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wrote:
C J Campbell wrote:


You were taught a very popular myth. No doubt you were also taught the
myth
(spread by Rod Machado and others) that your fuel gauge is only
required to
be accurate when it reads zero fuel. The FAR require you to have a
fuel
gauge that shows the quantity of fuel in each tank, whether you trust
it or
not.

Which FAR covers this?

I can find the one that requires a fuel indicator, but nothing about
accuracy.


Answering my own question...

23.1337 Powerplant instruments installation.

(b) Fuel quantity indication. 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);


So, (b) would imply that the 172 I rent that shows the right tank at
about 3/4 full when the tank is actually full is not airworthy since
it is not indicating the quantity of usable fuel in the tank.

That is correct. However, there is no regulation specifying exactly how
accurate the gauge is supposed to be. The one case I know of was a FSDO
inspector doing a ramp check who noticed the gauge read 2/3 full when the
tank was actually full. He said that was unacceptable and wrote it up. He
also failed my instructor on his assistant chief flight instructor check for
coming to the check ride with an aircraft that was not airworthy.

23.133 only defines zero fuel on a fuel gauge. After all, the manufacturer
could say that "empty" means zero total fuel if there were no regulation
defining what zero fuel means. The gauge has to indicate the quantity of
fuel in gallons or pounds, none of this business of unlabelled marks at each
quarter level like you see on cars. It does not give a blanket allowance for
the gauge to be inaccurate at any other level. 91.205 says:

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