Possible Weight & Balance Calculation Error

[ManGar]

Hey everyone,

Hoping to be steered in the right direction.  I have a 1963 M20C.  It was last weighed in 2000 before I owned it.  At the time the calculations were made, it came out with an empty weight of 1736.7, Moment of 86181.2, CG of 49.62(seems really aft to me) and a useful load of 838.3 lbs.  Since then, it has had several avionics replaced and most recently I had the Alternator STC done which left me with a CG of 50.12 which is outside aft of the envelope.  With no baggage or rear passengers and full fuel, and me as a 210 lb pilot, I am at the inside edge of the envelope.  This can't be right can it?  I'm thinking that maybe the arm used for the landing gear when it was weighed may have been wrong.  Does anybody know what the arm distance for the main and nose gear is?  They useded 65.75 for the main and 3.06 for the nose.  I've called around to see who could weigh it but no one I've talked to has the scales as they are expensive.  They offered to review the calculations but I'm of the mindset that if it wasn't done right the first time then it's still not going to be right.

Any information would be greatly appreciated.

 

[N201MKTurbo]

Do your log books have the wheel weights from the weighing so you can redo the numbers?

[ManGar]

I'll have to look but when they weighed it, they listed left main at 731.3 lbs with a arm of 65.75 and the right main at 784.3 lbs with and arm of 65.75 and nose at 533.1 lbs with an arm of 3.06.

[EricJ]

14 minutes ago, ManGar said:

I'll have to look but when they weighed it, they listed left main at 731.3 lbs with a arm of 65.75 and the right main at 784.3 lbs with and arm of 65.75 and nose at 533.1 lbs with an arm of 3.06.

I weighed a C earlier this year and the arms I used were 66.75" for the mains and -0.5" for the nose.   These were measured with a plumb bob from the datum, chalk lines, etc.  I think these dimensions are not published in the manual because they can potentially vary from airplane to airplane.

Do you know how much fuel was in your airplane when those weights were taken?   2048.7 lbs is awfully high for an empty weight.

I have a spreadsheet already set up from weighing a C model, if we can sort out the conditions under which yours was weighed (fuel content, etc.), I could give you ballpark numbers using the arms we measured.   It won't be valid for your airplane, but it should give an idea of whether there's a big problem in your w&b or not.

[ManGar]

10 minutes ago, EricJ said:

I weighed a C earlier this year and the arms I used were 66.75" for the mains and -0.5" for the nose.   These were measured with a plumb bob from the datum, chalk lines, etc.  I think these dimensions are not published in the manual because they can potentially vary from airplane to airplane.

Do you know how much fuel was in your airplane when those weights were taken?   2048.7 lbs is awfully high for an empty weight.

I have a spreadsheet already set up from weighing a C model, if we can sort out the conditions under which yours was weighed (fuel content, etc.), I could give you ballpark numbers using the arms we measured.   It won't be valid for your airplane, but it should give an idea of whether there's a big problem in your w&b or not.

The W&B sheet says it was weighed with full fuel (52 Gallons) 312 lbs. and 8 qts at -7.4 inches.

[Skates97]

This might help?

[ManGar]

18 minutes ago, EricJ said:

I weighed a C earlier this year and the arms I used were 66.75" for the mains and -0.5" for the nose.   These were measured with a plumb bob from the datum, chalk lines, etc.  I think these dimensions are not published in the manual because they can potentially vary from airplane to airplane.

Do you know how much fuel was in your airplane when those weights were taken?   2048.7 lbs is awfully high for an empty weight.

I have a spreadsheet already set up from weighing a C model, if we can sort out the conditions under which yours was weighed (fuel content, etc.), I could give you ballpark numbers using the arms we measured.   It won't be valid for your airplane, but it should give an idea of whether there's a big problem in your w&b or not.

By the way.... 1736.7 is the empty weight not 2048.7

[ManGar]

5 minutes ago, Skates97 said:

This might help?

Thanks for the info. I was wondering if the arm for the gears that they used seemed accurate.  I just can't believe that I have such an aft CG that I can't ever have anyone in the back or even carry luggage.

[EricJ]

10 minutes ago, ManGar said:

The W&B sheet says it was weighed with full fuel (52 Gallons) 312 lbs. and 8 qts at -7.4 inches.

For those weights I'm getting, with the gear arms measured on a different airplane:

EW: 1721.7

UL:  853.3

Total empty moment (in-lbs):  85909.95

Empty arm:  49.89"

Edit:  so not a lot different than what you have already.

 

Edited September 23, 2020 by EricJ

[ManGar]

1 minute ago, EricJ said:

For those weights I'm getting, with the gear arms measured on a different airplane:

EW: 1721.7

UL:  853.3

Total empty moment (in-lbs):  85909.95

Empty arm:  49.89"

Thanks for doing the calculations.  That seems really aft.  Guess I'm gonna have to get her re weighed.  Have any idea about how much it costs?  Also, do you know if some sort of balance weights are a possibility?

[mooniac15u]

1 hour ago, ManGar said:

I'll have to look but when they weighed it, they listed left main at 731.3 lbs with a arm of 65.75 and the right main at 784.3 lbs with and arm of 65.75 and nose at 533.1 lbs with an arm of 3.06.

I have no experience with weighing airplanes but a 50 lb difference between left and right mains looks odd.  Is it normal to see that much difference?  Is it possible the right main number got recorded wrong?

[EricJ]

2 minutes ago, mooniac15u said:

I have no experience with weighing airplanes but a 50 lb difference between left and right mains looks odd.  Is it normal to see that much difference?  Is it possible the right main number got recorded wrong?

It's pretty easy for a 3 to turn into an 8, so assuming 734.3 lbs instead of 784.3 brings the EW down to 1671.7, but the arm only moves up to 49.4".     Doesn't make that much difference on the arm.

 

[ManGar]

2 minutes ago, mooniac15u said:

I have no experience with weighing airplanes but a 50 lb difference between left and right mains looks odd.  Is it normal to see that much difference?  Is it possible the right main number got recorded wrong?

I was wondering why the difference if both tanks had the same amount of fuel.  I may not have any choice but to get her weighed.  I just need to find someone who has scales to do it.

[ArtVandelay]

I was wondering why the difference if both tanks had the same amount of fuel.  I may not have any choice but to get her weighed.  I just need to find someone who has scales to do it.

If you’re going to do it make sure it’s done right. Filling the tanks is a bad way to do it because they usually hold more than spec. My case I could squeeze another 3 gallons above spec so that would cost me 18 pounds.

[ArtVandelay]

For those weights I'm getting, with the gear arms measured on a different airplane:

EW: 1721.7
UL:  853.3
Total empty moment (in-lbs):  85909.95
Empty arm:  49.89"
Edit:  so not a lot different than what you have already.
 

Wow, that’s pretty far aft, did you move the battery from engine compartment to tail or some other mod that would affect it?

[mooniac15u]

2 hours ago, ManGar said:

Hey everyone,

Hoping to be steered in the right direction.  I have a 1963 M20C.  It was last weighed in 2000 before I owned it.  At the time the calculations were made, it came out with an empty weight of 1736.7, Moment of 86181.2, CG of 49.62(seems really aft to me) and a useful load of 838.3 lbs.  Since then, it has had several avionics replaced and most recently I had the Alternator STC done which left me with a CG of 50.12 which is outside aft of the envelope.  With no baggage or rear passengers and full fuel, and me as a 210 lb pilot, I am at the inside edge of the envelope.  This can't be right can it?  I'm thinking that maybe the arm used for the landing gear when it was weighed may have been wrong.  Does anybody know what the arm distance for the main and nose gear is?  They useded 65.75 for the main and 3.06 for the nose.  I've called around to see who could weigh it but no one I've talked to has the scales as they are expensive.  They offered to review the calculations but I'm of the mindset that if it wasn't done right the first time then it's still not going to be right.

Any information would be greatly appreciated.

 

Have you looked at the original data from when it was weighed at the factory?  That information was in the POH for my 1963 M20D. 

[EricJ]

45 minutes ago, ManGar said:

I was wondering why the difference if both tanks had the same amount of fuel.  I may not have any choice but to get her weighed.  I just need to find someone who has scales to do it.

Race scales are not that expensive, so I'd hope the scales wouldn't be too big of a hurdle.    This is what I have and I've used them to weigh and adjust suspensions on race cars and weigh airplanes.   They're good scales.

https://www.amazon.com/ProForm-67644-7000lb-Wireless-Scale/dp/B01ETY2W44

[David Lloyd]

My airplane is the fattest C in existence at 1771 lbs.  The CG is 46.15, so I think the 49" numbers are most likely incorrect. My weights were 615 for the left main, 614 for the right main and 542 nose wheel.  The OP said the nose arm was 3".  That has to be incorrect. Mine was close to 0.  This is the distance between the center of the nose trunnion and the center of the nose axle.  Can't be 3". My distance the nose and main axle centers was 66.5, so 65.75 is believable. 

If you want to check the math, print and read the weight and balance work sheets, go to the airport with those sheets, a plumb bob, a level and a tape measure.  

Park your airplane on a nose downhill slope and check for level.  Deflate your nose tire as needed. Measure the distance between the nose wheel axle center and main wheel axle centers.  Fill out the first part of your worksheet (assuming your weights were correct). Measure the distance from the center of the nose gear trunnion and main wheel axle centers. This is where you will find the 3" difference.   Fill out the second part of your worksheet.

This will not be an exact calculation as the gear discs are slightly compressed with all the stuff in your airplane that should not have been weight and the 30 gallons of fuel you didn't drain.  But, it will give you a pretty good idea.

My airplane was weighed earlier this year after an installation by Pee Dee Avionics in Cheraw, SC. They had the scales, they removed a fuel line and pumped out most of the fuel, then drained the remainder of the fuel. Then added back the unusable fuel. I think they charged a few hundred dollars.

 

[carusoam]

MG,

1) You are about to embark on the same journey that everyone else has gone through...
2) WnB checking is the first thing... many new owners didn’t get the paperwork during the transition...

3) Your paperwork is pretty far off base... somebody probably lost sight of where 0.0 is located.... it is a bolt on the nose gear...

4) Weighing the plane needs to follow a procedure to get it right... that procedure is in the maintenance manual...  

5) Chances are, somebody didn’t know these details and put the details together in a close but not quite right kind of way...

6) Find who made the errrors to see what else they touched....

7) No need to blame anyone.... the factory has messed up quite a few as well...
 

8) Most mechanics that have Mooney experience are going to know how to weigh a plane...

9) there are a few threads around here where people have spent pages trying to find where their errors are and came from in their WnB...

10) It is important to know... doing the WnB prior to flight is still important... especially when the plane is new2U...

11) To know if a mechanic has scales... call and ask...

12) To save a ton of dough, organize all the things that are now included in the weight... a dated record that can be updated each time something gets added or removed...

13) expect to find a level surface... then realize, your nose is going to be a couple of inches high...

14) Be ready to Jack the plane up to help with leveling... it helps to have jack points for your plane...  ask your mechanic what he has...

15) you will become familiar with a seam, some seat tracks, and a bolt, by the time you are done with this project....
 

16) Work on getting a copy of the MM, Parts Manual, and latest POH printed for your plane.... All stuff that is handy to have...

 

PP thoughts only, not a mechanic...

Best regards,

-a-

[N201MKTurbo]

1 hour ago, David Lloyd said:

they removed a fuel line and pumped out most of the fuel, then drained the remainder of the fuel. Then added back the unusable fuel. 

I know there is a specified amount of unusable fuel, but if you pump the fuel out from the tank pickups with the airplane level, isn't the fuel left in the tanks, by definition, the unusable fuel?

[ArtVandelay]

I know there is a specified amount of unusable fuel, but if you pump the fuel out from the tank pickups with the airplane level, isn't the fuel left in the tanks, by definition, the unusable fuel?

Yes, if it’s level. Pretty sure that step gets skipped most of the time.

[David Lloyd]

1 hour ago, N201MKTurbo said:

I know there is a specified amount of unusable fuel, but if you pump the fuel out from the tank pickups with the airplane level, isn't the fuel left in the tanks, by definition, the unusable fuel?

Reread, pumped most out, then drained the remainder.  Tanks were completely empty before adding unusable.   Later on, I ran the engine on the right tank with only the unusable fuel for about 10 minutes at idle before realizing the fuel pressure had gone to zero. Unusable meant sucking air and just enough fuel for idle.

If you only pumped fuel out, only reusable would remain more or less.  The shop said the got less than a gallon from both sides total when they drained what wasn't pumped out. They added back 1.25 gallons per side to follow the instructions.

[N201MKTurbo]

1 hour ago, David Lloyd said:

Reread, pumped most out, then drained the remainder.  Tanks were completely empty before adding unusable.   Later on, I ran the engine on the right tank with only the unusable fuel for about 10 minutes at idle before realizing the fuel pressure had gone to zero. Unusable meant sucking air and just enough fuel for idle.

If you only pumped fuel out, only reusable would remain more or less.  The shop said the got less than a gallon from both sides total when they drained what wasn't pumped out. They added back 1.25 gallons per side to follow the instructions.

So, its unusable fuel +2.5 gallons. That's 15 Lbs!

[Hank]

8 minutes ago, N201MKTurbo said:

So, its unusable fuel +2.5 gallons. That's 15 Lbs!

When I resealed the 52 gal tanks in my C, both sides were filled to the cap with less than 53 gal. But it was 2010, I no longer have the receipt from the FBO.

[markgrue]

It seems really heavy for a C.  Did they reweigh it after the repair for the gear up landing in 2018?  Is there a lot of remote avionics in the area behind the baggage compartment?  Perhaps a big fire extinguisher in the baggage or hat rack area?  My 63 D/C is right at 1500lbs and 42.5 inches empty.  I think a reweigh is good advice.  I weighed mine when I did the annual in august and actually lost a few pounds with some cleaning and removing unused wire and components.

 

Mark