Daleaholic
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How Do You Balance And Aircraft?

Sun Oct 19, 2008 11:29 pm

Now I'm not an engineer or the like, I'm merely an enthusiast, so please don't jump down my throat  Silly

I often hear people saying that an aircraft needs to be balanced and this is very obvious. My question is, how do pilots/rampers/agents know how much weight is where? Do the landing gears have some kind of measuring device which can determine how much weight is being put on top of it?

Just something I thought about...

Thanks in advance  Smile
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Starlionblue
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RE: How Do You Balance And Aircraft?

Sun Oct 19, 2008 11:49 pm

They do it the old fashioned way: paper and pencil. Or with a computer.

That is, they do the calculations without help from weight sensors on the gear. They estimate given number of pax and cargo in sections of the aircraft.
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Soku39
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RE: How Do You Balance And Aircraft?

Mon Oct 20, 2008 12:37 am

Well which one are you talking about? There's a weight and balance done for every flight, and there is also a weight and balance done every time permanent equipment is added or removed from the airplane. The first is the responsibility of the pilot, the second is done by an A&P.
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wilco737
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RE: How Do You Balance And Aircraft?

Mon Oct 20, 2008 1:46 am



Quoting Daleaholic (Thread starter):
I often hear people saying that an aircraft needs to be balanced and this is very obvious. My question is, how do pilots/rampers/agents know how much weight is where? Do the landing gears have some kind of measuring device which can determine how much weight is being put on top of it?

Hi Paul.

the loadmaster gets a loading plan where it says how many baggages he has to put. Or container or pallets.
I explain it on our cargo flight. We have several positions in our airplane. Upper deck, lower deck. There are so called "positions" on those decks. On that loading plan he can see where to put which container or pallet. He does exactly know the weight of the container and pallets. So, he puts them everywhere on the airplane, according to the loading plan. The weight and balance guys (and girls) did the plan where to put them that the aircraft is properly balanced or "in trim" as we call it  Wink THey have a computer telling them it is fine or not. If not, they have to change and swap the load until the airplane is "in trim". Once loading is finished, we get a "loadsheet" where we can see which position has which container on it and it says exactly how heavy those containers are. And of course our total weight including fuel and the pilots etc.

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HAWK21M
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RE: How Do You Balance And Aircraft?

Mon Oct 20, 2008 9:25 am

Out here The Aircraft is weighed once in three yrs as per schedule,unless a major alteration in the aircraft necessitates a fresh Weight schedule either through actual weighing or calculation method.
During regular service the L&T [Load & trim sheeter],uses a printed checklist or computer programme & fills up the aircraft accordingly & ensure that the CG is within the permissible band.

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airxliban
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RE: How Do You Balance And Aircraft?

Mon Oct 20, 2008 1:19 pm

Out of curiosity, what percent of MTOW does a full passenger and cargo load constitute on a larger airliner i.e. 340/777 sort of thing - and is this ratio constant from small aircraft to large aircraft?

Do loadmasters of large aircraft ever have to shift passenger seating around to maintain a desired balance? What about a loadmaster on, for example, a Saab 340?
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futureualpilot
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RE: How Do You Balance And Aircraft?

Mon Oct 20, 2008 1:35 pm



Quoting Airxliban (Reply 5):
Do loadmasters of large aircraft ever have to shift passenger seating around to maintain a desired balance? What about a loadmaster on, for example, a Saab 340?

Yes, sometimes they do in order to move the CG within acceptable limits.
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Tristarsteve
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RE: How Do You Balance And Aircraft?

Mon Oct 20, 2008 2:52 pm



Quoting Airxliban (Reply 5):
Do loadmasters of large aircraft ever have to shift passenger seating around to maintain a desired balance?

We carry a lot of freight on our B763s, and all the pallets have to go in the fwd hold because the rear hold door is too small.
If we have a light passenger load, it is very important that the pax sit down the back to balance the freight. The aircraft is usually near the fwd CG limit anyway.
But this is done at the planning stage a few hours before departure. Seats are blocked off in the DCS computor to force the pax to sit towards the rear. Even then, with a few no-shows, we have to offload a pallet for CG reasons, or call Cargo and ask them to break one down into containers, that can go in the rear hold.
Don't have this problem on A330 or B777 as they have large cargo doors fwd and aft (well ours do) so freight can go in the aft hold if required.
 
Tristarsteve
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RE: How Do You Balance And Aircraft?

Mon Oct 20, 2008 2:55 pm

p.s a lot of B767 have a small fwd freight door and cannot carry pallets
 
tdscanuck
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RE: How Do You Balance And Aircraft?

Tue Oct 21, 2008 12:13 am



Quoting Airxliban (Reply 5):
Out of curiosity, what percent of MTOW does a full passenger and cargo load constitute on a larger airliner i.e. 340/777 sort of thing - and is this ratio constant from small aircraft to large aircraft?

On a 777-300ER it's about 20% (154000/775000). I suspect it's pretty similar for other large twins.

I think the percentage goes up with smaller aircraft since the payload, relative to the aircraft, gets higher and the range goes down (so more of your total weight is actual people/cargo, as opposed to fuel).

Tom.
 
wingscrubber
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RE: How Do You Balance And Aircraft?

Sun Oct 26, 2008 4:37 am

Weight and balance! Very important, I have no experience with loading aircraft for scheduled passenger service but this is what I know...

For my final year design project at uni (remote controlled plane) I had to calculate the weight and balance; I did so by estimating my centre of lift at the quarter chord of the (straight) wing, I then lumped my various components into their own specific categories and calculated their weight based on volume and density of the materials, and then estimated their individual CofGs. Once I had the CofG and weight of each 'module' or component, such as engine, centre fuselage, tail, battery, fuel tank, landing gear etc, I then worked out the distance between each of the CofGs and the quarter chord which is where I wanted the whole aircraft CofG to be. With the distances worked out, I could then work out the moments of each by multiplying the weight by the distance. For moments behind the quarter chord you take the distance as a negative number, so that gives a negative moment, for moments in front of the quarter chord the distance is a positive number which gives a positive moment.
After calculating all the moments, with negative for behind the target CofG and positive for in front, you add them all up. If the design is 'balanced', the sum of all the moments should come to zero or very close to it. If the number is greater than zero, you know your CofG is in front of your estimated centre of lift, if it's less than zero then your CofG is behind your estimated centre of lift.
You can create an excel spreadsheet to do all of the above very quickly, and then you can also make a pretty graph of the weight distribution of your aircraft.

I'm also learning to fly, and as a student pilot, you're taught a very similar process, although with a Cessna 172 pilots operating handbook there are lots of helpful tables and graphs to help you work out whether you'll be in balance much more quickly; no excel spreadsheets necessary, just a calculator.

And finally, in a real world design environment, at work (I'm an aircraft systems design engineer) if I make a design change which has any kind of weight impact I have to go talk to the 'weights engineers', who will then tell me if my design change is catastrophic or insignificant, hehe... Mostly they're happiest when the airplane is getting lighter  Smile
Seriously though, weight and balance all of a sudden gets more complicated when you have swept wings, variable flap settings, spoilers and fuel tank cross-feed to think about, so the weights engineers obviously go into more detail than I did with my remote controlled plane.

And finally, google turned up this-
http://www.eflite.com/software/wb_demos/301/84/
It seems you can buy software to balance the aircraft for you, and I imagine most airlines have something similar.
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FredT
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RE: How Do You Balance And Aircraft?

Sun Oct 26, 2008 7:11 am

http://www.airliners.net/aviation-fo...5&searchid=118697&s=OBWBS#ID118697

On board weight and balance systems (i e weight sensors in the struts) have been tried a number of times with less than satisfactory results. There are too many potential disturbances for it to be the primary means of doing the weight and balance.

Wingscrubber,
so you're one of those people who are always trying to add 2 kg gizmos!  Wink
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