How much fuel is left when the bottom is dry?

[Wildhorsetrail]

On an M20J, I'm trying to get a handle on what empty and full look like. If no fuel is visible straight down through the filler, what is the maximum amount that could still be in the tank?

[Skates97]

The easiest answer is to run one tank dry, or if you aren't comfortable with that close to what you think is dry. Then fill it and see how much it takes to fill it and do the math.

[Ragsf15e]

We also have a long thread about making calibrated sticks to dip the tanks before flying.  That’s probably your best bet. 

[jaylw314]

1 hour ago, Wildhorsetrail said:

On an M20J, I'm trying to get a handle on what empty and full look like. If no fuel is visible straight down through the filler, what is the maximum amount that could still be in the tank?

I think it's about 8 gallons, IIRC.  Just fill up the tank and subtract it from 32.  Not as precise as running the tank dry, but we are talking about one significant digit.

Edited May 24, 2022 by jaylw314

[N201MKTurbo]

When you can see a little fuel looking straight down ir is about 5 gal. If you can’t see any all bets are off.

[Wildhorsetrail]

It did end up coming out to about 5 gallons in my airplane.

[pkellercfii]

For my standard-tank K model, it’s about five gallons.  A standard tank J model should be very similar, since my understanding is that the J vs K fuel tank difference is outboard of the fuel caps.

—Paul K

 

[carusoam]

When comparing this value between various Mooneys….

Always know where the fuel cap is located….

The LBs moved the fuel cap up hill a ways from the smaller tanks of other Mooneys….

Some Mooneys have two fuel caps on each wing….

Others have bladders…

Cool ones have digital fuel floats….

When you look in the tank, and the bottom of the tank is uncovered….. nothing is going to show on the stick, no matter how calibrated it is….

If you have the 26gal tanks…. And no fuel is under the fill neck…. That is pretty empty…. And subject to the oddities of attitude and uncoordinated flight….

 

Everyone gets to learn the specifics of their own fuel tanks… 

Run each one dry, fill it up, one gallon at a time… take notes….

See how many gallons you can fit in it…

 

PP thoughts only, stuff that is written about often….

Best regards,

-a-

[FlyingDude]

Running one tank dry might create temporary vapor lock situation before you can switch to the other tank.  2-3 seconds of engine sputter might be bad for one's morale.

If you want to get the tank really dry, you can burn gas until it barely vanishes from your sight through the fuel cap, then lift the drain ring in the fuel selector and stick a plastic pen through it to keep it up and drain all the fuel from the fuel selector/gascolator.  If you want to be extra accurate, you can level the plane (remove nose wheel and put a 2x4 wood beneath, if you don't want to jack the plane up).  After maybe 30min, the fuel stops dripping: what remains is exactly the unuseable fuel in your tank with the plane in flight attitude.  If you collected all the fuel into a container with a hose (no evaporation), then you'll also know how much there is between the "barely visible through fuel cap" level and "no more useable fuel left" level.  Then, fill it up when the wings are level (if your fuel farm is sloped), and measure how much it takes up to the brim.

Given the fuel prices, I'd recommend collecting the draining fuel with a hose into a good container.  If you use a pan, you'll be watching $$ signs evaporate.  The environment and fire hazard, too, of course.