Fuel Flow Discussion, Moved from Float Mounting

[Battson]

I understand you are not a 'gravity feed' fuel system if you need pumps to keep the engine running. You are pumped. In that case, you run the risk of losing all fuel supply to the engine, if one vent is blocked.

[Archer39J]

Correct, I only mention sucking pumps because I haven't looked into that at all, since it's not what I'm running. You can equalize the pressures by having them vent to the same open space.

[whee]

There is still confusion on this topic but I don’t know how to resolve it.

The issue has nothing to do with a pump sucking air. It affects both gravity feed carburetor systems and fuel injection systems. Matt’s failure was with Continental Fuel injection. The two times I experienced the failure it was in a Lycoming O360 powered plane. Bendix fuel injection might actually be more tolerant of this issue because of the types of pumps used.

The issue is having the higher air pressure in the airspace of one tanks vs the other. IDK how you equalize the pressure in the airspace of each tank without connecting them.

Putting a check valve in the cross vent line, like pretty much all certified high wing planes do, easily solves the parking on a slope fuel transfer issue. Worth nothing that most fuel valves connect the tanks together when in the off position so you need to put the valve in either right or left when parked on a slope to prevent fuel transfer even if you don’t have a cross vent. Or verify that you valve maintains tank isolation when in the off position. My SPRL valve connects the tanks when placed in the off position.

[Archer39J]

You mean like carrying two complete independent ignition systems, stabilizer struts as well as flying wires, tires larger than really necessary, a backup battery for electric ignition?
Sometimes you need an backup plan.

Clearly this is a real risk, we have two firsthand accounts right here. So anyone with fuel injection should be worried about differential pressures - especially high pressure systems.

If a mason bee lodging itself in a tank vent can bring the whole plane down, then I'll be installing a backup system.

What are the "simpler ways around that" - I am keen to hear what other options I have, which are simpler than a bit of tube between tanks.

The designer who made one of those decisions also said a cross vent isn't necessary. The risk is there fuel injection or not. But I'm sticking with gravity feed to the pumps so my solution would only be applicable to a otherwise gravity fed system, if you're sucking at the pump good luck and I don't see a cross vent affecting that anyway off the top of my head.

My solution is a check valve on each tank, ones that open at a pressure well below the pressure head of the fuel. Air gets in if your cap vents get clogged, doesn't let fuel out, doesn't remove your ability to isolate tanks on a slope, and won't let fuel settle in a low point for those wanting to go outboard side to outboard side with their cross vent (dihedral isn't shown in any of those fuel diagrams).

Not 100% on this solution but it's where I'm leaning.
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[Archer39J]

So I think the conclusion is, if you have fuel injection - you NEED to install these vents to be on the safe side, or avoid unbalanced flight if you have a low fuel tank.

I wish I knew this before the plane was assembled! Going to be a difficult mod to get right without easy access.

That said, I slip all the time with low fuel, I have no cross vent and never had a problem with the Bendix system... because I never had a blocked vent. If the vent blocks, then I run the risk.

"To be on the safe side" is a great way to add unnecessary weight to an airplane.

​​​​​​Fuel injection doesn't inherently necessitate a cross vent. If you're worried about differential pressures there are simpler ways around that.

[Bcone1381]

Here is a collection of fuel tank diagrams of certified aircraft I found on-line that I thought may help visualize this discussion.

Below is (with a 95% confidence feeling) a C-152. Lycoming O-235 engine. The C-152 merely has a shutoff valve, so its always on BOTH. One vented cap + a cross vent + a tank vent with a check valve.
Screen Shot 2020-02-06 at 11.00.26 AM.png

Below is a Cessna 174N with one vented cap. The C-172 N has a carbureted Lycoming O-320 engine.

I note two things here, the cross vented tanks and the vent line between the fuel supply line and the cross vent. I see this on all the later model Cessna's, C-172's, 180's, 182's, If parked on a left leaning slope, some fuel will drain from the right tank. If the vent line was run back to the right outboard of the right tank it would not drain out. What happens if the check valve fails closed? The R tank's vented filler cap should relieve a pressure differential.

Screen Shot 2020-02-06 at 10.47.39 AM.png


Cessna 172S. This is the latest model of C-172 that has a Lycoming IO-360 engine instead of a carburated O-320. Cessna designed the system with a header tank (fuel reservoir tank). Note how the header tank is vented. I don't see vented fuel caps on this application. What happens if the Left tanks vent's check valve fails closed? I think negative pressure would build up and the engine would fail with fuel in the tanks. I question if the schematic is accurate by showing the unvented fuel caps.

Screen Shot 2020-02-06 at 11.06.18 AM.png

Cessna 177 Cardinal with header tank. Cardinals were made with both carberated and injected Lycoming 360 CI engines. I'm not sure which one this set up was designed for. No check valve. No vented caps. Will fuel drain onto the ground if parked on a slope? I don't think so.

Screen Shot 2020-02-06 at 10.55.10 AM.png


Cessna 182S with a 230 hp Fuel Injected Lycoming IO-540 engine. Note fuel venting is all over the place. It has two vented fuel caps, a cross over vent, fuel supply lines vented, and a vent in each tank. This fuel system does NOT have a header tank.


What conclusions should we draw?

[Archer39J]

Then you have to think about how to purge fuel that gets into the low spots or you'll not be equalizing the pressure between tanks.

[Bcone1381]

Yes.......

[zkelley2]

Just an fyi, not all fuel sector's "off" will prevent cross flow. For example on a maule you have to put the selector in r or L to prevent cross flow. Off just blocks the port going forward but is just as open to both tanks as the both position is.

[AKKen07]

Would putting the crossover vent attachments at the top outside corners of the tanks help with fuel inadvertently transferring?

[AKKen07]

Thanks for that!

[Archer39J]

More like their pressures need to be equalized, which you can do in ways other than a cross vent.

[svyolo]

I read about someone that had a valve on the crossover. I can't remember if it was certified or EAB. I think I might do the same.

[yateselden]

I am considering install of the crossover vent from top of sight glass connections, with a small Parker shut off valve for parking unlevel. Crossover may not be needed, but shouldn't hurt anything either. I'll add it to check list. I do return more fuel to the right tank maybe a crossover will help with that.

[Gerhard Rieger]

Thanks Bob/Mark,