Hey all, just a quick one been doing the BT practice qs and some online and the statement "usable fuel" has be confused. Does this include or exclude reserves?
thanks guys!

"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 - www.ecfr.gov/cgi-bin/text-idx?SID=d158f5.../14cfr23_main_02.tpl
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 You may need to go to the FAA website and look up superseded rules to make sense of things ...

The light aircraft Flight Test Guide may be found in this document - www.faa.gov/documentLibrary/media/Adviso...ircular/AC_23-8C.pdf.

Some specific guidance for establishing the unusable fuel quantity may be found in this document - www.faa.gov/documentLibrary/media/Adviso...rcular/AC_23-16A.pdf

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).

I messed up an online exam question which also had the term useable fuel. The question said the ECHO had 40USGAL 'useable' in the main tanks which I assumed was in a 'legal' sense useable fuel i.e. flight fuel... So I added variable and fixed.

Any time you see a reference to "usable fuel" it means the fuel available until the engine might falter/fail. I say "might" because, due to the way unusable is determined, usually there is some (anywhere from none to an undetermined bit of) additional fuel (in the unusable quantity) which, functionally, may be usable physically providing that the aircraft is flown gently and, generally, straight and level.

"Usable" definitely is used in a legal sense. The term is defined in the design standards and the certification requires you to comply with those standards (via the flight manual or POH). Just keep in mind that, when you exhaust your usable fuel quantity, you are either on, or very close to, flying on fumes.

Therefore, the total required fuel (ie burn off and ALL reserves plus anything else you might like to keep up your sleeve) MUST be taken from the usable fuel quantity. Total fuel = declared usable + declared unusable fuel. From a practical point of view, as pilots, we aren't really fussed about unusable .. it's usable that counts for keeping the noise going.

Given that most light aircraft only have a minor sump amount of unusable, it really is getting to be a question of exploring the semantics and definitions rather than practicality but that's the way the examinations go.

Not much use having reserve fuel if it’s not useable!