how much confidence do you have in fuel gauges?

Discussion in 'Hangar Talk' started by readytocopy995, Jul 30, 2022.

  1. steingar

    steingar Taxi to Parking

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    Of the three aircraft I've owned only one had fuel gauges who's readings bore some some relationship to the quantity of fuel aboard.
     
  2. Dan Thomas

    Dan Thomas Final Approach

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    These gauges worked remarkably well when the airplanes were new. I flew a six-year-old 172 in 1973, and a Cherokee that was a bit older than that, and a Cardinal that was maybe five years old. I don't remember any problems of inaccuracy with any of their fuel gauges. Most airplanes now are OLD and they need work. Would the fuel gauge in your 1967 Ford still be working accurately after 55 years and half a million miles or more?

    If they are maintained, and worn parts replaced, they will be quite accurate. I've done it. There is no magic to it. And endless statements claiming that "every airplane I fly has bad fuel gauges" are not proof that they were always useless; they're just proof that maintaining some systems is not a priority. What else in the airplane is being ignored?
     
  3. readytocopy995

    readytocopy995 Pre-Flight

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    This is basically what I'm talking about. I have a 172S. When I'm looking at flight planning if you plan to land with say 8-10 gallons, you could have 4/4 or 5/5 in each tank. That's supposed to get me 45+ minutes of fuel. The gauges in my plane were surprisingly accurate, if it shows 5/5 and I stick it, it's got 5/5. I guess what I'm asking is will the plane really perform just fine with 1 gallon of fuel in the wing? Personally I prefer to land with about an hour of fuel minimum depending on conditions and what I'm doing
     
  4. Rgbeard

    Rgbeard En-Route

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    Are you familiar with the term "unporting"?

    Not sure if/how it applies to high vs. low-wing?

    If you do simple maneuvers with too little fuel in the tanks, it can be problematic for most from what I know.

    I'm no expert - but you won't catch me flying with fewer than 5 gallons in each side. I will have landed well before then.
     
  5. Stewartb

    Stewartb Final Approach

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    The best reason for changing to a L-R-Both fuel system is to prevent fuel starvation due to unporting. Header tanks are another way to deal with it.
     
  6. Gary Ward

    Gary Ward Cleared for Takeoff

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    I don't know and hope I never have to find out if it will fly with 1 gallon on each side. Only twice did I run them to 3 gals each. Both times I was uncomfortable to say the least. First time was early on with my wife in the plane after 3.5 hrs of flying. I did not say anything to her about it and I was heading home to my airport and crossed over 2 local airports that I knew I could stop at if need be. But my fuel gauges kept moving so I knew I had some fuel in the tanks. I landed at home OK except my wife was unhappy cause she needed to use the bathroom. Note to self "don't fly 3.5 hrs with her in the plane again". When my gauge needles settled down they were reading 3 and when I stuck the tanks there was 3 gallon left according to the gauge stick.

    Second time was just last winter flying with my buddy in his Cherokee and me in my 172. We both landed at Zaneville Ohio on a 10°f night at 6:10pm and the FBO had just closed on a week night so no fuel truck driver. FBO doors were locked and no code to get in to warm up. Snow on the ramp. I had planned to get fuel there. There was no self serve pumps. I was nervous and annoyed because we got a late start no fault of my own.
    We planned to go west into the wind which is toward home in the dark at 5° it was fricken cold. I went first and he followed me in his "tanker" cheerokee with 50 gal tanks! lol
    I didn't feel I was gonna make it to the airport with the cheaper fuel(not that I cared) so I told him on the radio I was deviating a bit south and landing at a closer airport. Stupid head wind. I made it and stuck my tanks first before filling up. One was 3 gals and the other maybe 4 gals just what the gauges said on the ground, that looks empty in the air. I was scared and a bit influenced by my friend wanting to push on. I made the right call and he landed with me. Said he would never criticize anyone for landing early to get fuel. That was a scary cold night as I was under an hour of fuel at night. Try my best not to let that happen again.
     
    Last edited: Aug 1, 2022
  7. Dan Thomas

    Dan Thomas Final Approach

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    Your 172S AFM will tell you what the unusable fuel is. That's the minimum you should ever have in the tanks unless you're draining them for maintenance. Unusable fuel is the fuel that won't reach the engine in the attitude most critical for flight, such as a full-flap, power-off approach, or a VX climb.

    This is from the TCDS for the 172S:

    upload_2022-8-1_15-38-21.png

    So three gallons unusable, 1.5 per side.

    Note 1, mentioned in that excerpt, has to do with the weight and CG of the unusable fuel. It's part of the empty weight.


    Any dipstick should be calibrated to read Zero at the unusable fuel level, NOT when the tank is totally empty. Some airplanes have significant unusable fuel. Calibrating the stick to Empty means that sooner or later you will rely on that unusable fuel, and might be sorry you did.
     
  8. Gary Ward

    Gary Ward Cleared for Takeoff

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    My tanks hold 21.5 gals each and 1.5 is un usable per side I believe. I figure 20 per side for flight planning.
    Wish I had the larger tanks like your S model. It I did I probably would put a 360 in the plane a couple years ago?

    It's only 25000 bucks a pair to swap wings with Williams for wings that accept larger tanks not counting the tanks and tank covers or labor. Close to 30Gs for larger tanks for my plane.
     
  9. readytocopy995

    readytocopy995 Pre-Flight

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    Yeah your first story is what I'm driving at. What's really the line for safe vs dangerous in fuel levels? Everybody seems to have a different opinion. Some say half tanks which I think is too conservative.

    Correct the plane has 53 gallons usable over 56 gallons total. But does that literally mean until you hit 0 the plane is going to run just fine? That's kind of the point of this thread since I have no intention of trying that out in the wild. Just always wondered. Because if so, then planning to land with maybe 8-10 gallons of fuel is a perfectly fine idea. But currently I try and land with like 15 gallons or so (usable) because I'm not sure how "confident" I am in those last 5-10 gallons. Does that make sense?....

    Essentially I would like to have a better sense on if with 1 gallon usable on each side how close it is to the engine hesitating or dying? Does 2 total usable gallons literally mean you can burn those 2 gallons and not be able to tell the difference? Again this is PURELY from the perspective of the engine hesitating or dying when you're making your turns in the pattern or whatever, I am not suggesting this is a safe practice when considering all other factors (go arounds, missed approach, etc...)
     
  10. Dan Thomas

    Dan Thomas Final Approach

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    I wouldn't run it down to 1.5 per side. If you had to abort the landing and go around, you could be in real trouble.

    If the tanks are on Both, if one unports due to uncoordinated flight the other will still feed. Any air that gets into the line will be vented back to the right tank from the header tank. Useable fuel means just that: it will continue to flow, but landing with 15 gallons remaining might be a bit extreme. That's nearly a third of full tanks.
     
  11. Rgbeard

    Rgbeard En-Route

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    Half tanks is too conservative in my opinion. 3 gal per side is too few in my opinion. unporting is a risk that increases with any type of maneuvering, slips being especially troublesome.

    I have my risk tolerance levels that are mine. You’ll need to make your own decisions.
     
  12. readytocopy995

    readytocopy995 Pre-Flight

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    To be completely clear, 1.5 gallons each side is too few to be flying around on. It's a few minutes worth of fuel. Terrible idea. But I want to know, when does the engine start having fuel starvation issues when you're doing normal maneuvers. That's how I would set my limits, a buffer from that point.

    Like for instance say on a day VFR flight, no chance of IFR, normal conditions, etc... I decide I'm comfortable landing with 30 minutes of fuel. In a 172S that's about 5 gallons to make the math easy, and let's round to 6 to split evenly across two tanks. If the plane starts having fuel starvation issues with normal maneuvers at 2/2, then my own personal risk tolerance I would want +3/+3 from that. So I'm not going to want to land with less than 10 gallons.

    ....But does that start at 2/2? Does it start at 0/0? 4/4? It changes things quickly.

    Agree on all accounts. But I am having a tough time making a call on this because I don't know. Does "0" fuel mean that's when it will start having problems? Or will that start way sooner like when you have a couple of gallons in each tank?
     
  13. Rgbeard

    Rgbeard En-Route

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    What kind of maneuvering are you considering in this. You find circumstances get you close to the airport 1,000 ft above the traffic pattern. Perfect setup for a nice slip. But should you?

    There’s also the carbs vs injection considerations.
     
  14. readytocopy995

    readytocopy995 Pre-Flight

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    sure a little slip, normal turns in the pattern.
     
  15. Pinecone

    Pinecone Pre-takeoff checklist

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    As has been said, but seemingly not read, is that unusable fuel is fuel that cannot be used IN ALL FLIGHT ATTITUDES.

    So you may be able to burn that fuel in level flight, but when you push the nose down to descend, the fuel pickups unport and the engine quits. Or you have to go around, and when you pitch up, the engine quits.

    The gauges are supposed to read 0 (Empty) when you are down to unusable fuel.

    The only way to truly calibrate the gauges and your dipstick is to fully drain the tanks, then add in the unusable as specified in your POH. That should show 0 on your gauges (if your plane is in level flight attitude).

    And your dipstick should read 0 in the normal ground attitude.

    Most of this discussion has been around Cessnas. But for low wing, I would prefer to have 6 gallons on one side with near 0 on the other, versus 3 and 3.
     
  16. PaulMillner

    PaulMillner Line Up and Wait

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    Interestingly, Cessna used three different suppliers at different times for fuel gages in the '70's... two of them operated the way you outline, the other operated the other way around.

    As far as accuracy, aircraft manufacturers aren't required to use TSO'd fuel gages... but the TSO for fuel gages requires 3% accuracy, of full range I believe. So if you install aftermarket TSO'd gages, that's what you'll get, which is a bit better than the implicit accuracy in the certification standard.

    Paul
     
  17. Stewartb

    Stewartb Final Approach

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    What plane do you fly that unports in a normal flight attitude? My planes have front and back ports on each tank. Banking can unport a tank but wings level should not. That’s part of the usable fuel determination.
     
  18. Clip4

    Clip4 Final Approach

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    Depends on the model year of the 172.
     
  19. Dan Thomas

    Dan Thomas Final Approach

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    The "other way around" would imply that with the master off, the gauges go to full. I never, ever saw that in any of the many 172s I worked on. What am I missing here?
     
  20. Dan Thomas

    Dan Thomas Final Approach

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    Yes. Exactly. The Cessna should have the selector on Both for takeoff and landing. If you slip, you unport one tank but the other has the fuel piled up against the port (or ports, in later models). Gravity flow means that the fuel will still get to where it needs to go. That applies to the lighter singles; some, like the 210, do not have a Both position, and unporting and fuel starvation can occur just like in a low-wing airplane. The minimal dihedral of the 210's wings make that more likely.

    One should know his/her POH or AFM very well, but I regularly hear that the owner doesn't have such a manual. That's not only illegal, it's asking for an accident due to ignorance. From a 210 POH:

    upload_2022-8-2_9-50-35.png
     
  21. Stewartb

    Stewartb Final Approach

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    Fuel injected Continental-powered Cessnas use an accumulator tank. Unporting isn’t really a threat. The fuel shutoff is not on the tank selector. It’s a separate valve downstream of the accumulator tank. Know your fuel system.
     
  22. PaulMillner

    PaulMillner Line Up and Wait

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    Yeah, that's not the interpretation I had in mind. The other way around means that grounding the fuel gage wire gives an empty tank indication... opening the wire gives a full tank indication. When the master is off, there's no power to move the meter off the zero peg... so it still reads empty with no power.

    Paul
     
  23. mryan75

    mryan75 Pattern Altitude

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    I know you’re speaking hypothetically, but 1.5 per side would be illegal. And 30 minutes’ fuel remaining is legal, but as my first instructor always says, legal isn’t always safe. Everyone has their own risk tolerance but I think trying to land with 30 minutes’ fuel in the tanks is nuts. You just never know. I was doing pattern work once and a guy ground-looped his Wilga (not for the first time) and tower closed the field. Nearest airport is 25 minutes away. I’d hate to have been doing pattern work thinking “no problems, I’ve got 30 minutes’ fuel and the airport is RIGHT THERE.” Yeah, well…

    To me this is a huge blind spot for GA. On average once a week in this country, someone runs an airplane outta gas. About 50 times a year; once a week. For something that should literally never happen that just blows my mind. If people think half tanks is too conservative, I respect that. But I will damn well never run my plane outta gas. Besides, going up and coming down is the fun part.
     
  24. mryan75

    mryan75 Pattern Altitude

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    I said specifically legacy GA aircraft.
     
    Last edited: Aug 2, 2022
  25. mryan75

    mryan75 Pattern Altitude

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    No, it’s a simplistic piece of advice you disagree with. Which is fine. It’s not bad advice.
     
  26. Zeldman

    Zeldman Touchdown! Greaser!

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    I trust fuel gauges more than this...

    upload_2022-8-2_12-7-8.jpeg
     
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  27. readytocopy995

    readytocopy995 Pre-Flight

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    again, i'm not suggesting at all that I intend to land with 1.5 gallons per side. that is foolish. i am 100% with you on the "you never know" part, but the times I'm saying I would be comfortable with 30 minutes of fuel is when I'm totally VFR, totally good weather, totally good TAF, and in my area of SoCal I have something like 8 airports within 10 minutes. if i'm going somewhere where the nearest diversion is 25 minutes away, then yes you plan for more fuel.

    but again my main question was centered around "where does the engine start to show signs of fuel starvation" which has sort of been answered to be once you are hitting the "unusable fuel" and the reason for that question is not because i intend to fly to that point, but because i wanted to set my personal fuel minimums above that point, not just above zero fuel.
     
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  28. FPK1

    FPK1 Pre-takeoff checklist PoA Supporter

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    Isn't this what we are all taught?
     
  29. Tarheelpilot

    Tarheelpilot Final Approach PoA Supporter

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    It was/is commonly taught but not accurate.
     
  30. Pinecone

    Pinecone Pre-takeoff checklist

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    That is the definition of unusable fuel. Fuel that cannot be fed to the engine at the worst normal attitude.

    So if the fuel ports are near the back of the tank, they could unport in a nose low attitude.

    But it could be that there is no attitude where the fuel pickups unport at a higher level than level flight. But their may be. Do you know the fuel system of EVERY plane?
     
  31. Tarheelpilot

    Tarheelpilot Final Approach PoA Supporter

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    I think it is bad advice. To each their own.
     
  32. Pinecone

    Pinecone Pre-takeoff checklist

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    So, you would land with 50 gallons of fuel, since that is half of the 100 gallon total usable fuel? LOTS of legacy aircraft have 100 gallon or high fuel capacities.
     
  33. Stewartb

    Stewartb Final Approach

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    Fly with one tank selected and fly it empty. Find out what it does.

    My Cub’s sight gauges are horrible indicators. If I rock the wings I can see that something’s there. Fuel flow instruments have an im
    I know mine. Fore and aft ports on both sides, both planes. The question is what plane you fly that doesn’t? Old stock Cubs only had two ports on one side, thus takeoff and landings were with the left tank only.

    As has been said, know your fuel system.
     
  34. Dan Thomas

    Dan Thomas Final Approach

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    A Cessna 150 fuel tank:

    upload_2022-8-3_11-13-34.jpeg

    This is for the right wing. The outlet is on the bottom or the inboard side, 2/3 of the way back from the front. You can see that a fuel-flap-power-off approach would unport it earlier than in a climb.

    That fuel line runs from the port and down inside the rear doorpost, under the floor and tees together with the left tank's line to the fuel shutoff valve.

    A Cessna 152 tank:

    upload_2022-8-3_11-16-24.jpeg

    This one is for the left wing. It has outlets at the front and rear of the inboard side. The aft port feeds a line that runs down the aft doorpost, and the front port feeds a line that runs down the front doorpost. They tee together under the floor. This arrangement reduces the unusable fuel level by quite a margin.

    Similar mods were made to the 172 in later legacy model years. The POHs give details.
     
  35. mryan75

    mryan75 Pattern Altitude

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    I think the relative inability to determine how many gallons are still in the tank is kind of my point. I’m any event, to reiterate, I fly until the gauges read about half, then I pick somewhere to go for fuel.
     
  36. mryan75

    mryan75 Pattern Altitude

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    Do you have a specific reason why?
     
  37. SoonerAviator

    SoonerAviator Final Approach

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    Only fuel gauge I trust less than an aircraft fuel gauge is the one in a boat. However, the one in the aircraft generally has more dire implications when it comes down to accuracy.
     
  38. Maxnr

    Maxnr Line Up and Wait

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    Yup. A Barron that I used to fly was totaled by an unthinking pilot. He made a brief passenger drop enroute. He reentered the runway in a right turn and accelerating. #1 quit at rotation. The Flight Manual does say "No turning takeoffs." It also says "No take offs with selector on aux." He failed to switch to mains. BTW the aux tanks only have a single port, the mains have fwd & aft ports. He became unemployed and had some certificate action.

    I once flew a J-3 for over an hour and noticed that the float type fuel gauge still indicated full. I twiddled with it after landing and it operated more or less normally, but with some binding.

    Capacitance fuel gauges are usually dead on if they are recalibrated as scheduled. One type that I'm familiar with has both capacitance fuel level indication and float type low fuel level warning. Sometime the Low Fuel light comes on when descending, goes off when level off, but the fuel gauge is dead on.
     
  39. Dan Thomas

    Dan Thomas Final Approach

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    A classic example of Primacy here.

    PRIMACY - Present new knowledge or skills correctly the first time. (Teach it right the first time.)

    (a) When students are presented with new knowledge or skills, the first impression received is almost unshakeable. This means that what you teach must be correct the first time. Students may forget the details of lessons, but will retain an overall image of the skill or knowledge for a long time. Frequently you will be required to perform manoeuvres in the aircraft before a student has had the necessary background training. You must perform those manoeuvres correctly or the student may imitate any errors you make.

    From https://tc.canada.ca/en/aviation/pu...e-tp-975#part-i-learning-and-learning-factors
     
  40. Tarheelpilot

    Tarheelpilot Final Approach PoA Supporter

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    Yes.