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Fuel Measurement


Jim Meade

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It takes some time to get to know a new airplane, and how the fuel gage reads related to fuel burn. After you fly your airplane a while you will learn to judge how much fuel you have left by using the sight tube.

 

Too right, Tom.

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Agree. The fuel sight tubes are in the same location where the fuel outlets of the tanks are. This is where fuel flows to the engine. If you see fuel here, you have fuel available to the engine. If the fuel is at one's predetemined reserve mark (mine's one hour+), it's time to land and take more on. Can't be much more simple than this.

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Let me reiterate that when we did a very careful calibration of our fuel quantities, we found both sight gauge placards applied so the fuel levels indicated were off and by different amounts. I removed and replaced the placard so that zero on the sight gauge lines up with zero on the placard. The numbers on the placards still don't line up with the numbers that I marked right on the sight tube based on actual fill levels. There may be some parallax or other factors involved.

 

The Sight gauges are of course accurate in that they show what they show. Some of the markings may not be, so if the placard says 3 gallons it may be more or less. At least they were not accurate on my airplane.

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Regarding the reliability and value of fuel sight tubes (gauges), I am reminded of the American humorist, James Thurber, who wrote, "There is no safety in numbers, or in anything else".

 

As noted by others, the sight tubes are what they are. While in flight, they provide information about the amount of fuel in the tanks. The information I get from my sight tubes is not perfectly accurate nor is it perfectly inacurate. That makes them very much like all the other in-flight information I have about the amount of fuel in my tanks.

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I was just talking about looking at the tube. As far as I can tell it will never match the placard.

 

I flew a Thorp T-18 to Oshkosh this year. It has a fuel gage in the panel that reads E, 1/4, 1/2, 3/4, and full. It uses a float type sending unit. The tank is "V" shaped and holds 28 gallons. I can tell you that when the tank is at 1/2 or below you need to start thinking about landing for fuel. When it shows 1/4 you better be on the ground, because you have no reserve.

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I was just talking about looking at the tube. As far as I can tell it will never match the placard.

I flew a Thorp T-18 to Oshkosh this year. It has a fuel gage in the panel that reads E, 1/4, 1/2, 3/4, and full. It uses a float type sending unit. The tank is "V" shaped and holds 28 gallons. I can tell you that when the tank is at 1/2 or below you need to start thinking about landing for fuel. When it shows 1/4 you better be on the ground, because you have no reserve.

 

Do you mean in the Thorp, or the CT?

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From what I can gather from other pilots a fuel fudge on a small plane is only accurate at F and E. (And sometimes not even there.)

 

I believe the part 23 certification standards require fuel gauges to only be accurate at full and empty. If you think about it, with fuel sloshing around there is not really a way for them to be accurate at other levels.

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I don't have the references for LSA, but for standard certificated there are pleny of FAR references to show that gauges are to be accurate across the range and they are to show zero when there is no more usable fuel. Start with 91.205 and 23.1337. Again, this is for certificated. I'd have to look more for LSA.

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I don't have the references for LSA, but for standard certificated there are pleny of FAR references to show that gauges are to be accurate across the range and they are to show zero when there is no more usable fuel. Start with 91.205 and 23.1337. Again, this is for certificated. I'd have to look more for LSA.

 

I wonder if the FAR's were changed for standard certified aircraft over the years. If it has been that way for decades then I know for a fact that many (most) 1960's & 1970's 0ne hundred series Cessnas did not meet that requirement.

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I wonder if the FAR's were changed for standard certified aircraft over the years. If it has been that way for decades then I know for a fact that many (most) 1960's & 1970's 0ne hundred series Cessnas did not meet that requirement.

 

Nor would most modern Cirrus. With each of the four newer generations there was hope for improvement, but they still allegedly suck.

 

I'm fortunate that the gauge in my Sky Arrow seems spot on. Single tank in the fuselage behind the passenger. No sight tubes, but the tank is translucent and the visible portion is marked with gradations. No help in flight, but let's one confirm that the low end of the gauge reflects accurately.

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23.1337

(B) Fuel quantity indication. There must be a means to indicate to the flight crew members the quantity of usable f.uel 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 § 23.959(a);

 

(2) Each exposed sight gauge used as a fuel quantity indicator must be protected against damage;

 

(3) Each sight gauge that forms a trap in which water can collect and freeze must have means to allow drainage on the ground;

 

(4) There must be a means to indicate the amount of usable fuel in each tank when the airplane is on the ground (such as by a stick gauge);

 

§ 23.1553 Fuel quantity indicator.

 

A red radial line must be marked on each indicator at the calibrated zero reading, as specified in § 23.1337(B)(1).

 

91.205(B)

(9) Fuel gauge indicating the quantity of fuel in each tank.

 

I, too, have heard the "only needs to be accurate when empty" but have not seen a reference for that. Based on the above, it seems to me to be common sense that the gauge is required and is expected to be usable during flight, which would seem to me to have reasonable accuracy. 3/4 should not be 1/8 in calm, unaccelerated level flight, for example.

 

Again, I don't have the regs on LSA.

 

Which brings up another point - this is thread drift - how can ASTM or the FAA hold us to documents that are not in the public record? You have to pay for the docs. That gripes me.

 

 

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Jim,

 

The way I figured it is:

 

1. If the fuel gauge needle was moving around, I had fuel. :)

 

2. If it was stuck on the bottom, the tank was probably empty. :(

 

In reality, if you fly the same plane you pretty much learn what it is telling you, and you also use all the other tried and true methods. Of course, if fuel is being sucked out of an unsecured fuel cap and you don't see it, only the gauge will tell you. So. it needs to have some accuracy other than when the tank is empty.

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Can't be simpler.

calibrated from fully dry at 10 litre and 20 litre levels, (with a couple of litres put in first for "un-useable").

each flight filled with enough 1 hour jerry cans (20 litres each) to ensure adequate fuel for the intended flight

fuel management in a CT isn't rocket science.

 

a bit of yaw either way and it is pretty easy to know what you have.

anything under the 10 mark is good reason to anticipate your landing, IMO.

post-474-0-89423900-1380235595_thumb.jpg

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