Unusable Fuel?

ananoman

New Member
It is not possible to burn all of the fuel in the aircrafts fuel tanks. Some of this is done on purpose. The fuel pickup is not at the absolute bottom of the tank. Some space is left in case there is some water or sediment. This is what you are checking for when you sump the tanks.

When an airplane is certified the manufacturer has to do 'run out test' to see how much of the fuel carried is usable. If normal banking or maneuvering would cause fuel to move away from the fuel pickup then it is 'unusable'. Manufacturers obviously want to keep the amount of unusable fuel as low as possible. On small training aircraft, it is usually only a few gallons. More fuel tanks or large tanks will increase the quantity of unusable fuel.
 

EricH

New Member
Ah, I get it. I thought that might have been a possibility, but I was also thinking that unusable meant that it was the amount of fuel required by regulations to fly to the destination, and thereafter for 30/45 minutes, and being marked as unusable.

But it all makes sense now, thanks!
 

Derg

Cap, Roci
Staff member
You know when you chug a can of pepsi, but after you're done, there's just a little bit more you can't get out? Very roughly the same thing.
 

ananoman

New Member
[ QUOTE ]
Ah, I get it. I thought that might have been a possibility, but I was also thinking that unusable meant that it was the amount of fuel required by regulations to fly to the destination, and thereafter for 30/45 minutes, and being marked as unusable.

But it all makes sense now, thanks!

[/ QUOTE ]

What you were thinking of is referred to as 'reserve'

If you read airplane test reports in magazines they will often list the range/endurance with reserve at different power settings. This is somewhat meaningless, as the reserves you are required to carry is different for day/night VFR and IFR.
 
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