Empty Weight

Discussion in 'Maintenance Bay' started by birdus, Sep 17, 2019.

  1. birdus

    birdus Pre-takeoff checklist

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    Is empty weight normally determined with full tanks or empty tanks? I assumed empty, but the most recent W&B of the Luscombe 8E which I just bought has the weight at 950 with full fuel. That's heavier than I would've expected the plane to be. By the way, the first W&B I could find in my pile of paperwork (1946 or 47) had the empty weight at 860.

    Thanks,
    Jay
     
  2. kyleb

    kyleb En-Route PoA Supporter

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    Empty weight is zero fuel.
     
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  3. Greg Bockelman

    Greg Bockelman Administrator Management Council Member

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    Empty weight is normally computed with unusable fuel only. Not full fuel.
     
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  4. ja_user

    ja_user Pattern Altitude

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    Greg has the “most correct” answer.

    Sorrry, just discussing the instrument written earlier.
     
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  5. birdus

    birdus Pre-takeoff checklist

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    As part of trying to solve this mystery, I've been reading the FAA Weight and Balance Handbook, and just got to this part in the section: Preparing for Weighing

    "Drain fuel from the tanks in the manner specified by the aircraft manufacturer. If there are no specific instructions, drain the fuel until the fuel quantity gauges read empty when the aircraft is in level-flight attitude. Any fuel remaining in the system is considered residual or unusable fuel and is part of the aircraft empty weight."

    So you guys are right on the money. Now I just need to figure out how the plane could weigh 860 pounds empty when new, and weighs 950 pounds now with full fuel. It just doesn't make any sense (2 x 12.5 gallon tanks). The only way it could add up is if the empty weight when new was calculated with full fuel and 90 pounds of other crap have been added to the plane over the years. The plane is very simple and I've been poking around getting to know my new plane and I would bet a hundred bucks right now there's no way 90 pounds of equipment have been added to it. So, I'll keep trying to figure it out.

    Thanks.
     
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  6. birdus

    birdus Pre-takeoff checklist

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    Here's something more that verifies that the empty weight should absolutely be without full fuel:

    "If it is not feasible to drain the fuel, the tanks can be topped off to be sure of the quantity they contain and the aircraft weighed with full fuel. After weighing is complete, the weight of the fuel and its moment are subtracted from those of the aircraft as weighed. To correct the empty weight for the residual fuel, add its weight and moment."

    And yet more interesting info:

    "The empty weight for aircraft certificated under the Civilian Air Regulations (CAR) part 3 does not include the engine lubricating oil. The oil must either be drained before the aircraft is weighed, or its weight must be subtracted from the scale readings to determine the empty weight."
     
  7. kyleb

    kyleb En-Route PoA Supporter

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    Have you personally weighed the airplane? Occasionally, people make mistakes somewhere in the weighing and recording process...
     
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  8. birdus

    birdus Pre-takeoff checklist

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    No. I just got the plane on Thursday and am getting things figured out. I'm going to be reupholstering the seats and I think I'll get it weighed after that. Thanks.
     
  9. Salty

    Salty En-Route PoA Supporter

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    Be prepared to lose useful load. I’ve helped weigh 3 planes this year, and the best gained 10 pounds and the worst gained nearly 50. I’m convinced the factories lied about empty weights when they came out of the factory. Most of the stuff we install nowadays is much lighter than what came out.
     
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  10. Doc Holliday

    Doc Holliday Pre-takeoff checklist

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    During aircraft production, not every airplane gets weighed. The manufacture weighs a comparable model and that weight is used (plus or minus installed equipment). For QA, every now and then one model will be pulled off the line for weighing to check the numbers are still correct.

    Over the years the typical GA airplane gets equipment added or subtracted. This results in sometimes no w&b info being updated, or the person updating the w&b screws up the math, thus resulting in the numbers being off. If the aircraft is conformed and flown Part 135 it gets a current w&b, and Twins get reweighed IIRC every 36 months.

    Unfortunately some GA aircraft never see scales and never have the original w&b updated.
     
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  11. MooneyDriver78

    MooneyDriver78 Pattern Altitude

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    I went through all W&B entries, found 1 missing but was able to figure out from the logbook what was missing. I put them all into a spreadsheet, out of around 100 entries, 15 were wrong: missing, transposed number, bad math, etc
    In the end most of the errors canceled, was only 2 lb difference.
    My only experience weighing the plane, the result was ~120 lbs overweight. My 1 fuel gauge was bad at the time, it read about ~20 gallons low, the mechanic stuck it on the scales and used erroneous fuel gauges (which read in pounds) and subtract the fuel weight shown. Another mechanic after extensive changes didn’t want to do the math, so wanted to weigh the plane. He proudly said he would not do that way, he was going to fill the tanks and subtract the POH capacity because it’s accurate.
    I obviously said NFW. Later doing a fuel calibration for the JPI I found out my plane holds 3+ gallons more than specified.
    I will never let a mechanic weigh my plane.


    Tom
     
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  12. Bell206

    Bell206 Pattern Altitude

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    A couple points: if the aircraft must be configured in a certain manner for empty weight and balance it will be stated in the TCDS and/or the OEM manual. If none exist then most follow the W&B Handbook, AC43.13-1B, or other acceptable guidance to configure/perform the actual weight and balance. As to what is residual fluids/draining. Part of the wt/bal process is to level the aircraft. Once leveled drain all needed fluids and what remains is considered residual.

    As a side note, Luscombe's also have certified empty weight/bal CGs which are dependent on aircraft configuration and need to be looked at. Plus don't forget all aircraft need a current equipment list to go along with the empty weight/bal. Luscombe's can be be difficult at times as there were quite a few factory options available over the years (and TC owners) which can make things interesting at times. A quick look at your TCDS listings will give you the idea.
     
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  13. MauleSkinner

    MauleSkinner Final Approach

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    Scrub the plane inside and out while you’ve got the seats out.
    while you’ve got the seats out, scrub the airplane down well, inside and out.

    Make sure there’s nothing “extra” in the airplane when it’s weights, like headsets, non-required documentation, etc.
     
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  14. Doc Holliday

    Doc Holliday Pre-takeoff checklist

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    I walked in on a mechanic and owner weighing a Mooney once. I had to laugh because as they were setting it up, the cabin and baggage compartment was full of the owners stuff he flew around with. They were oblivious.
     
  15. Salty

    Salty En-Route PoA Supporter

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    oblivious isn't the word I'd use. I did a thorough scan and removed every little thing that wasn't part of the equipment list.
     
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  16. MauleSkinner

    MauleSkinner Final Approach

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    How much you want to bet they took the stuff out, weighed it, and then put it all back in, with the owner still not adding it into his flight weight?
     
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  17. Bell206

    Bell206 Pattern Altitude

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    The quickest way to get a 5-10% price reduction on a pre-buy is to grab the weight/bal report and equipment list and point out the errors. Fortunately while empty weight/bal is one of the most misunderstood features on a aircraft it is one of the easiest to physically correct. Unfortunately I've seen some radical departures from the norm by both mechanics and pilots which in some cases didn't end very well.
    Curious, then who? Your only option is an A&P or repair station.
     
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  18. Salty

    Salty En-Route PoA Supporter

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    My interpretation is that he won't have it weighed at all.
     
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  19. Weekend Warrior

    Weekend Warrior Pre-takeoff checklist

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    I've owned several planes through the years. I've had each one weighed when I bought them. The CLOSEST a W&B has been was about 30 lbs under its actual weight. The worst was about 70 lbs under its actual weight. Every plane has come in over what the W&B said, between 30 and 70 lbs over.

    When weighing, the mechanics I've used don't drain the fuel, they just subtract it from the calculations. Maybe that's what happened, but he didn't show it (or he didn't show it correctly).
     
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  20. birdus

    birdus Pre-takeoff checklist

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    Anyone can buy scales and weigh their own plane.
     
  21. Salty

    Salty En-Route PoA Supporter

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    But they can't change the W&B without an A&P
     
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  22. birdus

    birdus Pre-takeoff checklist

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    So, after starting this discussion, I took a closer look at the latest W&B. I hadn't looked carefully enough before. I did see that it had been weighed with full fuel. What I hadn't noticed was that the weight came in at 1,100 pounds on the nose. They subtracted 150 pounds (2 twelve-gallon wing tanks = 6 x 25) and came up with 950. The oldest W&B (original, possibly) says 860. I absolutely cannot imagine where there's an extra 90 pounds in this plane. I'll try to figure it out, though. Thanks for all the interesting and helpful comments!
     
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  23. Greg Bockelman

    Greg Bockelman Administrator Management Council Member

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    Airplanes do tend to gain weight over time. Orphaned wiring, a radio or two that wasn’t originally installed, an electrical system.

    If you can find it, compare the original equipment list with the current one and see what the difference is.
     
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  24. Bell206

    Bell206 Pattern Altitude

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    Unfortunately, in my experience that is the easiest way to throw off the empty CG especially on aircraft with any type of gravity feed header tanks. You can never calculate exact weight/arm/moment of the fuel for a number of reasons. I've always found going through the hassle of draining everything required gave the best results for me and the owner.
     
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  25. Bell206

    Bell206 Pattern Altitude

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    FYI: just by going through the weight/bal calculation process over time can alter your empty weight as the "moment" is only a mathematical product and not related to anything physical like weight or arm. As you round off, things change and over time add up which is why certain aircraft require reweighs every 3 years to "reset" the record. Throw in one missed decimal and it can easily add 90 pounds over that length of time.
     
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  26. birdus

    birdus Pre-takeoff checklist

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    Ah. So you're saying that each W&B tends to be based on the last one, not from scratch, and so you get stacking errors (or the product of multiple errors)?
     
  27. Greg Bockelman

    Greg Bockelman Administrator Management Council Member

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    That definitely can happen.
     
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  28. Bell206

    Bell206 Pattern Altitude

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    Yes. Until the next actual weight and balance is performed.
     
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  29. Weekend Warrior

    Weekend Warrior Pre-takeoff checklist

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    I agree, but its usually close (at least MUCH closer than the old w&b with additions over the years), and if anything, coming out slightly lighter than actual, due to old planes gaining weight, but loosing fuel capacity. Over time tanks have a way of slightly caving in on themselves, and tend to seem to hold more fuel than they actually do.
     
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  30. champ driver

    champ driver Line Up and Wait

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    As a new owner you should be as least somewhat familiar with the TCDS for your plane. If you haven't already, you should make copies of the relevant parts that apply to your specific model. Here's a link to your TCDS, in it are CG limits, fore and aft, equipment lists and weights and other info you may need to know how to access when necessary.

    http://rgl.faa.gov/Regulatory_and_G...8be44c186257c9200786a6a/$FILE/A-694 REV25.pdf
     
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  31. Salty

    Salty En-Route PoA Supporter

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    I found about a half dozen math errors in my W&B records over the years, but as someone said above, they were minor and mostly cancelled each other out.

    My favorite one though was were the mechanic removed 8 pounds in front of the datum meaning he needed to subtract 8 pounds with a negative ARM, so it should have been a positive moment, but apparently he couldn't stand to add when he had removed weight, so he made it a negative moment which moved the CG forward instead of backward like it should have.
     
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  32. MauleSkinner

    MauleSkinner Final Approach

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    I’ve found at least a dozen bad ones over the years...bad math on the calculations, bad arms for the scales a couple of times.

    Only had one that I actually flew out of CG because of it...at least that I know of.

    When I get in a new-to-me airplane, I always check superseded weight & balances for a CG shift...anything over a couple tenths of an inch bears scrutiny IMO.
     
    Last edited: Sep 18, 2019
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  33. birdus

    birdus Pre-takeoff checklist

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    It wouldn't be official, no.