That's an unreasonable expectation, since no gauge measures exactly.
One - I didn't say it would have to measure exactliy. I said that the regulation did not require the gauge to read an exact quantity in flight. Just that it is required to indicate a quantity. The only place the regulation requires the indicator to read an accurate quantity is at zero.
However, addressing the "no gauge measures exactly", I'd certainly hope that's not a fact. I would really like to know what my ITT is in each engine...same with my N1 and N2 speeds. I don't want the engines vibrating or burning apart because of an inaccurate instrument.
They're typically calibrated at a particular point
Yes, and per the regulation that point is at the end of the usable fuel or "zero".
and grow increasingly inaccurate at points far removed from the calibration point.
Tough to say that without seeing the mx requirements of the individual instruments and installations. Do you have any you could post or a link to some? I'd enjoy reading about it during my down time (my all-too-much down time lately).
The regulation specifies that the calibration point is zero usable, meaning that the gauge is most accurate as the fuel quantity gets smaller. You're reading some other intent into that paragraph.
No. It means that it is required to be calibrated at zero, just like the paragraph says. When it's at zero, you're out of usable fuel. Period. There's nothing else to read into that paragraph, though I think you might be doing that yourself. It has nothing to do with how accurate the indicator is at other quantities.
The regulation says it must "indicate to the flightcrew members the quantity of usable fuel in each tank during flight," which it would not do if the gauge were not reasonably accurate.
Yes, there must be a gauge and that gauge must be marked in gallons or pounds or fractions "of the tank" (1/2 tank, 3/4 tank), but you'll find no regulation requiring the calibration of the fuel quantity indicator to actually
read 1/2 when the tank is 1/2 full.
The only thing that would require that is the mx specs or installation requirements of the specific fuel quantity indicator. 23.1337 doesn't require it. It may happen (reference most transport category planes and even some of the larger part 23 aircraft), but it isn't
required.
The accuracy doesn't have to be defined in the reguations; it may be defined by the courts, or perhaps by maintenance tolerances based on the technology being used to measure the fuel. Again, it's doesn't follow that because the tolerances aren't in the regulation that no tolerances apply.
That depends on the installation. Take your typical 172 fuel gauge. What are the mx requirements and tolerances of that gauge? I can only imagine how they're different from a LR-35 or a B-737's gauges. Even different from other part 23 airplanes like a King Air 300 or Citation 525 or 501/551. That doesn't change the fact that the regulation only requires the gauges to be calibrated at zero.
Moreover, that line of the regulation would serve no purpose if the only thing required was for the gauge to be accurate at zero. And the whole idea of requiring a fuel gauge that's only accurate at zero is absurd. You can recognize that by your engine quitting.
I agree 100%, but that's what the regulation says. We don't have to like it, but that doesn't change that the verbiage of the rule only requires calibration at zero.
That is the minimum. Can your fuel gauge tell you when you've got 600lbs in the tank? Sure. Can it tell you when you're down to 1/4 tank? Yes. Can it tell you that you've got 82.6 gallons left? You betcha. Does it have to by part 23 and 25 regulations? No. Only at zero.
-mini