23.959—Unusable fuel supply.
(a)
The unusable fuel supply for each tank must be established as not less than that quantity at which the first evidence of malfunctioning occurs under the most adverse fuel feed condition occurring under each intended operation and flight maneuver involving that tank. Fuel system component failures need not be considered.
(b)
The effect on the usable fuel quantity as a result of a failure of any pump shall be determined.