Fuel quantity with bladders

[AndreiC]

During my recent very long flight I tried to run one tank (left) empty so I could get an idea how much it actually holds. I have bladders, installed by some previous owner. The tanks are placarded "Fuel quantity 27.4 gallons" each side.

I left with the tanks completely full. The engine quit after my fuel totalizer showed 27.2 gallons used, which is well within the tolerance of my totalizer. So far so good.

When I got home I filled the tanks back to the brim, starting with the tank that had been emptied out. What puzzles me is the fact that the fuel pump showed only 26.2 gallons pumped when the tank filled up.

Can it be that the airport fuel pump is so inaccurate? I find that hard to believe. Do the bladders somehow constrict if they are not full, and maybe when I go back to the plane I'll find a bit of room in the left tank? Or is it possible that 1 gallon of fuel somehow got pumped back from the right tank to the left when I had the electric pump on? I am imagining all kind of things.

Thanks for any suggestions.

[Fly Boomer]

5 minutes ago, AndreiC said:

What puzzles me is the fact that the fuel pump showed only 26.2 gallons

I would trust the gas pump.  Those things are closely regulated.

[jetdriven]

When you run the bladders totally empty they tend to kind of sag a little bit and then when you fill them back up, air is trapped behind them and it takes a couple of minutes for it to squish out so if you go back in the morning, you will see they hold one more gallon per side

[Taildraggerpilot]

Try filling up one tank to the collar, then fill up the other tank to the collar. Then, go back to the first tank. You may be able to put in another gallon or so. Repeat for second tank……

[teethdoc]

So you ran the tank until it died, in the air?!

[AndreiC]

1 hour ago, teethdoc said:

So you ran the tank until it died, in the air?!

Yes, at 15,500 feet. At first signs of engine hiccups I switched tanks, and engine came back in a few seconds. I think this is a Mooney-recommended way of fuel management for long trips: fly one hour on one tank, then fly till the other tank runs dry, then fly on the first tank the rest of the way. That way all your available fuel is in one tank at the end. 

YMMV, make sure to check your POH for the recommended method for your plane. 

[AndreiC]

On 6/4/2024 at 5:55 PM, jetdriven said:

When you run the bladders totally empty they tend to kind of sag a little bit and then when you fill them back up, air is trapped behind them and it takes a couple of minutes for it to squish out so if you go back in the morning, you will see they hold one more gallon per side

You were right. I went yesterday to the plane, and without having touched it the level in the left tank was low enough that it looked like it could hold another gallon. Thanks.

[MB65E]

1 hour ago, teethdoc said:

So you ran the tank until it died, in the air?!

Common practice. I do it when I need to. 
Some are not comfortable with it. No sense having an unknown but low quantity in a tank. I want to know if a tank is done or not. Most flights it doesn’t matter. Some flights it does.
I’m also comfortable landing with <8g of gas or so. That’s an hour of fuel. But 4g  in each wing would not be as ideal as 8g in one wing. The fuel flow meters are very accurate, you can also sense when the engine will stumble by watching the fuel pressure flicker. 
-Matt

[Mooneymite]

5 hours ago, AndreiC said:

You were right. I went yesterday to the plane, and without having touched it the level in the left tank was low enough that it looked like it could hold another gallon. Thanks.

Yes.  I noticed this phenomenon many times on my C's bladder tanks.  I would fill the tanks chock-a-block full the night before an early launch, but the next morning the tanks were "less than full".  I supposed that the bladders "bulged" overnight creating a bit more capacity.   However the cooler temperature overnight may have been responsible for some of this as well.

[A64Pilot]

Ref fuel tank capacities.

Of course there is some variation between aircraft. Aircraft I used to build we quoted a 228 Gl capacity and of course it was checked on every aircraft we manufactured, but on average they all held a little over 230, a couple maybe even as high as 235, none held less than 228.

The FAA is fine if one holds slightly more than advertised, but if one holds even one Gl less it’s a pretty big deal starting with paperwork that Engineering has to write a deviation for, that aircraft gets its own special by serial number POH and all placards have to be special made to reflect actual capacity. We never had to do that.

So I suspect it’s common for most aircraft to hold a little more than the POH and placards say they do.