Empty Weight

[Dan Thomas]

How about just draining at the inlet to the fuel pump. Or the fuel selector valve. Someplace where what is not usable won’t get to and remain in the tanks. To be weighed as ‘unusable.’

Nope. With the airplane level you will drain out more than the unusable fuel, and there will still be fuel in the tank around the sump drain. The tank's outlet to the system is a little above the bottom of the tank so that debris and water tend to stay in the tank and find their way to the sump, where they're drained off. So you have to drain the system at the strainer or whatever, then drain the sumps.

If you don't get all the fuel out before adding the official unuable fuel, you end up with a higher empty weight and a smaller "useable" capacity that might show up when you're filling the tank five gallons at a time and marking a dipstick you're calibrating. If the TCDS unusable is small it might not show, but if it's significant (and some are) it surely will. Doing the job right is worth the extra few minutes.

Unusable fuel is defined as that fuel which will not reach the outlet in the attitude most critical for flight. This is typically a steep Vx climb attitude and/or a full-flap, power-off glide. With the outlet usually near the center of the side of the tank, the unusable fuel is that which can't leave via the outlet when the nose is high or low, and in some airplanes it's a lot. Five gallons, sometimes. With the airplane more or less level, that fuel will drain thru the outlet, so using your method will result in less than unusable. Calibrating a dipstick using that level as zero could get you into trouble. I've seen it personally.

 

[birdus]

For what it's worth, my Luscombe 8E's fuel tanks don't have sumps. Vx probably isn't that different than its attitude while sitting on the ground, maybe a bit steeper. We drained the fuel from the gascolator in the plane's 3-point attitude. In level flight, all the fuel is usable, although I don't know how much that differs from sitting 3-point on the ground

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[Dan Thomas]

This 172 left wing fuel tank will have significant unusable fuel. You're looking at the tank from the forward inboard and above, as if you were standing on the cowling. Note the single outlet at the bottom of the inboard wall about two-thirds of the way back. This arrangement would have a smaller amount of unusable fuel in a steep climb than in a steep nose-down attitude.

Later 172s had two outlets, similar to this tank, which I think is from a 152. Very little unusable fuel here. The two outlets will have lines leading down the doorposts and teed together at or near the selector valve so that fuel can flow even in extreme pitch attitudes.

Low-wing light airplanes can't do that. If one of the ports is exposed to air in the tank, the pump will suck air and not generate the suction needed to lift the fuel to the engine. Unusable fuel will be a larger number due to the need for a single outlet. One would need a more complicated system to reduce the unusable fuel, probably involving two outlets with a dedicated pump for each outlet, feeding a header tank that would send back to the tank any air that a pump sent to the header.