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Unit used to measure ARM in Airlines

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Unit used to measure ARM in Airlines

Old 01-11-2013, 06:23 PM
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Default Unit used to measure ARM in Airlines

"INCH" is the unit usually used to measure ARM in a weight and balance calculation. However airlines with it's longer planes, "inch" would cause the arm numbers to be too great.

Having said, what unit of measurement do Airlines use to measure ARM in a W&B???

Thanks
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Old 01-11-2013, 06:58 PM
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I don't fly jets yet, but from what I have seen in the Aircraft W&B manual (FAA-H-8083-1) the arm is calculated by inches from the reference datum... But I might be wrong as well...
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Old 01-11-2013, 08:16 PM
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Originally Posted by Squawk87 View Post
I don't fly jets yet, but from what I have seen in the Aircraft W&B manual (FAA-H-8083-1) the arm is calculated by inches from the reference datum... But I might be wrong as well...
Thanks for the info. However I think on a B747, measuring thing in inches would bring the arm to a rather big numbers.
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Old 01-11-2013, 08:28 PM
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Research mean aerodynamic chord, that's the unit that has been used on both the Saab and CRJ I've flown.
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Old 01-11-2013, 08:55 PM
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Originally Posted by SongMan View Post
"INCH" is the unit usually used to measure ARM in a weight and balance calculation. However airlines with it's longer planes, "inch" would cause the arm numbers to be too great.

Having said, what unit of measurement do Airlines use to measure ARM in a W&B???

Thanks
It's still inches. Moments, however, are normally divided by a constant and then called "Moment Index". This simplifies weight and balance computations where the numbers would otherwise become really big.

Last edited by OSAVIP; 01-11-2013 at 09:01 PM. Reason: typo
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Old 01-12-2013, 05:14 AM
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Still inches. But most airliners and larger aircraft use approved weight and balance programs that use "zones" to facilitate faster computations. The CRJ I flew had 4 zones: A, B, C and cargo. I don't remember specifically, but zone A was something like rows 1-4, B: rows 5-8, C: rows 9-13.

The FA counted passengers in each zone, and we used a "whiz wheel" to spin the numbers and figure out if we were in limits or not.


(the calculator has two other layers not depicted that allowed us to move stuff into the computation on the top half that resulted in a change on the bottom half)
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Old 01-12-2013, 01:56 PM
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From a recent weighing - BOW = 403029, arm = 1371,
moment = 552712534 lb/in. So yes, maintenance uses inches to figure the moment. But the crew uses index units to compute W&B so we never see those numbers.
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