MINIMUM zero fuel weight

[Mad (Flt) Scientist]

Originally Posted by westhawk

I have never before heard of a "minimum zero fuel weight" limitation. Without any wish to oversimplify, I must still ask whether it is possible that this limitation may be applied for the same reason as a "maximum zero fuel weight" limitation, but in the opposite sense? Could there be a structural reason for this limitation as there is with Max ZFW? Suppose full wing fuel were assumed with too little weight in the fuselage between the wings. Might the structures require a minimum weight be carried in the fuselage with wings full in much the same way that a maximum weight to be carried in the fuselage is established with zero fuel?

min Zero Fuel Weight is essentially the minimum design weight....the aircraft simply cannot be any lighter

Originally Posted by FAR. 25.321

(b) Considering compressibility effects at each speed, compliance with the
flight load requirements of this subpart must be shown--
(1) At each critical altitude within the range of altitudes selected by the
applicant;
(2) At each weight from the design minimum weight to the design maximum
weight appropriate to each particular flight load condition;

[glhcarl]

I found a definition of Maximum Zero Fuel Weight: Maximum weight of an aircraft with no disposable fuel or oil.

While I could not find a definition for Minimum Zero Fuel Weight: I would assume it is the minimum weight of an aircraft with no disposable fuel.

[Hufty]

So then, if you do your weight and balance calculations and the aircraft is outside the ZFW envelope but within the take-off and landing envelopes when you have added fuel, does that mean you are exceeding a strucural limitation? If so then, replacing fuel with payload (to raise the ZFW) would solve the problem right?

[westhawk]

Originally Posted by Mad (Flt) Scientist

min Zero Fuel Weight is essentially the minimum design weight....the aircraft simply cannot be any lighter

That's what I found when I checked part 25 too. In AFMs I have reviewed, "minimum flight weight" is the term used to describe the design minimum weight referred to in part 25. No reference is made to any term known as "minimum zero fuel weight" here that I can find. Perhaps that term is a misnomer or even applicable to the regulations of some aviation authority other than the FAA? In any case, min ZFW is not noted as a limitation in the AFMs of the part 25 aircraft I am familiar with, nor can I find any reference to it in the FARs. "Minimum flight weight" is, and I thank you for your attempt to research the meaning of the term which is the subject of this thread. (min ZFW)

Best regards,

Westhawk

[formulaben]

Originally Posted by Mad (Flt) Scientist

I don't think you're understanding my point.

The actual thrust available on a given plane is not relevant to the calculation of VMC.

What matters is the thrust levels which were defined for certification - which can indeed be a function of altitude (and temperature too) - but those may be either above OR BELOW the thrust you have on any given day, on any given aircraft.

This is quite interesting, so I'll add my 2 cents:

I think everyone agrees that actual VMC is based on certain variables. Given that, (and this might be applicable to only small aircraft with props) isn't published VMC calculated with the following criteria?

1) Sea level
2) Standard pressure
3) Max Gross
4) Aft CG (less rudder arm)
5) Banked 5 degrees into the operative engine (or perhaps wings level?)
6) Operative engine at takeoff power
7) Critical engine inoperative, propeller windmilling
8) Out of ground effect
9) Gear down & flaps at Takeoff position

Essentially, this is simulating a V2 cut, where you'd most likely encounter the dynamics of flight at or below VMC. 3 factors tend to help, or bring VMC down: gear down (stabilizing effect, lower CG), gross weight (banking effect greater with high weight), and 5 degrees into the operating engine. All the other factors tend to represented a "worst case" scenario, or tend to raise VMC. Changes in the other factors make VMC go down, such as altitude (less engine power), failed engine not windmilling (feathered; less drag) and operating engine not at max power.

The above is what I recall from my ME training. With larger aircraft, I tend to doubt the supposed stabilizing effect that gear down may have. Also, we've discussed the possibility of gross weight (high AoA) having on rudder effectiveness, possibly offsetting it in some models. And lastly, I've not seen instructors enforce the 5 degrees into the operating engine in the jet realm when training. Am I off-base here?

[john_tullamarine]

Great to see a head down tail up thread ...

Some thoughts ..

..and bank angle into the good engine ..

.. and, indeed, away from the good engine(s). Be very aware that the effect is non linear and, for the adverse bank case, can see the real world Vmca propelled into the stratosphere .. scratching into the memory here but I seem to recall something in the vicinity of a 34kt increase for a old, many-engined bomber for the 5 deg adverse bank case ?

.. Is this the lowest weight for which certificated performance data have been developed ..

Think of it from the other side of the table .. the minimum for which the applicant has elected to show compliance .. hence a limitation.

.. I've not seen instructors enforce the 5 degrees..

Note that the 5 deg is a certification limit (mainly to prevent excessively enterprising endeavours to reduce Vmc) and is relevant mainly at/near the Vmc situation. It is not particularly relevant for general OEI climb .. for this more general case, a somewhat lesser bank angle is appropriate to remove sideslip. Presuming that the "dynamic" Vmc case is more critical than the "static" (and this need not necessarily be the case), the hapless pilot who might permit himself to be caught out might get very embarrassed if he chooses not to use some bank (and rapidly) into the good engines .. the real world effect of bank on Vmca is significant.

.. isn't published VMC calculated with the following criteria ..

It would be better if you read your regs before posting words purporting to be indicative of regs .. think "worst case" and revisit your schedule. Subject to the normal caveat that one needs to pick the regs appropriate to the particular Type's certification basis .. the present FAA rules (25.149) suggest (we could put a few more qualifications on presumptions but the main concern is) ..

(c)(4) The maximum sea level takeoff weight (or any lesser weight necessary to show VMC)

(c)(5) .. except with the landing gear retracted.

.. banked 5 degrees into the operative engine (or perhaps wings level?) ..

If the OEM so chose .. but I would be very surprised. The OEM's aim is to minimise Vmc and wings level doesn't do that ...

[formulaben]

Originally Posted by formulaben

Given that, (and this might be applicable to only small aircraft with props) isn't published VMC calculated with the following criteria?

Originally Posted by john_tullamarine

It would be better if you read your regs before posting words purporting to be indicative of regs...

I wrote it as a question, not a statement. Statements end with periods, not question marks, which you conveniently left out of my quote. I also stated that it was what I recalled from multi-engine training...just hoping to see some discussion on those factors as they relate to VMC. I shall refrain from that though...

[westhawk]

I don't have any idea what a "head down tail up" thread might be, but I suspect it might be not be considered dignified in polite company! Oh, the coin fip thing. Allright.

MINIMUM zero fuel weight

Is this an officially recognized term or isn't it? Or is it like calling an aeronautical chart a map? Or a jet engine a motor? We know what you mean, even though that is not the properly recognized term.


How about it? I'll promise to give up now!

Best,

Westhawk

[john_tullamarine]

formulaben,

My apologies .. not endeavouring to stir up a hornets' nest .. be a good idea to revisit the various parameters ...

Westhawk .. head down tail up .. Oz expression indicating some effort being expended .. cf a student at the desk busily working away at a script.

[westhawk]

Originally Posted by john_tullamarine

Westhawk .. head down tail up .. Oz expression indicating some effort being expended .. cf a student at the desk busily working away at a script.

Thank you John! It is all of that.

Best,

[formulaben]

Originally Posted by john_tullamarine

Westhawk .. head down tail up .. Oz expression indicating some effort being expended .. cf a student at the desk busily working away at a script.

I was curious about that too, John. Thanks, you motivated me to look into the actual rules. I simply hadn't thought to go straight to the source. Interesting stuff. The regs address 5 degrees of bank for Vmca, but nothing about max rudder inputs for the takeoff roll regarding Vmcg.

[411A]

...except that those rudder inputs, in the event of an engine failure, must not exceed a certain value which, as I recall, is a maximum of 150 foot/pounds applied to the rudder pedal by the pilot.
This is from CAR4b...I suspect 14CFR25 is the same.

Older types required a push that a few might find hard to imagine.

[Mad (Flt) Scientist]

Originally Posted by westhawk

Is this an officially recognized term or isn't it?

Just spotted the term "minimum OWE" on a CRJ wieght/cg limitation chart. It's significantly below minimum flight weight. OWE would seem to be pretty similar to ZFW (crew vs no crew?)

[Hufty]

On the aeroplane I fly (EMB 145) the MTOW envelope is obviously the largest, with the MLW envelope inside it and the ZFW envelope inside that. The lower limit for all three is represented by one line, I'm just not sure what the consequences are of operating outside it. Thanks to Mad (Flt) Scientist - you have been offering up a lot of good information.

[Mad (Flt) Scientist]

Originally Posted by formulaben

1) Sea level
2) Standard pressure
3) Max Gross
4) Aft CG (less rudder arm)
5) Banked 5 degrees into the operative engine (or perhaps wings level?)
6) Operative engine at takeoff power
7) Critical engine inoperative, propeller windmilling
8) Out of ground effect
9) Gear down & flaps at Takeoff position

Actually, there's a degree of interrelation between 1, 2 and 6. #1 and #2 aren't actually part of the regs, and the "at takeoff power" part is subject to quite a bit of debate. Exactly how you define the "maximum available power" for the purpose of VMC testing has been changed a few times, plus there's the option to schedule thrust with altitude (and hence provide VMC alleviation at higher alts)

[john_tullamarine]

but nothing about max rudder inputs for the takeoff roll regarding Vmcg

FAR 25.149(e) suggests a max rudder load of 150lb for the current standard. This had been the case for many years. I'll leave it for you to track down but an earlier standard used 180lb for max rudder load.

[gaunty]

j_t

an earlier standard used 180lb for max rudder load.

Aaaaah them were the days, when men flew 'aeroplanes' and the girls served the tea and bickies.

runs and hides

BTW try standing on one leg with the knee slightly bent for more than a minute.

[Mad (Flt) Scientist]

Originally Posted by john_tullamarine

FAR 25.149(e) suggests a max rudder load of 150lb for the current standard. This had been the case for many years. I'll leave it for you to track down but an earlier standard used 180lb for max rudder load.

Voila!.....

Originally Posted by Sec. 25.149 - 1965 original text

(d) The rudder forces required to maintain control at VMC may not exceed 180 pounds nor may it be necessary to reduce power or thrust of the operative engines.

Originally Posted by Sec. 25.149 - Amdt. 25-42, Eff. 3/1/78

(d) The rudder forces required to maintain control at VMC may not exceed [150] pounds nor may it be necessary to reduce power or thrust of the operative engines. ....

The rationale may be found in ...

Originally Posted by Notice of Airworthiness Review Program No. 6; Notice No. 75-25; Issued on 05/29/75.

The proposal for Sec. 25.149(d) would reduce the maximum permissible rudder force used in determining VMC, from 180 pounds to 150 pounds, because flight test experience indicates that 180 pounds may make control too difficult for some pilots under some flight conditions.

All available on the FAA website ww.faa.gov - Historical Federal Aviation Regulations
then select the specific section under "Historical FAR by section", and you'll be presented with the history of the amendments to any given rule.

[non iron]

A penny im the pot.

Interesting ! - in a fake nazi accent - ,
politicians aside, this does`nt make a lot of sense in relation to water ballasted ( possibly a quarter tonne of water in the wings ) sailplanes. They don`t have the benefit of Vspeeds, with Vcs and Vne crossing above 25K, or do they ?

[Mad (Flt) Scientist]

Originally Posted by non iron

Interesting ! - in a fake nazi accent - ,
politicians aside, this does`nt make a lot of sense in relation to water ballasted ( possibly a quarter tonne of water in the wings ) sailplanes. They don`t have the benefit of Vspeeds, with Vcs and Vne crossing above 25K, or do they ?

They certainly don't have to worry about VMC which is applicable only to multi-engined aircraft ...