Everyone has it wrong about Unusable Fuel

[Ragsf15e]

2 hours ago, Shadrach said:

I've always wondered what the difference was.  Can you shed some light on the weight difference between electric and manual gear?

Looks like 13.4lbs…

[David Lloyd]

Notice also the weight of the fuel in that pic.  That was for 64 gallons usable at some temperature, I think about 85 degrees.  Not the 6 pounds per gallon we were taught but something near adjusted for temp.

[PT20J]

1 hour ago, David Lloyd said:

Notice also the weight of the fuel in that pic.  That was for 64 gallons usable at some temperature, I think about 85 degrees.  Not the 6 pounds per gallon we were taught but something near adjusted for temp.

Avgas only weighs 6 lb/gal at 15 deg C. It weighs less per gallon at higher temperatures and more at lower temperatures because a gallon is a unit of volume and the same mass of gasoline expands and contracts with temperature. See attached.

My M20J AFM/POH and SMM both specify using 5.85 lb/gal. which would occur at 34 deg C (93.2 deg F). Well, it is hot in Kerrville in the summer...

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VCF_Aviation-gasoline.pdf

[Shadrach]

4 hours ago, David Lloyd said:

Notice also the weight of the fuel in that pic.  That was for 64 gallons usable at some temperature, I think about 85 degrees.  Not the 6 pounds per gallon we were taught but something near adjusted for temp.

Perhaps @Ragsf15e’s plane and mine were done on the same 91° day...

Edited November 23, 2021 by Shadrach