The Cessna 120-140 Association Forum

Sorry folks, my intended question was, after the tanks are drained from the firewall in the 3 point or flying configuration, how much unuseable fuel is left in the tanks.

In the "flying condition" not very much fuel is left. Years ago when I was young and adventurous, I ran the tank until the engine quit. Then I was able to get over 12 gallons into it after landing. So, I'd say 1/2 gallon or less while straight and level will be "un-usable".

IN the 3 point stance, probably a lot more is un-usable, because the tank "pick up" where the outlet of the tank heads into the airframe and on to the fuel selector, is in the middle of the tank, not in the back corner, where the fuel sump is. I have no idea, I've never measured it, but I would bet at least a gallon, perhaps more.

If you want to do it that way, level the plane first, then drain it through the gascolator.

If you want to use the FAA definition of unusable fuel the answer is 1/4 of each tank, or 1/4 of 25 gallons. OTOH, as someone has already mentioned, in level flight you can burn just about every drop of fuel in the tank. Just don't try to do it with both, and, if you land with both tanks below 1/4, be sure you land somewhere where there is fuel available!

A couple of days ago, I purposely flew on one tank until the gauge quit bouncing, switched tanks and landed. I drained the tank to check for contamination, and it only had about 2.5 quarts left.