W&B - maybe a mistake???

Jaybird180

Jaybird180

Final Approach
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Jaybird180
Prepping for my upcoming checkride and I'm going over the W&B sheet for the aircraft I will be using.

The sheet records the following:
Empty: 1,527.8
Arm: 38.9
Moment: 59,534.4
Gross: 2,400
Useful: 872.2

My calculated moment arrives at the figure of 59,431.42

I don't think my math is off, I even checked it with multiple calculators. What should I do about this:confused:
 
Older airplane?
It could be compounded rounding errors from different revisions. I'm constantly correcting errors on planes I'm inspecting. You could try and trace back though the superseded records to see where the error occurred, then an airframe mechanic can make the required corrections.
 
Jaybird180 said:
Prepping for my upcoming checkride and I'm going over the W&B sheet for the aircraft I will be using.

The sheet records the following:
Empty: 1,527.8
Arm: 38.9
Moment: 59,534.4
Gross: 2,400
Useful: 872.2

My calculated moment arrives at the figure of 59,431.42

I don't think my math is off, I even checked it with multiple calculators. What should I do about this:confused:
Click to expand...

Nothing.........
 
Ahem- Tom, I have to demonstrate this to my DPE. You suggestion is to use the incorrect data?
 
Jaybird180 said:
Ahem- Tom, I have to demonstrate this to my DPE. You suggestion is to use the incorrect data?
Click to expand...

Work the problem with both, and you'll probably find that the difference is insignificant. Divide the moment by the weight and you arrive at an arm of 38.9674041 inches - Yup, 6/100ths of an inch different.

Forget the arm of the empty aircraft. You're adding moments. When you finish, divide by the total weight, and you have your solution, which will hopefully be more than 6/100ths of an inch from the edge of the envelope.
 
flyingcheesehead said:
Work the problem with both, and you'll probably find that the difference is insignificant. Divide the moment by the weight and you arrive at an arm of 38.9674041 inches - Yup, 6/100ths of an inch different.

Forget the arm of the empty aircraft. You're adding moments. When you finish, divide by the total weight, and you have your solution, which will hopefully be more than 6/100ths of an inch from the edge of the envelope.
Click to expand...
Not only is the difference between the two moments insignificant, the entire concept of calculating moments to the hundredth of an inch pound is ludicrous. We didn't have these inflated resolution issues when the calculating was done on a slide rule.:yes:
 
Okay, I get the idea about significant digits.

Regarding W&B problems, I thought that empty weight was to be included in the calculation. Meaning, total of all wights= ramp weight and total of all moments (incl empty moment)= ramp moment. rM/rW=rArm (CoG) and that all I had to do was ensure we do not bust Max Gross or go outside the CoG envelope full and empty (fuel).

Did I get that right?
 
Jaybird180 said:
Okay, I get the idea about significant digits.

Regarding W&B problems, I thought that empty weight was to be included in the calculation. Meaning, total of all wights= ramp weight and total of all moments (incl empty moment)= ramp moment. rM/rW=rArm (CoG) and that all I had to do was ensure we do not bust Max Gross or go outside the CoG envelope full and empty (fuel).

Did I get that right?
Click to expand...
You got it.
 
Our FBO's Cherokee is the same. I reviewed all the maintenance records and found a change that was reflected in logs but not in dispatch folder (it happened when they removed the rear seats).
 
Jaybird180 said:
Ahem- Tom, I have to demonstrate this to my DPE. You suggestion is to use the incorrect data?
Click to expand...

Which is the correct data? the W&B for that aircraft or your calculations?
 
Jaybird180 said:
Okay, I get the idea about significant digits.

Regarding W&B problems, I thought that empty weight was to be included in the calculation. Meaning, total of all wights= ramp weight and total of all moments (incl empty moment)= ramp moment. rM/rW=rArm (CoG) and that all I had to do was ensure we do not bust Max Gross or go outside the CoG envelope full and empty (fuel).

Did I get that right?
Click to expand...

You do include the empty weight.

But, you are provided with weight, arm, and moment on the empty plane. You only need two of three to find the third. You add the weights and the moments (adding arms is meaningless) to get a full weight and moment, and you divide that full moment by the full weight to get the arm to compare with the weight and balance envelope for the airplane.
 
Jaybird180 said:
Yes, if it were my airplane I would ask the IA who performed the annual reweigh the airplane for free.
Click to expand...

Yeah, not practical.
1 the w&b isn't something he's required to inspect, that's PIC territory.
2 you are likely looking at compounded rounding, not a mistake.

However in this case just fill out a discrepancy sheet (your school has those right) on it and see what happens.
 
Skylane81E said:
Yeah, not practical.
1 the w&b isn't something he's required to inspect, that's PIC territory.
2 you are likely looking at compounded rounding, not a mistake.

However in this case just fill out a discrepancy sheet (your school has those right) on it and see what happens.
Click to expand...


Whaddya know...a phone call and now they're going to look into fixing it.

Thanks.
 
There are advantages to letting someone else deal with the problems!
 
Jaybird180 said:
Prepping for my upcoming checkride and I'm going over the W&B sheet for the aircraft I will be using.

The sheet records the following:
Empty: 1,527.8
Arm: 38.9
Moment: 59,534.4
Gross: 2,400
Useful: 872.2

My calculated moment arrives at the figure of 59,431.42

I don't think my math is off, I even checked it with multiple calculators. What should I do about this:confused:
Click to expand...

Based on math rules, anything more than 3 significant digits is invalid and should be rounded off based on the fact that your starting ARM only has 3 significant digits.

Anything that would be rounded off is fair game for variance. As others have said, .06 of an inch is generally insignificant unless it's on the wrong side of the envelope.
 
I read somewhere that A&Ps can use "FAA Approved" tables instead of doing the math and some of those tables have some "slop" in them.

Just passing the old wife's tale along.

I prefer the math be right, but it's the FAA. Who knows! ;)
 
denverpilot said:
I read somewhere that A&Ps can use "FAA Approved" tables instead of doing the math and some of those tables have some "slop" in them.

Just passing the old wife's tale along.

I prefer the math be right, but it's the FAA. Who knows! ;)
Click to expand...


Wt x arm=moment, I use a FAA approved Texas Instuments.

Honestly when I went to ground school and saw the W&B tables they confused the hell out of me. I still do all my calcs the "long" way and ignore the charts other than once I've figgured my weight and arm to ensure it falls within the limits.
 
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