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John.Heddles replied the topic: Usable fuel definition
"Unusable fuel" is a certification definition and is included in the empty weight of the aircraft - ie, it doesn't come into operational calculations at all. Hence, usable fuel is any fuel in the tank in excess of the declared unusable.
Establishing the unusable fuel quantity involves a bunch of tests of which most pilots are totally and blissfully unaware.
The basic idea behind unusable fuel is that, with fuel levels above the declared unusable quantity, functioning engine operation should not experience any fuel supply interruption problems during operations which are normally to be expected. Depending on the aircraft's fuel system configuration, that may mean you can milk a little more fuel out of a tank in smooth conditions - but you aren't permitted to consider that in planning.
Some light bedtime reading for pilots in the training loop -
The current light aircraft design rules may be found in this document -
Note that the rules were revamped substantially in recent times. You will find lots of references to different rule numbers in your general reading. You may need to go to the FAA website and look up superseded rules to make sense of things ...
Unusable fuel quantity has absolutely nothing to do with fuel planning and operational fuel reserves. Operational fuel and reserves are based on usable fuel quantity. Reserves are there to provide you with some backup in the event of problems. Reserve fuel certainly can be used but such should only occur in unexpected conditions (variable reserve) or in extremis (fixed reserve).
Engineering specialist in aircraft performance and weight control.