W&B fun...math's not for everyone (including me).

[Ross Taylor]

Hi all,

I had noticed that our CG was way aft, when using the most recent W&B sheets and entering those data into Garmin Pilot.  According to the sheets, our CG was 48.83 without any passengers or load...and that, clearly, is unusually far aft.  I had gone through the old W&B sheets and found an error, which I corrected in my calculations.  A recent question by another MSer brought this to mind and, since I'm going through all the logs and docs this weekend, I thought I'd share my findings in the event they help someone else.

Here's the offending W&B calculation sheet after a prop replacement - it was in this calculation that the CG on our 1966 E moved aft from a reasonable 46.84 to an unusual 48.78.  See what they did?   They removed a prop and spinner, then installed a prop and spinner.  And they didn't deduct the moments from the items removed. 

Instead of this: 81834.16-(1621.10+105.05)+(1752.30+93.50) giving a new moment of 81,953.65...they added them all together, resulting in a new moment of 85406.10.  And to get the CG, you divide that by the weight.  And that, friends, started a chain of 3 other incorrect W&B calculation sheets because each subsequent one started with the figures from the prior one (as is normal).  So, our CG (on paper only) crept back to a very aft 48.83.  

There's another suspect sheet, too, where a shop removed the original windshield and installed a 201 windshield.  They listed a bunch of removed items, but didn't include the original windshield in that list.  But the 201 windshield is listed as installed.  So, unless the 201 figures are a net change (which I don't believe is the right way to do that) then not listing the original windshield removal throws that sheet off too.

Anyway...I hope this helps someone else digging through their calculations too.

Be well,

Ross

EDIT/NOTE:   Before anyone follows my example here, please read the comments below.  I didn't account for the negative arm figures, so my math is wrong here too...better than the prop shop, but still wrong.  

 

Edited April 13, 2020 by Ross Taylor
My math was wrong, too!

[Ross Taylor]

And, just for fun, here's the original W&B calculation sheet

[EricJ]

And the 50-60-or-whatever years of multiple W&B sheets that most of our airplanes have probably had have so much error accumulation in them as to not be very reliable.   It seems like rounding errors tend to lose weight, too, so the weight "gain"  that people usually see on the scales are probably more due to that than dirt/debris accumulation.    I have a couple significant mods planned yet on mine, but once those are done it's gonna get weighed again.

[carusoam]

+1 on weighing again... Early in ownership...

+1 on getting the stations set up properly in an app...

The level of math required is just on the edge of not really familiar... for everyone... even those trained in the engineering behind it...

Falling out the back of the envelope in calculations has got to be scary... but, falling out in real life, it’s going to be terrifying... 

You really get to know your plane...

There used to be an OWT regarding having to log the data...

If you get data from the scales that is not right... no need to log it... it takes a fair amount of effort and involvement to get it done right...

Goes well with finding Usable fuel vs. full up... 

Good time to build the fuel stick...

Best regards,

-a-

[Ross Taylor]

25 minutes ago, EricJ said:

And the 50-60-or-whatever years of multiple W&B sheets that most of our airplanes have probably had have so much error accumulation in them as to not be very reliable.   It seems like rounding errors tend to lose weight, too, so the weight "gain"  that people usually see on the scales are probably more due to that than dirt/debris accumulation.    I have a couple significant mods planned yet on mine, but once those are done it's gonna get weighed again.

In our case, the errors all happened after 1999 when she was last weighed.  But even in the relatively short time since, our CG "moved" a lot.   50-60 years of rounding...definitely.  Plus how many "insignificant" changes happened during that lifetime?

[Bob - S50]

Well...

Unless I'm mistaken, you still have a couple mistakes in there.  First, some general information.  If you remove a weight, that's a negative just like it shows.  Since the prop is forward of the reference point that makes the arm a negative too.  Negative times negative is positive so the moment arm increases.  When you add the new prop the moment will decrease.  Here are my calculations:

Weight / Arm / Moment

1747.10 / 46.84 / 81834.16  Starting point

-53.75 / -30.16 / +1621.10  Remove prop

1693.35 /         / 83455.26  Result

-3.6 / -29.18 / +105.05  Remove spinner

1689.75 / 49.45 / 83560.31  CG calculated by dividing moment by weight

+58.10 / -30.16 / -1752.3   Add new prop

1747.85 /     /81808.01  Result

+3.10 / -30.16     / -93.50  Add spinner

1750.95 / 46.669 / 81714.51  CG calculated

And as a way to verify that is correct, your moment should not increase (CG moving aft) when you are adding weight forward of the reference point.

 

However, I think you have another mistake.  When you removed the old prop and spinner, the arm was different for the prop than the spinner.  When you added the new spinner you used the same arm as the prop.  If you MEANT to use -29.18 for the spinner arm, the moment would be -90.46 instead of -93.50 with your final calculations of:

1750.95 / 46.67 / 81717.55

[ArtVandelay]

The error rate in my entries was about 20%, but in the end they mostly canceled out, leaving a 2+ lb difference.

Weighing is a waste of money because no one does it by the book in my experience, I had one 120lbs heavy, because the used erroneous fuel gauges to get the fuel weight. Another one suggested to fill the tanks to get an accurate fuel weight. To do it right takes time and money.
I don’t know anyone who actually asks their passengers to step on scale or weigh their luggage, so in the end knowing your plane’s weight to the pound is not really necessary.

[Jerry 5TJ]

About 1990 and armed with a Lotus spreadsheet program I went back through the logs of my 1964 E and found over the years at least three different mechanics had made errors in their W&B revisions.  Plus there were many changes logged which should have gotten W&B revision but didn’t.  And there were some items installed with no log entries. 

[Guest]

3 hours ago, ArtVandelay said:

“To do it right takes time and money.”
 

That sums up most of the issues with GA

Clarence

[Ross Taylor]

So, it's 5am here and I just woke up thinking...damn, I didn't account for the negative arms.  Like I said, math's not for everyone...including me!   @Bob - S50 is right on the money here, regarding the negative arms.  As to his question about the different arms for the spinners, I'm not sure why they're different...those figures came from the prop shop that did the work.  Hmmm.   But at least we're closer now.  Thanks, gang.

[Bob - S50]

6 hours ago, ArtVandelay said:

The error rate in my entries was about 20%, but in the end they mostly canceled out, leaving a 2+ lb difference.

Weighing is a waste of money because no one does it by the book in my experience, I had one 120lbs heavy, because the used erroneous fuel gauges to get the fuel weight. Another one suggested to fill the tanks to get an accurate fuel weight. To do it right takes time and money.
I don’t know anyone who actually asks their passengers to step on scale or weigh their luggage, so in the end knowing your plane’s weight to the pound is not really necessary.

Glad we are in the minority.  We drained the fuel and oil and leveled the plane with blocks on the scales before we took our measurements.  Nice to know we have a UL of 1013 pounds that way.

[ArtVandelay]

Glad we are in the minority.  We drained the fuel and oil and leveled the plane with blocks on the scales before we took our measurements.  Nice to know we have a UL of 1013 pounds that way.

According to my manual for the J, you’re suppose to fill oil to capacity?

[Guest]

1 hour ago, ArtVandelay said:

According to my manual for the J, you’re suppose to fill oil to capacity?

Something like this?

Clarence 

[ArtVandelay]

Something like this?
Clarence 

That’s it, I see no reason not to include oil, it’s not like it’s optional.

For TKS capable planes I can see draining the TKS fluid.


Tom

[Andy95W]

The "classic" Mooneys did not include engine oil in the empty weight.  At some point, probably about the time the FAA started requiring a true POH instead of the old Owners Manual, they also started requiring engine oil as part of the empty weight.  I would assume @Bob - S50 M20J would fall into that category, but not sure.

[Ross Taylor]

In the original Mooney docs I posted, above, 2 gallons of oil are listed in the "useful load" list...like passengers. 

[EricJ]

17 minutes ago, Andy95W said:

The "classic" Mooneys did not include engine oil in the empty weight.  At some point, probably about the time the FAA started requiring a true POH instead of the old Owners Manual, they also started requiring engine oil as part of the empty weight.  I would assume @Bob - S50 M20J would fall into that category, but not sure.

That changed in 1975, due to the General Aviation Manufacturer's Association standardizing definitions for Empty Weight, for POHs and the associated pilot W&B calculations.

So, yeah, if you have something older, your W&B probably requires addition of oil in the loading calculations.   Later aircraft include it.

https://en.wikipedia.org/wiki/Empty_weight

When weighing an aircraft it is legit to compute the appropriate influence of either oil or fuel from a known state.   The TCDS includes the arm for the oil sump in each entry and "unusable fuel" amounts to be used for all models in Note 1.  As mentioned, the most accurate weight may be obtained by draining all fuel and adding back the values shown in Note 1 in the TCDS, or starting from full fuel and subtracting usable.   Since fuel changes density slightly with temperature, draining fuel will remove any temperature-related errors converting volume to weight.

But also as mentioned, I don't think this is a process that lends itself to high-precision.   Getting as reasonably accurate as you can is definitely useful, but there's a diminishing return to high precision given natural uncertainties in passenger/cargo weights and arms as well as fuel weight (due to density), uncertainties in equipment arms, etc., etc.

Just my dos centavos.   I am not a lawyer, I did not sleep in a Holiday Inn Express last night, do not try this at home, your mileage may vary.

 

[Bob - S50]

The POH for our '78 Mooney, step 3 of weighing says to drain the oil.

[EricJ]

14 minutes ago, Bob - S50 said:

The POH for our '78 Mooney, step 3 of weighing says to drain the oil.

So does mine, but the Service and Maintenance Manual for the M20J, as posted by M20Doc above, says to fill the oil.    It can be done either way as long as you're consistent when you do the calculations.    In other words, don't delete the oil in the initial computation and then never add it back in in the load calculations.   Likewise don't add it during weighing and then add it again during loading, etc.   The W&B sheet in the POH should indicate somehow whether it was done with or without oil, I'd hope.

[Paul_Havelka]

Our Cherokee hasn’t had an updated weight and balance since the 70s. After our avionics installation I’m sure she lost a little weight so when I get off work in 4 weeks I plan to fly the plane to a guy at Riverside in Tulsa that is known in our area for weighing and providing a new w&b sheet.


Sent from my iPhone using Tapatalk

[Guest]

1 hour ago, Bob - S50 said:

The POH for our '78 Mooney, step 3 of weighing says to drain the oil.

That’s really interesting.  Which takes precedence, the PoH or the maintenance manual.

Clarence

[JimB]

Actually the POH would be considered the FAA approved document while the AMM is just accepted. But I've never seen a POH with a aircraft "weighing" procedure. The procedures for weighing the aircraft should be in the AMM to establish basic empty weight. The procedures for calculating wb and loading the aircraft should be in the POH.

And dont forget to add back in your unusable fuel. 

[Bob - S50]

Our html file that calculates W&B automatically adds in 6 quarts of oil which is the level to which we fill the engine.

[EricJ]

42 minutes ago, JimB said:

Actually the POH would be considered the FAA approved document while the AMM is just accepted. But I've never seen a POH with a aircraft "weighing" procedure. The procedures for weighing the aircraft should be in the AMM to establish basic empty weight. The procedures for calculating wb and loading the aircraft should be in the POH.

And dont forget to add back in your unusable fuel. 

Actually the only part of the POH that's "approved" is the limitations section.

Manufacturer's Maintenance Manual typically trumps everything for the FAA when it comes to maintenance and procedures.   Pilots aren't authorized to update W&B, empty weight, etc., so I don't think the POH really comes into it.   I've no idea why a weighing procedure is in the POH, or especially why it's different than the MM. 

 

Edited April 13, 2020 by EricJ

[ArtVandelay]

The POH for our '78 Mooney, step 3 of weighing says to drain the oil.

Are you sure?