How Accurate is Airplane W&B?

airdale

airdale

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airdale
The thread on who can sign off W&B reminds me of a question I have always wondered about:

Many of these old airplanes we fly were weighed 30-40 years ago with different paint, different interior, different avionics, ... etc. Also without all the crud that has accumulated over the years.

The W&B sheets attempt to track changes but after ten or more changes, including maybe a paint job, can we expect the numbers to be anywhere near reality? I can believe that they are close enough for practical use, but accurate to a couple of decimal places? I doubt it.

Certainly someone here has re-weighed an airplane and checked the result against its cumulative W&B calculation. What was the result?
 
airdale said:
The W&B sheets attempt to track changes but after ten or more changes, including maybe a paint job, can we expect the numbers to be anywhere near reality? I can believe that they are close enough for practical use, but accurate to a couple of decimal places? I doubt it.
Click to expand...

Sort of reminds me of the hierarchy of engineering estimates:

guess, wild guess, educated wild guess, calculated educated wild guess, and rectorandom guess (pulling a number right out of your a$$).

Jim
 
I weighed mine after I did the alternator and avionics upgrade, lost 200lbs of empty weight, much of it 3 generations of avionics in the nose.
 
airdale said:
The thread on who can sign off W&B reminds me of a question I have always wondered about:

Many of these old airplanes we fly were weighed 30-40 years ago with different paint, different interior, different avionics, ... etc. Also without all the crud that has accumulated over the years.

The W&B sheets attempt to track changes but after ten or more changes, including maybe a paint job, can we expect the numbers to be anywhere near reality? I can believe that they are close enough for practical use, but accurate to a couple of decimal places? I doubt it.

Certainly someone here has re-weighed an airplane and checked the result against its cumulative W&B calculation. What was the result?
Click to expand...

I would trust most mechanics to do the math "Removed 5 pound radio at station XX.YY, replaced with 3 pound radio at station XX.ZZ" I wouldn't trust them to accurately calculate the CG after an interior or paint job. IMO, that's time to re-weigh the airplane.
 
Henning said:
I weighed mine after I did the alternator and avionics upgrade, lost 200lbs of empty weight, much of it 3 generations of avionics in the nose.
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Not sure what you mean. Was the airplane 200# lighter than the cumulative W&B sheets said it was? Someone just forgot to recalculate when 200# of avionics was removed? That's a pretty big mistake!
 
Depending on how old your plane is, it has been known for decades that manufacturers back in the late 40's to probably the mid 60's flat out lied about how much their product weighed right out of the factory. I guess this is why we end up with regulations.

It's similar to how you could purchase a 120 volt air compressor 10 years ago with a 5 hp motor despite the fact that, since 1 hp = 746 watts, it is completely impossible to have more than 2.4 hp from a 120 volt electric motor running 15 amps. This is why the same compressor today, with the same exact motor is only 2 hp.

So without regulations what is a manufacturer gonna do? Tell the truth while his lying competitor has you believing his model has 200 lbs more useful load?

So, for anything really old, you're never going to be sure unless you just get it weighed. Even if not official, maybe you know a buddy with some race scales? At least you'll know.
 
We've held "weigh-ins" at several Grumman fly-ins, and our experience is that airplanes of the light single engine class gain about 1 lb of undocumented weight each year after their last actual weighing. IOW, if you take an airplane that's been 30 years since its last weighing, with everything done by computation since then, it would be unsurprising to find out it actually weighs 30 lb more than the paperwork says it does.
 
airdale said:
Not sure what you mean. Was the airplane 200# lighter than the cumulative W&B sheets said it was? Someone just forgot to recalculate when 200# of avionics was removed? That's a pretty big mistake!
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Didn't bother to calculate new W&B after the work, we just weighed it, much simpler. I personally think weighing the airplane should be part of a purchase, it's not expensive.
 
A typical older Bonanza weighing ends up gaining 50 to 100 pounds if it hasn't been done since leaving the factory.
 
My EAB Home built gained weight from when it was built. I can not remember the actual numbers but I believe it was in the 40 lbs range. Its 10 years old. I weigh it with every condition inspection.

Tony
 
I just had my 150 W&B updated. I lost 4 pounds! I was able to get the original W&B from Cessna, and we went from there. I was afraid to weigh it just because I need as much useful load as I can get, and as others have said you typically gain weight over the years.
 
So, OK, you want to weigh your aircraft or maybe a lot of aircraft on a single day. Where do you rent, borrow, or steal a couple of scales for a day?

Jim


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weirdjim said:
So, OK, you want to weigh your aircraft or maybe a lot of aircraft on a single day. Where do you rent, borrow, or steal a couple of scales for a day?

Jim


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Click to expand...

Check with a local EAA chapter. A lot of chapters have scales that you can borrow or rent.
 
weirdjim said:
So, OK, you want to weigh your aircraft or maybe a lot of aircraft on a single day. Where do you rent, borrow, or steal a couple of scales for a day?
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Local automotive racing supply shop -- necessary for sanctioned racing. But you only use three out of four pads. :D
 
weirdjim said:
Sort of reminds me of the hierarchy of engineering estimates:

guess, wild guess, educated wild guess, calculated educated wild guess, and rectorandom guess (pulling a number right out of your a$$).

Jim
Click to expand...

I had been saying "rectal extraction" but I like rectorandom better! :lol: :yes:

As far as Aviation W&B, the program does the math correctly but... GIGO. Garbage in, garbage out.
 
JB1842 said:
I just had my 150 W&B updated. I lost 4 pounds! I was able to get the original W&B from Cessna, and we went from there. I was afraid to weigh it just because I need as much useful load as I can get, and as others have said you typically gain weight over the years.
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Reminds me... My mechanics tell me that often when aircraft move from low elevation to being based in Colorado they lose a small amount of weight (without changes). Lots of theories as to why.
 
My plane hadn't been weighed in 50 years when I got it and had pages of add this, substract that corrections. Oddly enough when I weighed it, it was within a couple of pounds form what the computed W&B papers said it was.
 
petrolero said:
Reminds me... My mechanics tell me that often when aircraft move from low elevation to being based in Colorado they lose a small amount of weight (without changes). Lots of theories as to why.
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Most likely loss of moisture in the upholstery and belly schmegma.
 
petrolero said:
Reminds me... My mechanics tell me that often when aircraft move from low elevation to being based in Colorado they lose a small amount of weight (without changes). Lots of theories as to why.
Click to expand...
Well you would be further from the Earth's center of mass. The radius of the Earth is about 4000 miles so the decrease going from sea level to 5000 MSL would be something like 5 hundredths of a percent (1.5 lbs on a 3000 lb airplane). Of course that assumes you're using spring compression or load cell scales rather than a balance.
 
gismo said:
Well you would be further from the Earth's center of mass. The radius of the Earth is about 4000 miles so the decrease going from sea level to 5000 MSL would be something like 5 hundredths of a percent (1.5 lbs on a 3000 lb airplane). Of course that assumes you're using spring compression or load cell scales rather than a balance.
Click to expand...

I'm pretty sure that you are grossly over-estimating the force of gravity but nonetheless the error exists. I'm telling you right now that the reason is because the original number was complete bs. Take any airplane from San Diego to Denver and you're gonna tell me that there is some measurable significant change in the empty weight?

I don't care how dried out it gets I'm calling bull on that LoL

You guys are entertaining if nothing else...
 
flyingron said:
My plane hadn't been weighed in 50 years when I got it and had pages of add this, substract that corrections. Oddly enough when I weighed it, it was within a couple of pounds form what the computed W&B papers said it was.
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Had it been painted or gotten a new interior? It seems to me that those are the most likely to turn the calculations into fiction.
 
Silvaire said:
I'm pretty sure that you are grossly over-estimating the force of gravity
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F = G(Mm) / r*r

We just want the ratio, to compare the change in force. Let:

d1 = 4000 (sea level distance from center of gravity of Earth)
d2 = 4001 (About a mile farther away, 5000MSL).

F1 = G(Mm)/d1*d1
F2 = G(Mm)/d2*d2

F2 / F1 = (d1 * d1) / (d2 * d2) = (4000 * 4000) / (4001 * 4001) = 0.999500

0.9995 => 99.95%. Or, 3000lbs * 0.9995 = 2998.5lbs, a difference of 1.49lbs.
 
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We are less talking about F=MA which more relates to load factors in turbulence or steep turns.

We are concerned with shifting the CG forward or aft of the true CG of the current config of the aircraft. Shift it forward, and you raise stall speeds, shift it back and you lose or degrade elevator control.

True, exceeding overall gross weight is a problem too, but W&B is also about CG.
 
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kyleb said:
I would trust most mechanics to do the math "Removed 5 pound radio at station XX.YY, replaced with 3 pound radio at station XX.ZZ" I wouldn't trust them to accurately calculate the CG after an interior or paint job. IMO, that's time to re-weigh the airplane.
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I've found math errors on probably 30% of the W&B entries I've examined over the past 30 years. I've come to the conclusion that, when it comes to math, A&Ps are mechanics, not CPAs. My recommendation is to bone up on the math and formulas involved then go over the calculations yourself. Some of the errors I found were insignificant, others not much.
 
airdale said:
Certainly someone here has re-weighed an airplane and checked the result against its cumulative W&B calculation. What was the result?
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Our flying club just re-weighed our 1978 Archer. There are no re-weighs in our logs that we can find.

The airplane has recent paint and interior.

We've gained 32 pounds since 1978, pretty close to Ron's formula.
 
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gismo said:
Well you would be further from the Earth's center of mass. The radius of the Earth is about 4000 miles so the decrease going from sea level to 5000 MSL would be something like 5 hundredths of a percent (1.5 lbs on a 3000 lb airplane). Of course that assumes you're using spring compression or load cell scales rather than a balance.
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Except when you calibrate the scales by putting a reference mass on them you calibrate out any local variation in gravity.
 
Capt. Geoffrey Thorpe said:
Except when you calibrate the scales by putting a reference mass on them you calibrate out any local variation in gravity.
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That's interesting. But, I don't think it's entirely true. I admit I'm making a few assumptions about how the scale works, but see if this makes sense:

You'll change the scale's zero point, for example, certainly. But what you're not changing is the spring constant. The scale is designed so that a certain displacement equals a certain reading. Imagine some BIG difference, like in denver a mass that weighs a pound at sea level will only weight 0.5lb at altitude. The scale needs a certain force pressing on the spring to indicate a certain weight, but you have half as much as you need to indicate the correct weight.

Now, obviously (as pointed out earlier) balance scales don't have this problem, but does anyone use balance scales to weigh something that big?
 
Mistake Not... said:
That's interesting. But, I don't think it's entirely true. I admit I'm making a few assumptions about how the scale works, but see if this makes sense:

You'll change the scale's zero point, for example, certainly. But what you're not changing is the spring constant. The scale is designed so that a certain displacement equals a certain reading. Imagine some BIG difference, like in denver a mass that weighs a pound at sea level will only weight 0.5lb at altitude. The scale needs a certain force pressing on the spring to indicate a certain weight, but you have half as much as you need to indicate the correct weight.

Now, obviously (as pointed out earlier) balance scales don't have this problem, but does anyone use balance scales to weigh something that big?
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Actually, "Geoffrey" is correct assuming that the calibration is performed at the same elevation and somewhere near the place the scales are used to weigh an airplane (there are small variations in local gravity strength from one region to another even at the same elevation) and the reference weights were "calibrated" accurately (e.g. NIST traceable). Because the scales are "zeroed" prior to calibration and prior to use this amounts to a two point calibration which should remove all repeatable error except for non-linearity (non-linearity would introduce error at any weight other than that used for the calibration).

Calibrating a scale with a reference weight does amount to changing the "spring constant" but with an electronic scale the calibration simply changes the relationship between the amount of "spring compression" (i.e. load cell strain) and the displayed weight.

But I offered the gravity based error calculation a bit tongue in cheek. I don't have specs handy but I think you'd be hard pressed to find an affordable scale that was repeatable let alone accurate to .05% and chances are the scales owned by the local EAA chapter haven't been calibrated since they left the factory.

In any case what I really meant was that if you used the exact same scales (without re-calibrating) to weigh an airplane at sea level and then shipped the plane and scales to Denver there would be an approximately 1.5 lb change in the measured weight if the scales were repeatable to that level of precision.
 
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gismo said:
Because the scales are "zeroed" prior to calibration and prior to use this amounts to a two point calibration which should remove all repeatable error except for non-linearity (non-linearity would introduce error at any weight other than that used for the calibration).
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Yeah, I know what you meant. But I like problems like this. It's funny... the variation of gravity with radius makes it weigh a little less, but the effect of increasing latitude makes it weight more, and by a bigger factor. http://en.wikipedia.org/wiki/Gravity_of_Earth
 
Henning said:
Most likely loss of moisture in the upholstery and belly schmegma.
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:lol: belly schmegma

This is the largest part of the weight loss in my view. Part of it may be simply the fact that they clean out some of the belly schmegma and clean the exterior too. :dunno:

Sure there is a non-zero difference in the earth's gravity but it is not enough to account for the weight loss alone.
 
flyingron said:
My plane hadn't been weighed in 50 years when I got it and had pages of add this, substract that corrections. Oddly enough when I weighed it, it was within a couple of pounds form what the computed W&B papers said it was.
Click to expand...

That is odd. Ususlly, planes ( and pilots) gain a few pounds over the years.
 
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