings( bending relief) the little arrows on the wing tips.

now same ac above max zero fuel at max landing weight

less fuel in wings smaller little arrows on the tips. ergo more stress on roots.


on the L1011s with 1and 3"A" tanks. if the 1 and 3 are not full the "A" tank fuel is considered zero fuel weight. The A tanks are in the belly.

[Knold]

Such an error requires two corrective statements. It's a Tristar! ;)

[BoeingMEL]

Who cares? Just the sight of the name "Alex Whittingham" brings me out in a sweat thinking about doing Perfs A&E in an old timber shed at Lydd about 150 years ago! Great to see you're still ezplaining the inexplicable Al. Hope you're keeping well mate. BM

[nzer]

Just out of interest, which jurisdiction's ATPL syllabus requires anyone to know all sort of this stuff ? Seems archaic to me (albeit possibly of interest to academics)

[411A]

Seems to me that this is rather basic stuff that any professional pilot on large jet transports really should know.

A bit more to flying than just knowing (or think knowing) which button to push on the FMS.....:uhoh:

[Alex Whittingham]

Its the JAA ATPL. At the moment pilots just have to be able to say that MZFM is created because of 'stresses at the wing roots' but there are other objectives, as yet unexamined in much detail, that require a knowledge of the loads on the wing, fuselage and empennage on the ground and in flight and need a knowledge of the structural limits and their origins. This question is for me, so I can get the training material right.

G'day BIK, hi BM, do you still remember all that Performance? figure 18 temperature trap??!?

FE Hoppy, does MLW - MZFW = fuel in the wings? Not in my experience, but I'm prepared to be proved wrong.

[FE Hoppy]

If its not fuel what is it??

Anything that isn't included in the "zero fuel weight" must be "fuel".
I don't think you will find an aircraft that doesn't use centre line fuel first. Unless it's being used for logitudinal balance.
So it's "in the wings".

[Alex Whittingham]

Oh, I get you, sorry.

LM - ZFM = fuel in the wings, I agree with. I don't think its true of the limits, MLM and MZFM, though. If it is, that may be the answer to the question.

Not many aircraft count centreline fuel to the ZFM. The TriStar is the only one I can recall, apart from tankers.

[BoeingMEL]

Temp trap Al... never forgot it but not too important now. Diagnosed with MS Feb 97..but CAA medics were brill... let me back into the saddle for 2 years while it was in remission. Not too much call for drivers with walking sticks and useless right eyes anymore! Still smiling every day tho. Cheers mate, BM

[NigelOnDraft]

Alex...

<<Not many aircraft count centreline fuel to the ZFM. The TriStar is the only one I can recall, apart from tankers>>

Not in normal circs - I agree. However, when you get to "abnormals" it does - certainly our 757s, with all the TWA800 fuel pump problems - meant that if we got unusable C Tank fuel, with anything other than full wings, than we had to check that ZFW + C Tank Fuel was < MZFW.

NoD

[slice]

Come on then 411A - if it is so basic you should be able to pound out on your keyboard blindfolded, the details of the structural reasons for MZFM and MLM being somewhat different to us mere mortals. And master, while you are at it explain to my plebian mind the structural situation with regards to high wing jet aircraft (BAe146, C5, C141, IL76 etc) given that there is no bending moment from the undercarriage at touchdown.:8

[john_tullamarine]

I suspect that the centre tank issue is that the normal fuel usage specified incorporates consideration of wing bending.

It will only be when you are doing something non-standard with fuel loading that you need to worry about including centre/belly aux tank fuels in the ZFW.

Slice .... can we please keep it a bit nice ? I have no interest in your thoughts on any particular PPRuNe member but, in tech log, we like to keep things on a more rational plane ....

[Captain Stable]

Quite a few high-wing aircraft DO have a bending moment on landing - F27, F50, Dash-7, Dash-8...

And slice, I echo john t's plea.

[slice]

John and Captain - understood, but posted only to draw the individuals attention to the fact that the patronising tone of the post seems to belittle those just seeking information.

[john_tullamarine]

411A is an interesting chap and of the "old school" in many of his views. Whether you do, or do not, like that aspect of his makeup is for you to decide.

He does, of course, have a keenly honed and skilled talent for engendering spirited discussion ..... however he is, so far as I have seen, not prone to falling to inappropriate posts in this forum.

All we ask is that posters to this forum retain a reasonable level of decorum .. there are alternative places to express more emotive views on life, death, and the universe ....

So far as 411A's post is concerned, it does not appear to be patronising (a personal view only - by all means stick to your contrary view) but echoes a view held by many in the Industry that knowledge standards have fallen dramatically over the past 20 years.

[aztruck]

B737 manual states that wing tanks must be full if more than 800kgs in the centre tank, although, as previously mentioned, this can be varied as a result of the Fuel pump AD that is ongoing.
My money's on wing bending stress upward for the ZFW (sorry mass) and wing bending stress downward plus localised upward force from the gear attachments.
Interestingly, when you go from a 300 to a 700 series the wing changes and so does the fuel capacity in it(the 700 carries 1.4 tonnes less).
The 300 zfm is 48307kgs, and its mlm is 51709, a difference of 3402 kgs.
the 700 zfm is now 54657 and mlm is 58604, a difference of 3947 kgs.
The increase in zfm is 6350 kgs, the increase in mlm is 6895.
Perhaps the greater increase in MLM is due to the lesser amount of fuel in the wings for a max weight landing i.e. less bending at the root compared with the 300?
Ps...Alex...are you still doing courses at the Gatwick Hilton, and have you still got the dodgy XJS?

[Alex Whittingham]

Thanks for your replies. Does anyone have a reference book they can recommend for aircraft structures?

Hi aztruc. I'm pleased to say the Hilton courses ended when the old Perf A exam went out the window, no more Hilton breakfasts for me! The dodgy XJS is still with us, the engine's good, a TWR 6.0 litre, but the body is deteriorating. Its in the garage right now having the rust cut out of it.

[Alex Whittingham]

Its a TriStar? Sh*t, I thought it was a DC10!

Thanks for the references. FARs and JARs are nearly identical.

I'm with you on the gear stuff. I can believe that ZFM is limited by the in-flight condition but I can't see any direct link between the loads you quoted and the MZFM limit.

JAR/FAR 25.301(b), which follows the para you quoted and describes how load limits should be etablished for the whole of Subpart C, says that load intensities should 'closely represent actual conditions' which implies, to me, a normal fuel load. JAR/FAR 25.321(3), which deals specifically with flight loads, requires, as you say, any practical distribution of disposable load. Disposable load is traffic load and fuel.

I appreciate your effort, but I just don't see a clear link to MZFM unless you start off by assuming these loads affect the same part of the structure as MZFM and argue a posteriori .