Weight and Balance sanity check, 70's 172

Discussion in 'Hangar Talk' started by exncsurfer, Oct 19, 2017.

  1. exncsurfer

    exncsurfer Pattern Altitude

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    I made a couple posts where I was trying to see if anyone can get a 70's model 172 within CG limits for Utility category. Several replies confirmed that they could. So, I'm now questioning the empty weight numbers on this rental. Does anyone have a 1970's or similar model weighing report that I can compare to this one? It looks like they weighed it with full fuel, I don't know if that is typical. I don't know where they got their arm values for the nose gear and rear gear for the total weight arm, but I think if there is an error, this is where it is introduced. I'm not sure where they got their arm numbers for subtracting the fuel, or the quantity they used to obtain the empty weight and CG, but they used 40 gallons at 47.92, POH seems to suggest(based on the loading graph) that it should be 38 gallons at 48.21(or so). Maybe they were trying to average unusable fuel in with usable? Even using the other arm of 48.21, its still way aft of where the example 'airplane' in the POH CG is and still way out of range of ever obtaining Utility in any loading. Unless there is a bowling ball mounted in the tail, I don't see how the empty CG could be that far back. The 'example' airplane in the POH has an empty arm of 37.3 inches, with full oil.

    upload_2017-10-19_14-32-48.png
     
  2. Pilawt

    Pilawt Final Approach

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    FWIW, here's the one for my 172N (Lyc. O-360) in 1997:

    Screen Shot 2017-10-19 at 11.41.05 AM.png
     
  3. exncsurfer

    exncsurfer Pattern Altitude

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    Thanks for that, what jumps out at me is that your nose gear weight is higher, even though your main gear numbers are lower. So it seems there is a bowling ball in the tail(or something). Or maybe the engine is lighter in this one somehow?

    Based on that, it seems the number could be right, but now, why is it right? :)

    (by the way POA members are awesome, I don't care what anybody says about you)
     
  4. tlglenn

    tlglenn Line Up and Wait

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    I don't have the sheet in front of me, but a 172M that I fly (1973 manufacture year) has an empty weight of 1436 with an arm of 39.01. It holds 48 usable gallons with an arm of 47.08.
     
  5. exncsurfer

    exncsurfer Pattern Altitude

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    Thanks, I still haven't seen anyone reporting 40 or over.
     
  6. Greg Bockelman

    Greg Bockelman Touchdown! Greaser!

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    Look up the Type Certificate Data Sheet for the 172. You can google it just like that. All kinds of information there. Just scroll through until you find the right model number.
     
  7. dmspilot

    dmspilot En-Route

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    The 172P I used to fly had an empty CG of 39.9.
     
  8. exncsurfer

    exncsurfer Pattern Altitude

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    Curious where you get the 47.08 from? In all the type sheets I see 48 listed for fuel tank arm.

    This is what I'm looking at:
    http://rgl.faa.gov/Regulatory_and_G...3a36336effbcd38625738a004bbe52/$FILE/3A12.pdf
     
  9. Dan Thomas

    Dan Thomas Final Approach

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    There are only two possible solutions for your W&B discrepancy: do an actual re-weight, with actual dimensions taken for the gear locations, or go back through all the amendments and find the errors.
     
  10. Salty

    Salty Final Approach

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    I did this with my mooney. Boy there were a lot of them. I learned that A&P's aren't real good at math. In one case, they added where they should have subtracted and took 10 pounds off my useful load (should have been putting 5 pounds back on but removed 5 instead)
     
  11. flyingron

    flyingron Touchdown! Greaser! PoA Supporter

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    Did around in the maintenance books and service bulletins. I bet those "to two decimal place" numbers come from there.

    The arms for the nose gear, mains, and fuel tank look reasonable. The 240 comes from the published useable fuel. You top off the tanks to weigh, then subtract the weight of the usable fuel. The "unusable" fuel is part of the empty weight by definition.
     
  12. bnt83

    bnt83 Final Approach

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    Drained & sumped is better, a "topped tank" is likely overfilled by an unknown quantity among other variables.
     
  13. MauleSkinner

    MauleSkinner Final Approach

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    I've seen W&B errors on Cessnas where they rolled the airplane up on scales but used the arms for the attach points at the fuselage, which artificially moved the CG.
     
  14. tlglenn

    tlglenn Line Up and Wait

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    Oops typo. It should've been 47.80.
     
  15. Dan Thomas

    Dan Thomas Final Approach

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    Drained and sumped gives you empty tanks, not unusable fuel. The TCDS specifies the unusable fuel amount and its arm.

    Unusable fuel is that fuel that won't flow out of the tank when the airplane is in certain critical attitudes, like a Vx climb or a full-flap, power-off descent. The fuel sloshes forward or aft and the unusable amount won't reach the outlet, which is often around the middle of the tank and raised a bit from the bottom.

    Drain it, sump it, then add unusable fuel. Then weigh.

    As an example, here's an excerpt from the 172 TCDS, Note 1:

    Serial Nos. 17261578, 17261445, 17265685 through 17274009
    The certificated empty weight and corresponding center of gravity location must include unusable
    fuel of 24 lbs. at (+46) through 172M (17267584) or 18 lbs. at (+46) 17267585 and on and full
    oil of 11.3 lb. at (-14).
     
  16. Dan Thomas

    Dan Thomas Final Approach

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    And if they didn't level the airplane in accordance with the maintenance manual, they're still off. It's amazing how nose-low "level" is.

    Measuring the axle locations with respect to the datum is one of the bigger pains of W&B.
     
  17. JOhnH

    JOhnH Touchdown! Greaser!

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    I recently had my 50 yr old Bo reweighed. It gained around 80#s on empty weight. Years and years of rounding ng and errors added up.
     
  18. bnt83

    bnt83 Final Approach

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    Thanks for the addition I left out.

    The point I was trying to make was, which error is going to be worse?

    Scale: Drained & Sumped then add unusable via math

    or

    Scale: Topped tanks then subtract "usable" via math (subject to error via non-standard fuel density too)

    IME weighing with full tanks is noticeably heaver in the calculated empty weight than the other way.
     
    Last edited: Oct 20, 2017
  19. bnt83

    bnt83 Final Approach

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    How stable is earth's gravity?
     
  20. flyingron

    flyingron Touchdown! Greaser! PoA Supporter

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    Drained and sumped isn't any better. You are probably below 0 usable when you do that. The empty weight must include all the unusable fuel. Fill the tank to the stated "full" position. Tell me where in the Cessna documentation that it tells you how much unusable fuel there is?
     
  21. bnt83

    bnt83 Final Approach

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    It doesn't but if you can't subtract you shouldn't be weighing it.

    upload_2017-10-20_15-22-17.png
     
  22. bnt83

    bnt83 Final Approach

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    Ahhh but note 1 does

    upload_2017-10-20_15-30-45.png
     
  23. bnt83

    bnt83 Final Approach

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    FWIW almost all the TCDS I've looked at over the years has the total unusable fuel somewhere in the notes section. The only issue I see is older airplane like 1980s Falcons, when the wing planks are off and the tanks are DRY. Often they want to weigh it with dry tanks and there is no real good number to use. Later models show trapped fuel, total unusable, drainable unusable etc, so in theory you can find a decent number to work with.

    My favorite configuration is suction defuel, level it on jacks (on the scale) then sump.

    Of course small airplanes can have STCs that affect this number. In transport category there may be an optional tank that is still defined well in the W&B manual and its fairly easy to come up with a solid number. Not too many third party STC fuel tanks on these.

    That's why I say defueled and sumped is a better place to start. Can't overfill the fuel tanks when the are drained and sumped and essentially no fuel density error since not subtracting out the total usable.
     
    Last edited: Oct 20, 2017
  24. dmspilot

    dmspilot En-Route

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    The POH for a 172M instructs you to drain all fuel prior to weighing. I would say that following the manufacturer's weighing procedure is, indeed, "better", than not following it.
     
  25. Tom-D

    Tom-D Taxi to Parking

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    After reading this thread, isn't it amazing that there is a single aircraft out there that has a accurate W&B. sheet aboard?