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Weighing M20J


gacoon

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4B024E0A-8C2C-4CDC-A362-E6F34582AFE5.thumb.png.2162b6c4eb57ac98fafd8812518a80b3.pngWe are weighing my 1990 M20J following removal of a bunch of equipment.  Does anyone know why Mooney uses 5.82 lb for 100LL.  This weird value results in nearly a 12lb difference depending on the selected weighing procedure.  If you weigh it with full tanks you back out 64 gallons @ 5.82 or 372.5lbs.  if you drain tanks and add in unusable fuel, 2.5 gallons @ 5.82 it adds in 14.55lbs.  100LL weighs 6.01.  Full tank weighing procedure adds in an extra 11.5 pounds.... ???

 

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When we weighed our plane in 2013 we did it with fuel in and subtracted the fuel weight.  After all the equipment changes for the GFC500 install and other changes we'd made in between, I realized we might have made as much as an 8 pound error in our calculations.  We decided to re-weigh the plane with no useful fuel on board, just the 1.25 gallons/wing as specified in the procedure.  We gained 18 pounds of useful load.

As for the 5.82 lbs/gallon, I had done some looking on line and found specs for a few manufacturers:

Epic says their relative density is .65 to .75 at 15C.  If you take the average of that, .7, it gives you a value of 5.84 lbs/gallon.

Phillips says theirs weighs 5.83 lbs/gallon.

Shell says theirs weighs 700 to 730 Kg/M^3.  Using 715, that translates to 5.97 lbs/gallon.

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If you weigh with full fuel, you need to check the specific gravity of the fuel used to fill it. Avgas weighs between 5.7 to 6.1 pounds per gallon depending on temperature and the brand. 5.82 is an average weight at 59 degrees F.

And, there is no way to know if the tanks are perfectly "full", since the level depends on filling to the neck, not above or below it. You can have 2-3 gallons variance there. Also depending on how level the plane is. 

Best practice is to defuel the plane. 

Edited by philiplane
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FAA Weight & Balance Handbook lists standard weight for Avgas at 6.01 lbs/gal @ 59F. You could use that if you want since it’s an FAA reference. Really, the only way to know for sure is weigh it empty. I suppose you could fill it up and drain a carefully measured gallon and weight that.

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1 hour ago, PT20J said:

FAA Weight & Balance Handbook lists standard weight for Avgas at 6.01 lbs/gal @ 59F. You could use that if you want since it’s an FAA reference. Really, the only way to know for sure is weigh it empty. I suppose you could fill it up and drain a carefully measured gallon and weight that.

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Lot of  good info here.  If you look at older POH's you see they use 6 for the weight on the weight formula, somewhere along the line they changed the formula to 5.82.  As you point out the actual W&B calculation has used 6lbs since forever.  SO - as mentioned in another post never use the full fuel method.  I am going to have the mechanic just calculate the new weight from the last known aircraft weight.  If I ever put in the new CIES fuel senders I'll probably weigh it empty to get a good number.  Thanks everyone.

 

PS:  you really cant use the FAA reference as you mention as the Service Manual calls for using the formula in the POH and the POH calls out the 5.82.

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Even the procedure for draining the tanks and adding back the unusable fuel seems kind of odd.  Seems like the instructions we used earlier this year were to drain the fuel via electric fuel pump and disconnected line, then remaining fuel from the sump, then add back the unusable fuel, 1.25 gallons in my case. But the instructions above don't mention draining fuel out of the sump, which would seem like the unusable fuel.  ???

Just as a side note, with that unusable fuel added, after all the weight and balance stuff,  and the airplane sitting on the gear in front of the hangar, I started and ran the engine at idle for a while.  The ramp in front of my hangar angles slightly downhill.  This was the uphill tank, fuel would have been against the pickup.  After about 5 minutes or so, I notice the fuel pressure had gone to 0.  Engine was still running smooth.  I turned on the electric fuel pump and the pressure went back up to about 6 PSI.  Several more minutes idle before the pressure dropped again.  At that point I figured it was pretty much done. Put 27.1 gallons in the tank (bladders, 27.4 usable).  I could have put in a few more tenths if I was patient.

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Unuseable fuel is determined under specific conditions. That doesn't mean you cannot use it under other conditions. I ran one tank down until our Cies input digital gauge said zero. The engine ran another 5 minutes at cruise power.

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1 hour ago, David Lloyd said:

Even the procedure for draining the tanks and adding back the unusable fuel seems kind of odd.  Seems like the instructions we used earlier this year were to drain the fuel via electric fuel pump and disconnected line, then remaining fuel from the sump, then add back the unusable fuel, 1.25 gallons in my case. But the instructions above don't mention draining fuel out of the sump, which would seem like the unusable fuel.  ???

I think there's a mistake in those instructions, since "unusable fuel" is supposed to be the inaccessible fuel that's left after the pump runs it dry.   So if you drain the tank with the pump (step d), and then add back unusable fuel (step e), I'm not sure how much fuel you'll actually have in the tank, but it might be more than the "unusable fuel" amount expected.    If you drain the tank via removing the sump plug, you'll have "undrainable fuel" left, which should be close to nothing.   Some airplanes have specified "undrainable fuel" but I've never seen a spec for that for a Mooney.   The TCDS has "unusable fuel" in Note 1 specified in lbs with the appropriate moment arm.

Since the TCDS specifies unusable fuel in lbs with a moment arm, one way to do it is to drain the airplane completely via the sumps, weigh it, and add back the value specified in the TCDS to the weight determined on the scale, at the arm specified in the TCDS.    This is similar to weighing it with full tanks and subtracting out usable fuel, and there may be a small advantage one way or other depending on the specific gravity of the fuel.   Perhaps.  ;)     Weighing it full is easier and less messy, weighing it empty might be a bit more accurate and might give a few lbs more UL.

 

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AVGAS <> 100LL,  Avgas weighed ~6 lbs/Gal, but Avgas is no longer and 100LL weighs less and why modern Mooney POH's now say to use 5.82lbs/gal for 100LL

But be careful, Mooney still uses or assumes the AVGAS weight in their documentation including the TCDS and POH when specifying unusable fuel weight. They only clarify with a note that says use 5.83 Lb/Gal with 100LL but otherwise pretty much uses or assumes 6lb/gal when they give weight numbers for fuel in both POH & TCDS.

Of course such details are only of importance to owners/pilots wanting to get as much legal useful load as possible out of their airframes. 

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52 minutes ago, David Lloyd said:

Might be an unwelcome surprise for those that don't.

Not nearly as an unwelcome surprise to those pilots that simply ignore their POH weight and balance limits. In other words, the one that if they do a Weight and balance calculation use random internet data (not their planes) or those that load up there Mooney's with 5 pax or other gross deviations like a 4 people on board with 20 gal and beyond the forward aft limit. The NTSB records have lots of such examples of pilots ignoring their airframe limits, such as the 3 examples I listed. But I am still waiting to read about the first report of an aircraft official/legal weight and balance record being so wrong by either computation or weighing that it attributed in any way a mishap. I believe the record shows Pilots are quite capable of doing that all by themselves without any such help! If there is any such data to the contrary I would sure like to hear about it. 

Edited by kortopates
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Might be an unwelcome surprise for those that don't.

I don’t see how, let’s assume that there was an error in your W&B, it would have to be an egregious error. Small errors is not going to cause you to drop out of the sky.
I recommend sanity checking your W&B.
You’re more likely to have a larger error from weighing it.
I have a 100 changes, 16 errors, weight is off by 2 lbs and balance is 0.12.
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2 hours ago, RLCarter said:

I just don’t get how some don’t want to know the "actual weight and CG" of their aircraft. I would rather know where I stand than brag about a UL# that isn’t accurate 

With what degree of accuracy do you really think one is getting with a re-weighing of an aircraft? From what I've seen, they can be very error prone. Just from reading posting here, we know that commonly shortcuts are taken rather than following the guidance in the POH or Maintenance manual precisely. Are the scales accurate with a recent calibration? Was the plane properly leveled? Were the tanks entirely emptied? Was it re-checked to verify repeatability of measurement? ..... Errors here can can have much bigger impacts than gaining a little weight here or there,  after the years from small unaccounted changes. Re-weighing an aircraft accurately is not an easy operation.

There are some modifications which can cause you to lose confidence in an otherwise carefully done list of mods of equipment adds or removals; such as adding coats of new paint rather than stripping or other body or interior work that isn't properly accounted for. But without these concerns, attention to detail in weight and balance updates is going to be less risky method of determining "actual weight and CG". 

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I just don’t get how some don’t want to know the actual weight and CG of their aircraft. I would rather know where I stand than brag about a UL# that isn’t accurate 

You’re assuming the weighing process is accurate, I know from first hand experience it’s not.
UL will affect the price of the plane when you go to sell...almost every time a plane is listed for sale here on MS, the question about UL is asked.
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1 hour ago, kortopates said:

With what degree of accuracy do you really think one is getting with a re-weighing of an aircraft? From what I've seen, they can be very error prone. Just from reading posting here, we know that commonly shortcuts are taken rather than following the guidance in the POH or Maintenance manual precisely. Are the scales accurate with a recent calibration? Was the plane properly leveled? Were the tanks entirely emptied? Was it re-checked to verify repeatability of measurement? 

Weighing the aircraft by following the procedures with accurate scales is very repeatable, I had to do my E twice because the worksheet was misplaced. Both times the oil and fuel was removed and the aicraft leveled and weighed, my IA later found the first worksheet and they were identical. The reason I opted to weigh the aicraft was all of the errors in the on the weight & balance documents over 50 years of adding and removing things. 

 

1 hour ago, ArtVandelay said:

You’re assuming the weighing process is accurate, I know from first hand experience it’s not.
UL will affect the price of the plane when you go to sell...almost every time a plane is listed for sale here on MS, the question about UL is asked.

And why wouldn't it be accurate if done correctly? 

 

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2 hours ago, ArtVandelay said:

That’s a big IF, the answer is it’s usually not. Then you have to ask yourself how accurate the scales are, when were they last calibrated...just a 1% error is 18lbs.

And that 18lbs could be a positive or negative. To me weight & balance is important and the FAA must agree or it wouldn’t be required before each flight, knowing the empty weight & balance is the starting point. Will an aircraft fly outside the envelope? Yes it will, I’ve seen aircraft loaded past gross and so far aft that the tail had to held up, I wouldn’t do it but I know several that have. 

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We weigh aircraft all the time.  My personal Mooney I did a new interior, re-painted it, and a whole new avionics suite.  After doing all that kind of stuff you really want to re-weigh the plane.  I picked up 17 lbs UL.  We level the plane and we use digital jack scales that are calibrated annually.  The scales are wirelessly connected connected to a computer.  It’s very accurate.

I used the empty of fuel weighing method & added back unusable fuel.

 

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