"Gig Giacona" wrote in message ...
]
Empty... unusable... Not much difference in my book. If there is no fuel
flowing to the engine, it's empty!
Of course, if you know the Chinese fuel management technique, you can
extract some or all of the unusable fuel.

The regulation reads

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

23.1337 says
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
§ 23.959(a)