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Gallons are not pounds

Fuel is measured in gallons but weight and balance needs pounds. The FAA’s standard teaching figure for aviation gasoline is 6 lb/gal (jet fuel 6.7). Always use the figure in the aircraft’s own document — here, 6 lb/gal 100LL.

Usable fuel is what the engine can use; unusable fuel stays in the tanks and lines and is part of the empty weight. The HX-4 Trainer carries 52 gal usable.