Fuel gauges only need to be accurate when the tank is empty?

Discussion in 'Lessons Learned' started by Ravioli, Apr 17, 2021.

  1. Ravioli

    Ravioli Ejection Handle Pulled PoA Supporter

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    So I'm doing preflight. Normally I do it alone, but due to motor car issue my CFII picked me up at home and was walking and talking while I did the preflight. [Not germane to the story but it's good to live 2 miles from your hangar, easier to get rides ;)]

    I get my highly calibrated stick and measure 12 gals in each tank. I turn on the master switch and highly calibrated gauge indicates 12 gals each tank, 24 total.

    CFII and I jump in, taxi out, he gives me a psuedo clearance, I psuedo read back, I punch it in to the 430W, confirm that was all done properly, delete the flight plan, put in the flight plan we're actually going to do, run-up, take off

    still climbing he asks "what's that red light?" it's the low fuel on the right tank. But hey, there's 12 gals in there! We both saw it.

    I hadn't seen this before, but thought, maybe when I level off it will get its act together

    I say, nope, we're calling this sortie off. switch to the left tank and return to land. The whole time I'm thinking gauge problem. Get her back in the hangar, get out the highly calibrated stick, 6 gals. The gauge is correct. So where did that 6 gals go?

    I give myself high marks for ADM. It would have been so easy for me to blame the gauge, which had just been crossed check with the stick, but I said nope, ain't right - no flight.
     
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  2. asicer

    asicer Final Approach

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    Fuel drain stuck open that worked its way back shut when you landed?

    How much was in the left tank after landing?
     
  3. MauleSkinner

    MauleSkinner Final Approach

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    In response to the thread title, a definite “no” per certification (assuming a certificated airplane).

    Good job on the ADM.
     
  4. unsafervguy

    unsafervguy Pattern Altitude

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    as you fly an aircraft that has a experimental certificate, 91.205 does not apply and you do not even need fuel gauges in it unless your op limits require it.
    § 91.205 Powered civil aircraft with standard category U.S. airworthiness certificates: Instrument and equipment requirements.

    my op limits include:
    7:aircraft instruments and equipment installed and used under 91.205 must be inspected and maintained in accordance with the requirements part 91.
    So if they are installed they must comply with all regs.
    and, under phase II,
    3. after completion of phase I flight testing, unless appropriately equipped for nigh and/or instrument flight in accordance with 91.205, this aircraft is to be operated under day only VFR.
     
    Last edited: Apr 17, 2021
  5. Bell206

    Bell206 En-Route

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    To add to the above, no, the fuel system is required to provide an indication of all usable fuel available in each tank during flight. Depending on which specific CAR 3 or FAR 23 the aircraft falls under the exact wording varies a bit. In most cases the indicator should be calibrated in appropriate units and clearly marked to indicate those units, with an additional requirement the indicator is calibrated to "zero" when the amount of usable fuel in level flight equals zero. And while a TSO fuel indicator is not required for CAR 3 or Part 23, if one is used the TSO has a requirement that system error is less than a certain percentage of the full scale reading.
     
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  6. unsafervguy

    unsafervguy Pattern Altitude

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    Have you figured out where it went yet?
     
  7. Stewartb

    Stewartb Final Approach

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    Bad cap gasket? Improperly seated cap? Was there any sign of fuel escaping aft of the cap? I left a cap off on my Cessna once. I knew I had filled to half tanks but the left gauge registered full. I knew something wasn't right so I returned to the strip to find the cap off and the tank nearly empty.
     
  8. Dan Thomas

    Dan Thomas Final Approach

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    If the airplane was sitting on a sideslope and the selector was on Both, the fuel will run through the system from the high tank to the low. Checking that low tank is only half the diagnosis; the other side might have had a lot more fuel than it started out with.

    And the old "fuel gauges only need to be accurate when they're empty" canard is tiresome. Does nobody--even many instructors--know how to look up the relevant regs and see for themselves?

    The airplane would have been certified under FAR 23, and this would apply. Read it carefully:

    FAR 23.1337

    b) Fuel quantity indicator. There must be a means to indicate to the flightcrew members the quantity of usable fuel in each tank during flight. An indicator calibrated in appropriate units and clearly marked to indicate those units must be used. In addition--
    (1) Each fuel quantity indicator must be calibrated to read "zero" during level flight when the quantity of fuel remaining in the tank is equal to the unusable fuel supply determined under [Sec. 23.959(a);]


    And this:

    §91.205 Powered civil aircraft with standard category U.S. airworthiness certificates: Instrument and equipment requirements.
    (a) General. Except as provided in paragraphs (c)(3) and (e) of this section, no person may operate a powered civil aircraft with a standard category U.S. airworthiness certificate in any operation described in paragraphs (b) through (f) of this section unless that aircraft contains the instruments and equipment specified in those paragraphs (or FAA-approved equivalents) for that type of operation, and those instruments and items of equipment are in operable condition.

    (b) Visual-flight rules (day). For VFR flight during the day, the following instruments and equipment are required:
    <snip>
    (9) Fuel gauge indicating the quantity of fuel in each tank.

    (c) Visual flight rules (night). For VFR flight at night, the following instruments and equipment are required:

    (1) Instruments and equipment specified in paragraph (b) of this section.

    (d) Instrument flight rules. For IFR flight, the following instruments and equipment are required:

    (1) Instruments and equipment specified in paragraph (b) of this section, and, for night flight, instruments and equipment specified in paragraph (c) of this section.


     
  9. MauleSkinner

    MauleSkinner Final Approach

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    Which the majority of our GA airplanes are not.
     
  10. flyingron

    flyingron Touchdown! Greaser! PoA Supporter

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    It matters not CAR 3 for FAR Part 23. There's nothing in the regs that say anything about the "accuracy " of the fuel guages.

    All the regs say is that the E mark indicates zero usable fuel (as opposed to bone dry).
     
  11. unsafervguy

    unsafervguy Pattern Altitude

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    doesn't raviloi fly an RV? none of that applies in that case.
     
  12. Dan Thomas

    Dan Thomas Final Approach

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    A CAR 3 airplane is still subject to the FAR 91.205 requirement for a means for the flight crew to determine, in flight, the amount of fuel remaining. That would also apply to an E-AB aircraft, would it not? Where is the exemption for a homebuilt to fly outside of that?

    A dead fuel gauge that reads empty when the tank is empty is still a dead fuel gauge and does not comply with FAR 91. A wildly inaccurate gauge doesn't comply, either.

    Edit: Bell206 has corrected me on the E-AB compliance; seems that 91.205 doesn't apply to it. I'm a Canuck, and here CAR 605 applies to anything that flies under power, and fuel gauges must be installed and working, even in a homebuilt.
     
    Last edited: Apr 19, 2021
  13. Bell206

    Bell206 En-Route

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    How so? I guess you missed this part above for the old Part 23: "the quantity of usable fuel in each tank during flight. An indicator calibrated in appropriate units..." Quantity being an amount or number. So how does that not apply to accuracy? 6 gallons is 6 gallons. Same with Part 3: 3.672 Fuel quantity indicator. Means shall be provided to indicate to the flight personnel the quantity of fuel in each tank during flight. The CARS/FARs also require other instruments to show oil pressure, RPMs, etc but there is no stated "accuracy" requirements either???
    Not quite. This is an additional requirement to the above quantity requirement. Except in the new Part 23 rule, they dropped both the "quantity" and "zero" requirement for this: (4) Provide the flightcrew with a means to determine the total useable fuel available... How an OEM achieves this is further described in several Part 23 ACs which go over the previous fuel quantity rules in CAR 3 and Part 23.
     
  14. Capt. Geoffrey Thorpe

    Capt. Geoffrey Thorpe Touchdown! Greaser! PoA Supporter

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    Accuracy would be a requirement to indicate "within x% of the total fuel" or within "x gallons / pounds / units" or something similar. Also not included in the regulations (to my knowledge) are precision or resolution requirements.
     
  15. Cap'n Jack

    Cap'n Jack Final Approach

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    I've no good answer about what a tank is supposed to read, but good decision! Thanks for the real-life lesson.
     
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  16. Harry Myler

    Harry Myler Filing Flight Plan

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    When did that start? My ‘76 Cessna ha E 1/4 1/2 3/4 F until we digitized a few years ago...
     
  17. flyingron

    flyingron Touchdown! Greaser! PoA Supporter

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    But it says nothing about accuracy. If it had an accuracy requirement it would say it has to reflect the quantity plus or minus (either percentage or some absolute value).
     
  18. MauleSkinner

    MauleSkinner Final Approach

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    Yes, precision is required...
    That doesn’t mean “1/2” has to be 12 gallons in a 24 gallon tank, but it does need to mean the same amount every time it reads 1/2.
     
  19. Dan Thomas

    Dan Thomas Final Approach

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    I didn't say that. FAR 23.1337 did. That was the 1996 version of that FAR, long after your airplane was built. Furthermore, if you know how much your tank holds, then 1/2 should give you some idea as to how much is in there, right?
     
    Last edited: Apr 20, 2021
  20. Dan Thomas

    Dan Thomas Final Approach

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    I did find some specification once and can't find it again.

    The FAA knows full well that the typical spam can fuel gauge system CANNOT be as accurate as everyone would like. The tanks are usually in the wings, which means that they're long and wide and thin, and any flight or ground attitude other than just perfect, with no turbulence or other movement, is going to move that fuel around and make the sender float sit somewhere other than where it should. Dihedral makes it even harder to gauge, since the sender has to be at the inboard end to read the last of the fuel. Large airplanes (transport category) use multiple senders in each tank, and the results are summed to get a more accurate reading.

    A digital system still uses the old-school float-on-an-arm to actuate some sort of current controller or voltage divider. A capacitive probe has no moving parts but still suffers the inaccuracies caused by location in the tank. We can get the electronics more accurate but the physical realities of the tanks limits the whole affair anyway.

    The tank in your car is much deeper, shorter and narrower than the ones in your airplane, it has the float or probe in the middle where the level changes minimally, and the gauge is much more accurate because of that.
     
  21. Bell206

    Bell206 En-Route

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    By accuracy are you referring to scale error or a numerical quantity number like 16 gallons? The only scale error requirements I've seen are with TSO indicators as the scale error requirement is part of the TSOA.

    Keep in mind this indicating system is required to show the usable fuel in each take which is the accuracy side of the equation. Once the certified usable fuel quantity is determined under a different CAR/FAR the indicating system must show that amount. So if the usable fuel is 16 gallons the indicating system must show that amount. The usable fuel quantity is required to be listed in the TCDS or POH/AFM. If there are any limitations, e.g., level flight, etc. that are needed to provide an accurate usable fuel amount those limitations are also required to be listed in the TCDS, POH/AFM, placards, etc. as per the CAR or FAR.

    For example, on a Cessna 172A, the TCDS lists 42 gal total and 37 usable split into two tanks. Quick math puts it at 18.5 gal usable per tank. The installed fuel indicator is required to show that quantity in each tank. So that is where the accuracy comes into to play. Without any allowable scale error like with a TSO indicator it needs to show 18.5 gal on the gauge. In the case of the mentioned old indicators that were marked as F, 3/4, 1/2, E, the F would equal 18.5 and so forth with 0 usable fuel left at E, even though there may still be several gallons left in the tank.

    In both CAR 3 and old Part 23 the process for determining usable fuel for each tank was the primary quantity number and the accuracy requirment. All the performance calculations and pilot fuel reserve figures were based of this usable fuel determination.
     
    Last edited: Apr 21, 2021
  22. Bell206

    Bell206 En-Route

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    But the 1/2 is required to equate to half the certified usable fuel quantity left in that tank. Which can be converted to a numerical value as mentioned above.
     
  23. A Martin

    A Martin Pre-takeoff checklist

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    .

    No mention of aircraft type that I could find ... no mention of engine size .... no mention of typical fuel burn per hour ... no mention of time spent on ground doing warmup and mag checks and full power run-ups .... no mention of time or length of taxi .... no mention of time in the air or circuits while preparing to land..

    Would have used a couple of gallons at least .... and to determine it by using a manual dipstick is like splitting atoms with a broadaxe .... not very precise
     
  24. Matthew Rogers

    Matthew Rogers Line Up and Wait

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    No mention of any correct punctuation or complete sentences in your post. ;)
     
  25. Dan Thomas

    Dan Thomas Final Approach

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    We made custom aluminum dipsticks for the flight school airplanes, calibrated while the airplane was leveled and at every five gallons added from the certified fuel pump, starting the dipstick marks at zero with unusable fuel in the tanks. Those sticks were far more accurate than the aircraft fuel gauges.
     
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  26. A Martin

    A Martin Pre-takeoff checklist

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    The question was not about punctuation or dipsticks.

    It was about the amount of fuel burned from startup to after landing .

    My Lycoming 435 will drink 6 gallons in 30 minutes and I am not puzzled as to where the "missing fuel" went

    What engine is in the OP aircraft ?
     
  27. MauleSkinner

    MauleSkinner Final Approach

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    But the dipstick statement was inaccurate.
     
  28. AggieMike88

    AggieMike88 Touchdown! Greaser! PoA Supporter

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    The original "I don't know it all" of aviation.
    trying hard to resist inserting quaker state dipstick meme
     
  29. asicer

    asicer Final Approach

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    Were these constructed in the same manner as marking a wooden paint stick? Or was it something different? I would think it would be harder to read an aluminum dipstick constructed in the same manner.
     
  30. Dan Thomas

    Dan Thomas Final Approach

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    Aluminum, with a crossbar on top to rest on the filler neck instead of scraping the tank or bladder bottom. They were made to just clear the bottom. The numbers were marked as the fuel was added in steps, then the numbers were punched into the aluminum with number stamps, then the whole thing was painted with green epoxy primer. That primer is a flat finish, fuel-proof, and shows instantly the exact level of the fuel. The dipstick had left and right sides, since the tanks often vary a bit, and they were also stamped with the aircraft's registration. No confusion between airplanes.
     
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  31. asicer

    asicer Final Approach

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    Thanks. I was thinking that it would be hard to see the fuel on bare aluminum, so it would appear that the epoxy primer is the key ingredient.
     
  32. Dan Thomas

    Dan Thomas Final Approach

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    Yes, very hard to see on bare aluminum. The primer turns much darker when wet with the fuel, and there's no question as to the level. It also doesn't wick the fuel up higher as a wooden stick can.

    As with any dipstick, you can get an erroneously high reading if the fuel is sloshing around in the tank a bit. If you climb up on the fuel steps of a Cessna, that can get the fuel moving enough to make the stick overread a little. And on a hot day the fuel can evaporate off the stick fast enough to show a lower reading; you have to be quick about it.

    Compromises. Nothing in aviation is perfect, not even the ideal dipstick.