SAAFNAV
Topic Author
Posts: 350
Joined: Wed Mar 17, 2010 5:41 pm

Weight And Balance Measurements

Fri May 20, 2011 9:51 am

Good day,

How are aircraft weighed? I understand it's simple to just weigh them in totality, but how do they go about calculating the CoG etc?

Any explanation of the process with be appreciated!

Regards,

Erich
On-board Direction Consultant
 
kingairta
Posts: 454
Joined: Mon Feb 09, 2009 2:24 pm

RE: Weight And Balance Measurements

Fri May 20, 2011 2:06 pm

The aircraft is weighed with all the equipment necessary to fly minus the fuel and crew.

People smarter then me determine an aircrafts desired CG.

To compute weight and balance one needs a weight and moments.

Moments are drived from a given weight x the arm where the weight is. Arm being the location of the weight measured in inches from the reference datum line usually 30" or so in front of the nose. All measurements are taken from that spot.

Take my kingair 200 for example. It's weight with crew and gear minus fuel is 8,906 pounds with 16,514 moments.
(in this example moments are simplified by a 100 in larger airplanes like the C-130 moments are simplified by 1000)

So as it sits right now the CG is 185.4 inches from the RDL. Those people smarter then me have determined that a safe CG range at 8,906 pouns is 181.0 to 196.4.

Now when you add fuel you add the fuel weight and the moments it creates ( i don't have my chart handy so these numbers may not be all that accurate but enough for a lesson) If I had 2625 pounds of fuel that will give me a moment of 4848.

So now the airplane weighs 11531 with a moment of 21362 so now the plane has a CG of 185.3 Allowable range at this weight is roughly 181.3-196.4 ( i don't have my chart so bear with me)

Now add some passengers to the mix.

We'll say 2. 1 is going to sit at fusalage station 220(also known as the arm) the other one sits at 262.

We use a standard weight of 200 pounds per person and it keeps us pretty safe
So passenger one is 200 pounds and creates 440 moments passenger 2 is 200 pounds and creates 524 moments.

Now if we add in the 400 pounds and the 964 moments to our operating weight of 11531/21362 we get
11931 pounds and 22326 we get a cg of 187.1 the given range for this is roughly 182-196.4

So by knowing the weight and where it is placed on the airplane you do a little math and you find where the balance is. The more weight you add to the aft the farther aft that CG will go.

This is what I do every day. For every flight you also have to figure out the landing CG based on fuel on board. But if you don't do any air drops or tanking in flight if your with in balance at take off and your with in balance at zero fuel landing will be good.

There are other parameters to watch out for as well. Such as max weights, wing fuel limiting etc. Wing fuel limiting is basically a minum fuel load for a given cargo weight. You can't stuff a plane full of cargo and have a very little amount of fuel. If you exceed those limits then there is not enough fuel in the wings to hold the wing down in flight and will cause it over stress and then you could end up with the wing off light.

If you have any questions shoot em my way.

PS there is computer programs out there that will do all the math for you but I have not been privy to any of them and do this by hand for every leg. It gets to be tedious when you know from pure experience that if I put 2 passengers on this plane they could sit just about anywhere and I'll be well with in limits.

Some planes need to know where the CG is so the pilots can set the proper amount of elevator trim for take off. I have not crewed on any of those planes. So in my case all it tells the pilot is your either gonna have a heavy nose, a heavy tail or fairly neuteral feel on rotate. You can tell the difference from one extreme to the other but not to the point you have to worry about a tail strike if your with in limits.
 
SAAFNAV
Topic Author
Posts: 350
Joined: Wed Mar 17, 2010 5:41 pm

RE: Weight And Balance Measurements

Fri May 20, 2011 7:18 pm

Hi KingAirTA,

Thanks for your explanation. On the C-130 we use 'Index' instead of moment arm.. Same principle, but the guys in the white coats with thick glasses wanted a new name.

What makes my work simpler, is that we have loadmasters that do it for us. I just have to check and sign it off. I did a short course on the loadsheet though.

My question really is: during maintenance, how do the physically measure the weight (or I should say mass)?

Do they put a scale under each bogie? And if they do, how do they add it all up? Total the scales, or do you have to do some calcs first?

And after you've got the weight, you need to get to the centre... How is that?

Thanks for your trouble! I appreciate it.

Regards,
Erich
On-board Direction Consultant
 
B727LVR
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Joined: Fri Jul 11, 2008 7:54 am

RE: Weight And Balance Measurements

Fri May 20, 2011 8:04 pm

Quoting SAAFNAV (Reply 2):
My question really is: during maintenance, how do the physically measure the weight (or I should say mass)?

Do they put a scale under each bogie? And if they do, how do they add it all up? Total the scales, or do you have to do some calcs first?

When I was helping weigh MD-80's we put a scale under each tire, except for the nose, the scale provded was large enough to fit under both nose tires at the same time. One the acft I did we defueled the aircraft, and added ballast to the front end. Then totaled up all the weights.

I have been told that some aircraft like the A320 have scale systems that are used injunction with jacking the aircraft. The scale is a jack pad, and is connected electrically to a computer, and any calculations are computed then. I haven been lucky enough to use or see this set-up.

There are companies that specialize in this area. You call them up say what type of aircraft you have, and they come do it for you, with your help of course...
I'm like a kid in a candy store when it comes to planes!
 
kingairta
Posts: 454
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RE: Weight And Balance Measurements

Fri May 20, 2011 8:44 pm

I did weight and balance on C-130s as well. For 12 years. Index is used when you use a load adjuster via the slide rule. The Navy did away with those a long long time ago and went with moments. Spent a day on those at load school in LRF never did get the hang of them. Prefered my pencil and calculator.

The one time I was involved in weighing an aircraft we had a team come out to the unit and we gave them a hand. Basically we but weight sensor between the jacks and the jack pads on the air plane then used the given "axel weights" and figuered out the basic cg from there.
 
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Jetlagged
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RE: Weight And Balance Measurements

Fri May 20, 2011 8:58 pm

Some aircraft have integral weight and balance systems using force measurements in the landing gear struts to compute weight and CG position. It's an option on the the 747 for example. On the Classic it was an analogue system with a calibration and control panel on the F/E panel. It needed careful adjustment, but once set could compute the weight and CG before take off and use fuel quantity system data to display weight and CG throughout the flight. In the 744 the system is entirely automatic and is used as an input to the FMC so the pilot can choose to accept the prompted gross weight and CG or enter the values manually from the loadsheet.
The glass isn't half empty, or half full, it's twice as big as it needs to be.
 
aklrno
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RE: Weight And Balance Measurements

Fri May 20, 2011 9:19 pm

Quoting KingairTA (Reply 1):
There are other parameters to watch out for as well. Such as max weights, wing fuel limiting etc. Wing fuel limiting is basically a minum fuel load for a given cargo weight. You can't stuff a plane full of cargo and have a very little amount of fuel. If you exceed those limits then there is not enough fuel in the wings to hold the wing down in flight and will cause it over stress and then you could end up with the wing off light.

Maybe they could save some weight by omitting the wing off light and let the pilot figure it out by the extreme roll and large negative number in the vertical speed meter.
 
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SEPilot
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RE: Weight And Balance Measurements

Sun May 22, 2011 2:48 am

I weighed my 182 (in 1999) after installing a complete new instrument panel. According to the records, it had not been weighed (officially) since it left the factory in 1962; the weight and balance had been updated by adding and subtracting the weight of items added or removed. Calculating the CG is fairly easy; you take the weight on the nose tire, the weight on the two main tires together, and the distance between them, and it is easy to figure the CG. You then figure where that is relative to the firewall and that is the number you use. On my plane I discovered that the actual weight was about 50 lbs. more than it should have been based on the equipment that had been added and removed over the years plus the modifications I had done; i still don't know where all that extra weight came from. For a plane with an empty weight of around 1200 lbs it was a not insignificant amount.
The problem with making things foolproof is that fools are so doggone ingenious...Dan Keebler
 
SAAFNAV
Topic Author
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RE: Weight And Balance Measurements

Sun May 22, 2011 8:19 am

Thanks guys,

This is what I wanted.

I love learning about the maintenance side... Ground crew never get the glory, but they keep up in the air!

Regards,

Erich
On-board Direction Consultant
 
Tristarsteve
Posts: 3365
Joined: Tue Nov 22, 2005 11:04 pm

RE: Weight And Balance Measurements

Sun May 22, 2011 12:55 pm

Quoting SEPilot (Reply 7):
I weighed my 182 (in 1999) after installing a complete new instrument panel. According to the records, it had not been weighed (officially) since it left the factory in 1962

Airliners are weighed more often!
About every 3 to 4 years.

Last time I was there we used the bathroom scales method, 3 times for an average, moving the scales round each time.
 
kingairta
Posts: 454
Joined: Mon Feb 09, 2009 2:24 pm

RE: Weight And Balance Measurements

Sun May 22, 2011 3:13 pm

Quoting SEPilot (Reply 7):
On my plane I discovered that the actual weight was about 50 lbs.

Paint and dirt and few other things over the years add up.
 
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SEPilot
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RE: Weight And Balance Measurements

Mon May 23, 2011 12:03 am

Quoting KingairTA (Reply 10):

Paint and dirt and few other things over the years add up.

Dirt yes, paint no. The plane still has its original paint, which is considerably degraded from when it was new. Bare patches have been spray-canned, but I strongly suspect that the total paint weight is less, not more, than in 1962.
The problem with making things foolproof is that fools are so doggone ingenious...Dan Keebler

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