Fuel Flow Discussion, Moved from Float Mounting

[Archer39J]

Oh, this is the situation that only applies if your fuel pump is sucking fuel. We went over this already in the returnless FI thread.

In a properly constructed Bob fuel system this shouldn't be an issue, unless you're running FI with a full return. Remember that whole discussion about not over taxing the Bob systems ability to supply fuel?

[jaredyates]

Here's an experiment you can do at home that is the same phenomenon. Get 2 straws and a glass of water, put both straws in your mouth, one in the glass of water and one outside the water. Suck and see how much water you get in your mouth and how much air.

Thanks for this... It seems to apply especially in a low-wing situation. But whether our system is entirely gravity or not, it is still partially gravity-fed. If the pump is forward of the fuel valve, how can it be possible for one straw to be in the air? Wouldn't tank gravity pressure provide flow through the valve and up to the pump? Or is the issue that the pump is pulling more than the gravity pressure provides? Not trying to argue, but rather just to understand how this could happen in our case.

[jaredyates]

I moved the discussion out of the floats thread, but it looks like some of the comments didn't survive the move?

[zkelley2]

So you need to have a fuel pump for this to be a thing. It does not apply to gravity feed. If you have it on both and run 1 tank dry, it will suck the air from the empty tank and not the fuel from the fuller tank. If this happened in flight you can just switch to the tank with fuel in it, but you'll need to remember to do that and know which one that is. If you're not aware of this, you might think, it's on both, so clearly it has access to all the fuel.


The relevant FAR is 23.951b - Each fuel system must be arranged so that— (1) No fuel pump can draw fuel from more than one tank at a time; or (2) There are means to prevent introducing air into the system.

Then there's the differential head pressure issue as well. Which I think you could do by installing 1 fuel cap backwards and probably sideways and possibly some extent between 90 and 0 degrees.


Here's an experiment you can do at home that is the same phenomenon. Get 2 straws and a glass of water, put both straws in your mouth, one in the glass of water and one outside the water. Suck and see how much water you get in your mouth and how much air.

[whee]

The reg is 23.975(a)(4). It offers no discription so is not particularly helpful. If we look back to when the regs were written in blood, as they say, we can see the same requirement in CAR 3.446 but no reasoning. CAR 4a has some helpful info.

§ 4a.606 Tank installation. No fuel tank shall be
placed closer to an engine than the remote side of a
fire wall. At least one-half inch clear air space shall be
allowed between the tank and the fire wall. Spaces
adjacent to the surfaces of the tank shall be ventilated
so that fumes cannot accumulate or reach the crew or
passengers in case of leakage. If two or more tanks
have their outlets interconnected they shall be
considered as one tank and the air space in the tanks
shall also be interconnected to prevent difference in
pressure at the air vents of each tank of sufficient
magnitude to cause fuel flow between tanks.
Mechanical pump systems shall not feed from more
than one tank at a time except by special ruling from
the Administrator.

Also helpful is this NTSB finding. Note that Luscombe fuel system is essentially the same as those found on Bearhawks except that there is no unusable fuel in a Luscombe.

http://www.aopa.org/asf/ntsb/narrati...20090810X75326

[svyolo]

Other than the little vent tidbit, there really isn't enough info on his particular fuel system to speculate what happened. It would be nice to learn something here.

[svyolo]

It wasn't in AC43, it was in a different FAA (FAA-H-8083031A page 14-10) pub describing a single engine, gravity flow fuel system with 2 wing tanks and a fuel selector valve. It shows a different diagram for high wing/fuel injected. That diagram shows header tanks and no BOTH on the fuel valve. I believe this pub is an educational pub, not regs.

High-wing aircraft with a fuel tank in each wing are common. With the tanks above the engine, gravity is used to deliver the fuel. A simple gravity feed fuel system is shown in Figure 14-12.
Figure 14-12. The gravity-feed fuel system in a single- engine high-wing aircraft is the simplest aircraft fuel system.
The space above the fuel level is vented to maintain atmospheric pressure as the tank empties. The two tanks are also vented to each other to ensure equal pressure when both tanks feed the engine. A single screened outlet on each tank feeds lines that connect to either a fuel shutoff valve or multiposition selector valve. The shutoff valve has two positions: fuel ON and fuel OFF. If installed, the selector valve provides four options: fuel shutoff to the engine; fuel feed from the right-wing tank only; fuel feed from the left fuel tank only; fuel feed to the engine from both tanks simultaneously.

[jaredyates]

I would like to understand the science here.. for me it seems that when the valve is on "both" the left tank can drain to the right tank, and vice versa. Does that not allow the fuel to cross over?

[svyolo]

I believe I read the requirement in AC43. I will try to look it up again. I was planning on return lines to the mains for my EFI so I had 1/8 NPT ports welded to the top center of the inboard edge of each tank. I am probably going to use it to cross vent the tanks.

[JJohnston]

The only failure scenario I can think of is to have one cap vent hole plugged. If that happens the fuel won’t drain out of the tank with the plugged cap and an alternate air source would be necessary.

If that were the case, would a fuel crossover serve to equalize the pressure? Do the kit tanks have a provision for a vent crossover?

[Mark Goldberg]

One thing to keep in mind is that Matt's BH has a IO360 CONTINENTAL. Not a Lycoming. I am not sure if the fuel injection systems are the same. But worth noting. Mark

[Archer39J]

Only thing I'm finding has to do with Cessna 182s, 185s and their interesting venting scheme, maybe an AD for vented caps? What's the FAR that requires a cross vent between independently vented tanks?

[alaskabearhawk]

Ok, zkelley2 I admit, I don’t get it. If the fuel tank is vented through the tank caps, what would a vent line connecting both tanks do for you that the cap doesn’t already do? The only failure scenario I can think of is to have one cap vent hole plugged. If that happens the fuel won’t drain out of the tank with the plugged cap and an alternate air source would be necessary.

[zkelley2]

It's more or less exactly what he said. If you have a fuel pump and have a both setting on your fuel selector, you need to either a) never use it in both b) have a vent line running between the two tanks towards the top of the tank to equalize the pressure in the air above the fuel. I could do some math with it later to show how that happens, but that's what's happening. You can have one tank empty and the other tank completely full and not be able to get any fuel to the engine.

Every certified aircraft has this if they have a both selector. They are required to by FAR.

[jaredyates]

Sounds like a great Beartracks article.