Zero Fuel Weight

[paulsalem]

Can anyone explain the reasoning for Zero Fuel Weight?

Is it an inflight limit, or ground limitation or both.

Thanks

 

[KeroseneSnorter]

Could be both, however most of the stuff I think is an inflight structural limitation.

I have seen a couple that were limited due to gear loading(Wing spar issues at the gear attach points) with empty tanks on the ground.

 

[MauleSkinner]

Ever watch gymnast on the rings? The real hardcore guys do the "iron cross" thing, where they suspend themselves between the rings with their arms horizontal and body vertical?

With fuel generally being stored in the wings, it's like adding weights to their arms and wrists...doesn't have as big an effect on their ability to perform the routine.

Non-fuel items are generally put in the fuselage, which would be like adding ankle weights to our gymnast, severely limiting his ability to perform an iron cross.

The bottom line is the amount of lift that has to be transferred through the spars and wing connections to support the fuselage, as opposed to the amount of lift that's "used up" where it's generated in the wing.

Hope this helps!

David

 

[Workin'Stiff]

What he said....

 

[TonyC]

Can anyone explain the reasoning for Zero Fuel Weight?

Is it an inflight limit, or ground limitation or both.

Thanks

I'm not quite sure what you're getting at, since Zero Fuel Weight is a term that has a definition, not a limit. If it were MAX ZFW or MIN ZFW, it would be a limit, and we might discuss why.

Zero Fuel Weight is the sum of the weights of the empty airplane, the crew, hydraulic fluid, flight attendants, passengers, bags, cargo, and other furnishings. Basically, it's the weight of everything but the Usable Fuel.



If you could shed some more light on why you speak of it as a limit...





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[KeroseneSnorter]

If you could shed some more light on why you speak of it as a limit...
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I had a feeling you would appear on this thread!

 

[paulsalem]

Well in a 310 there is a zero fuel limit in the Limitations section. I understand you can't exceed it but I'm trying to understand why its a limitaion. (Ie what happens if it is exceeded)


We've all seen the video of the 777 with the wing being bent 30 feet or so above horizontal before it snapped.

So if there was fuel in the wings, could it have bent further up? That is what doesn't make sense to me

 

[MauleSkinner]

We've all seen the video of the 777 with the wing being bent 30 feet or so above horizontal before it snapped.

So if there was fuel in the wings, could it have bent further up? That is what doesn't make sense to me

Nope...if there was fuel in the wings, and the total weight was the same, they would have bent less...on the other hand, if you added fuel to the wings you'd get about the same amount of bend in the wings for the heavier weight.

Just to throw a little more confusion into the issue, on Beechjet 400's, fuselage fuel is actually INCLUDED in zero fuel weight. Again, this fuel is in the fuselage, increasing the bending moment on the wings in flight. On the 400A's and Hawker 400XP's, they changed the way the fuel is burned, so fewer Beechjet pilots accidently violate their limitations section in them

Fly safe!

David

 

[paulsalem]

Ok, its making more sense now.

Thanks

 

[KeroseneSnorter]

Ok, I just can't wrap my head around it.

We've all seen the video of the 777 with the wing being bent 30 feet or so above horizontal before it snapped.

So if there was fuel in the wings, could it have bent further up? That is what doesn't make sense to me

Think of it as a 12 foot long 2 x 4 laying across two saw horses. The middle of the 2 x 4 is where the Cabin would be and the board is the wing spar. In flight all of the weight is being supported by the wing spar.

If you push down on the middle of the board it will flex without too much pressure. If you press down further out on the board (wing) it does not flex as much as it does in the very center.

In flight the aircraft is rated for a given amount of g force. Typically +2.5 to 4 or so and -1.0 to 1.5 or so in a transport catagory airplane. As long as the weight is evenly distributed across the entire wing it will support the load. However if you concentrate too much weight in the middle (you sit or jump on the board) it will not be able to support near as much total weight as it could if it were spread out across the entire length of the board.

Basically it is the structural weight limit for the wing roots. Kind of the opposite of holding your arm straight out and putting 10 pounds on your hand or putting 10 pounds just off your shoulder. The 10 pounds in your hand feels like more than the ten near your shoulder. Same principle as a lever. You can exert more force with a longer lever than you can with a short one.

Another way to look at it would be to tie a rope as tight as you can between two trees trunks about 3 feet high. Sit or stand on the rope close to the tree and you only sag a little bit. Climb on to the middle of the rope with the same weight and the rope will probably sag all the way to the ground. Again it works much like a lever.

Not a very scientific answer, but hopefully it gets the point across.

 

[bigD]

I'm not quite sure what you're getting at, since Zero Fuel Weight is a term that has a definition, not a limit. If it were MAX ZFW or MIN ZFW, it would be a limit, and we might discuss why.

Zero Fuel Weight is the sum of the weights of the empty airplane, the crew, hydraulic fluid, flight attendants, passengers, bags, cargo, and other furnishings. Basically, it's the weight of everything but the Usable Fuel.

.

Heh - this is really pedantic, even for you, Tony!

 

[pilotmiketx]

Don't forget floor limits...

 

[USMCmech]

Basically it is the structural weight limit for the wing roots.

That is it in a nutshell.

Basicly the zero fuel weight stops you from loading all the load in the center of the span. Since the load is spread across the span somewhat more evenly, the wing attach fittings and hardware can be be smaller and lighter.

By placing the load of fuel in the wings the wing attach fittings do not have to cary that load, just the load of the fueslage and it's contents. When you are talking about airliners, and even buisness jets that adds up to several tons. Multiply that by the design load factor, and then the enginers fudge factor, and you are talking about some serious weight.

As stated any fuel in the fuselage tanks is included in the "zero" fuel weight, since it is adding to the load in the center.



FYI the wingroot of the 747 is attached by hundreds of 1 inch bolts streching from the front spar to the rear spar. I had to X-ray every single one back in July. After crawling into and out of the center tank 8 hours a day for 5 days I was a walking bruise.

Anybody want to consider a career in aircraft maint?

 

[TonyC]

Heh - this is really pedantic, even for you, Tony!

I guess it depends on how you use it every day. I look at Zero Fuel Weight every time I fly, but I never think of it as a limit. In fact, the only thing remotely related to a limit and Zero Fuel Weight for me and the 727 is the Min Flight Weight for some of them. If the Min Flt Wt is 100,000# and the Zero Fuel Weight is 95,000#, I must designate 5,000# of fuel as Min Flt Weight Fuel to comply with the FAA mandated restriction (something about floor loading & flexing).

Other than that, my dealings with ZFW are brief. My Flight Plan / Release is predicated on a predicted ZFW. If the actual ZFW varies from that, I have to consider how it will affect my fuel burn, and determine if I have enough fuel aboard.

In all of that, the ZFW is just a number, as the definition would imply. While there might BE a Max ZFW, I'm not aware of it, for it does not come into play in our operations. OR, I guess it would be more accurate if I were to admit that IF it comes into play, the ramp-provided Weight & Balance will have already taken that into consideration.

Let's save pedantic for something like the "with you" thread.





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[Donsa320]

In all of that, the ZFW is just a number, as the definition would imply. While there might BE a Max ZFW, I'm not aware of it, for it does not come into play in our operations. OR, I guess it would be more accurate if I were to admit that IF it comes into play, the ramp-provided Weight & Balance will have already taken that into consideration.

Let's save pedantic for something like the "with you" thread.


TonyC

The Maximum Zero Fuel Weights for our 727s were 140,000lbs for the B-727-2S7 and 141,000lbs for the B-727-2M7. In the manual under "Limitations".

As you say, the loading programs should have that built in. (I hope) <grin>

DC





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[TonyC]

The Maximum Zero Fuel Weights for our 727s were 140,000lbs for the B-727-2S7 and 141,000lbs for the B-727-2M7. In the manual under "Limitations".

They gave you a MANUAL!

Man, do I feel ripped off!











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[Donsa320]

They gave you a MANUAL!

Man, do I feel ripped off!











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Ha, I know what you mean...it was a different world back then. The bad part was...we were supposed to know what was in it.

 

[CutEmUp]

Don't you use your ZFW to figure your gross weight?

ZFW + Fuel on board = gross weight

 

[TonyC]

Don't you use your ZFW to figure your gross weight?

ZFW + Fuel on board = gross weight

The formula is correct, but I don't do it. The Ramp Agent plugs the numbers in the computer (the computer handles all the limits automatically) and the Weight & Balance manifest is printed out for me. We plug the TOCG and TOGW into the performance computer along with the environmental variables, and it spits out Takeoff Performance data. I sign it, and we close the door.




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[CutEmUp]

Wouldn't you need it if you get a hold or something and got to figure holding speed/epr/fuel flow? You never know