Fuel Flow Discussion, Moved from Float Mounting

[Archer39J]

Interesting discussion. Not sure if I'll put a cross vent in yet or not.

Honestly it's sounding like just a L and R would be the easiest. Don't have to worry about uneven venting siphons or uneven slopes. No more management than I deal with today, hmm.

[Nev]

Also found this which I’ve just done a screenshot of the pertinent points F75E8FC6-E92A-4705-831B-7AEAE9D46A36.jpegF75E8FC6-E92A-4705-831B-7AEAE9D46A36.jpeg

[Mark Goldberg]

This discussion about the fuel system is a little hard for me to understand. I have started to give Bob Barrows some of the comments and information from various people so he can evaluate. In a few days after he has been given enough info - I will post his comments/response. Mark

[Ed.Meyer]

Like several others on this thread, I am struggling some with this. Seems like there is a lot of theory given here and I can appreciate the theory stuff and in a situation like this, facts may be hard to prove. I do, like others, appreciate the discussion which I have re-read, some of it multiple times. Some stuff running through my mind:

First, I cannot see how a pressure imbalance can occur with two properly installed vented fuel caps, which Matt said was the case with his failure. With the two fuel caps only a few feet apart and moving at the same speed they would be applying equal pressure (pitot pressure) to vent both tanks. If one was installed backwards then I could see unequal pressure which would be less than one PSI. At 150 kts, pitot pressure is about 15 inches H2O or about.5 psi. The one benefit that I can see with an interconnected vent would be venting redundancy. This would come with the potential cross feed problem mentioned. I wonder if something else might have caused Matts failure. Also, I see in the photos that the airplane came to rest with the left wing low. Could some fuel have flowed through the fuel valve from the right to the left tank after the accident?

I am starting to grasp how differential tank pressure might cause fuel starvation such as the Luscombe accident Jon cited. Being a gravity fed system, the head pressure feeding the carburetor is very small with only the weight of the fuel in the system above the carburetor supplying the pressure. If one tank had low pressure, thus sucking fuel from the other, the suction might be greater than the head pressure at the carburetor causing flow into the carburetor to stop. If a single tank is selected and it is not vented it would also quit feeding fuel. It would seem that a system that includes a fuel pump would be less prone to this potential problem since the sucking from a low pressure tank would have to be greater than the gravity head pressure plus whatever suction the pump provides. Any time there is suction, especially when temperatures are warm, there is potential for vapor lock causing pump to stop being fed (which I have experienced in a low wing Cherokee 140 on a hot day with auto fuel, at least that is what I think happened. No way to prove since engine ran fine after landing.)

I will certainly keep following this to determine if venting modification might be needed on our Patrol. In 150 hours so far running on both most of the time there has not been any problem. I don't like running it very low on fuel though...

[whee]

Maybe worth noting that the NTSB did not determine what caused the vent pressure imbalance in the Luscombe wreck. They said possible a plugged vent, which they did not say they found and evidence of, or uncoordinated flight.

[Battson]

You can't have this problem if the tank cap vents are working correctly.

[Archer39J]

Running some numbers I've reached the same conclusions Ed. Of course we're all just speculating, hopefully Matt will continue to update us as more becomes known. But I'm still unsure how you can get, with correctly vented and oriented caps, a pressure difference between the tanks.

I also recall some folks talking about how their tanks "pop" with changes in altitude, and that it's normal/expected. I would say that tanks popping in flight are a sign of inadequate venting.

I'm looking hard at, if not putting in a cross vent, at least an extra vent (per tank) with its pickup on the top-outboard-forward corner of the tank.

[whee]

I've never heard my tanks pop.

Adding additional vents will not have any affect in regard to the failure mode which I am referring too.

[Archer39J]

If those vents are on both tanks so as to provide a bypass for unequal pressures it will. But let's not get too far in the weeds of detailed design, let's also stay away from absolute statements.

[Ed.Meyer]

Jon mentioned he had two engine failure in a Bearhawk while running on “both”. I see it did not have the standard Bearhawk fuel caps. I wonder about other details of this airplane. Like, were they vented caps, any other venting, fuel system details, FI or not, anything else different than standard Bearhawk. If any of this has been given, sorry I missed it.

[whee]

The BH I was flying had a O360 and the fuel system was built according to plans. The fuel caps were Luscombe or Atlee Dodge Cub style (half turn cap with a forward facing snorkel vent). Construction on this plane began before the kits were available so the Bob caps were not commercially available and the builder didn't have the machinery necessary to build them himself.

[N942VT]

The Super Cub has several STCd fuel valves to add the both position. Each one I’m familiar with requires a cross vent. The simplest way is to tie them together at the top of the site gauge. These STCs required flow testing in all flight regimes. No both setting, no cross vent. Both setting requires a cross vent. Should tell us something. This is one of the areas that following the certified world makes good sense. FWIW I flew 942vt for 500 hrs always on both. Lots of hard slips into tight places, steep banking turns down the valleys etc. Never a problem but it is carbureted so add that to the equation.

[Archer39J]

For technical correctness, 14 CFR 23.975 has been superseded by 14 CFR 23.2430: https://www.law.cornell.edu/cfr/text/14/23.2430

A cross vent is no longer required by the language of the regulation. The requirements now address possible adverse effects of incorrectly designed fuel vent systems 23.2430 (a)(1), (a)(3), (a)(6), and (b)(3). (b)(5) may be of note to those of us with 2700 GTOW, but I doubt anyone will go out of their way to meet that "reg" (which of course don't apply to experimental AC). I personally like the move away from dictating specific designs by regulation, let folks figure out how to avoid the undesirable outcome rather than lock them into a solution that might not be right for everyone.

In any case, a cross vent is no longer required by the "regs".

[Archer39J]

In principle I agree. Though I recall some POHs prohibit this.

[zkelley2]

I mean, a POH for an experimental aircraft isn't what sets any rules. They're for your own info only, or more likely the next owner.
As far as on the certified planes, you can always land overweight. It usually triggers an inspection.
At work we generally talk about landing overweight after takeoff every flight. Not normal ops, but if you're in the mindset of dumping fuel, you're in an emergency and none of it matters.

[Archer39J]

Of course not, and none of these "regs" apply to EAB, so there's no point in even discussing this. I just noted it exists, that'll teach me...

[whee]

I am hesitant to talk about this much more before hearing what Bob has to say; I don't want to cause and additional confusion.

I don't think it will hurt anything to state the constraints that must be met for this failure mode to occur and perhaps that will identify why we have only seen this failure a few times in the fleet.

1. Tanks must be independently vented with no cross tank vent line. ie, tank airspaces are independent from each other.

2. The fuel valve must be in the "BOTH" position.

3. The vent pressure in one tank must be sufficiently different than the vent pressure of the other tank.

4. The fuel quantity in the tanks must be low. One tank must be below approximately 4ish gallons. How low the fuller tank must be depends on how large the vent pressure differential is.

5. The above conditions must be met and remain that way until the fuel remaining in the fuel lines downstream of the fuel valve is consumed. Example; the fuel in the carburetor bowl must be consumed before the engine stops running.

There are probably few that run sufficiently low on fuel while somehow inducing a prolonged vent pressure differential. With Bob caps a leak thought the O-rings isn't likely so you'd have to misalign the cap vent or fly uncoordinated to cause a sufficient difference in vent pressure. Or fly really really low on fuel.

[Battson]

Additional constraint, albeit the most obvious - the tanks must be independently vented and not interconnected.

[whee]

🤦 good catch.

[Battson]

Jon mentioned he had two engine failure in a Bearhawk while running on “both”. I see it did not have the standard Bearhawk fuel caps. I wonder about other details of this airplane. Like, were they vented caps, any other venting, fuel system details, FI or not, anything else different than standard Bearhawk. If any of this has been given, sorry I missed it.

Another interesting side note, all the planes which have reported this issue are scratch built, and may have modifications from the basic design which we aren't aware of.

[whee]

As noted above, the fuel system in the plane I flew was build according to Bob's specs. The Cub caps were the only difference. I do think Bob's caps make this failure less likely when properly installed because there orings seal well and aren't likely to leak. Replacement of the cork gasket on the Cub caps is almost a preventative maintenance item.

[Battson]

FWIW I flew 942vt for 500 hrs always on both. Lots of hard slips into tight places, steep banking turns down the valleys etc. Never a problem but it is carbureted so add that to the equation.

I hear that.

I have done 750hrs of the same in NJB, we are ALWAYS running on both tanks. We are often flying with little more than 30 minutes fuel and doing aggressive slips, steep turns, etc. We have bendix fuel injection.

We do always keep our tanks pretty well balanced though, I don't give it full power flying out of balance either.

Never had a problem with our tank vents either. I've flown for 45 minutes with a fuel cap off too, like STOL flying, steep turns, patterns, long cruises at high altitude - all sorts. We hardly lost any fuel at all. Flew back and recovered the misplaced cap....

I think this failure is a very remote possibility.

[JJohnston]

One thing that hasn't been adequately explained is how, without a malfunctioning tank vent, such a pressure differential arises. Technically, if both tank vents are working, the tanks ARE cross-vented - via the atmosphere - even if that doesn't satisfy the reg.

[Ed.Meyer]

I was thinking the same thing. You said it very succinctly.

One additional thought that came to me is this: if there is significant difference in air volume in the tanks, like when one is near empty and the other is not, and there is altitude change then the tank with more air will have to vent more, either air coming in or going out, depending on increased or decreased altitude. This could create some imbalance until venting is complete. I have no idea how much this would or could be.

[Bcone1381]

Hummm....intersting.... And if we changed angle of attack, would the pressure on top of the wing change and submit a change of pressure inside of the tanks, and then would the change in the two unbalanced tanks change at different rates which might cause an unbalance? The fuller tank would change quicker....the fuller tank's pressure would go down quicker.

[Nev]

My understanding at this stage is that the cross tank venting is added for two reasons:

1. Provide alternative venting in the case of a tank vent blockage.
2. Aid dispersion in the case of a fuel line vapor lock.

If the tank vents are operational, it could perhaps be a vapor lock issue that is only presenting itself very occasionally in the right conditions.

If certain geographical areas are providing the right conditions of high ground temp, low temp at altitude, low pressure (individual fuel system intricacies) etc then it may only manifest itself in specific countries/states, and during certain times of the year.

Just another 2c worth in what is a very interesting discussion.

[Rollie]

I think I might be starting to grasp this concept. I really want to understand it because I routinely fly down to about 10 gallons on board. And I always select both tanks.

After reading everything here, this is where I'm at: At first I thought that if there is enough pressure diff for one tank to suck the fuel out of the other and basically override the fuel pump and gravity, wouldn't it be enough to stop the fuel flow out of that tank even if the selector valve was on that tank instead of both? Maybe not. With the vent partially clogged on say the left tank, as the fuel level decreases, the air pressure in the left tank will decrease, maybe not to the point that it would stop the fuel from draining out of the tank but when you add in the right tank with the vent clear and oriented forward, the airflow might be partially pressurizing that tank. So now we have a differential. Fluids take the path of least resistance. (the one thing I understand about fluid dynamics) Usually that path is down from our tanks and into the engine, but in a rare case of the one tank being pressurized enough to be pushing fuel out and the other tank not having any significant pressure, the path of least resistance could become going down from the tank with the properly functioning tank, through the selector valve and up into the other tank. Once the fuel is flowing that direction, it would probably keep going that way and bypass the engine.

That is what I got from this discussion. I might be way off, I'm actually looking for clarification to see if I got it, I'm definitely not trying to say this is what happened.

Now I am also curious if check valves in the fuel lines just upstream from the selector valve would have the same result as a cross over vent line. It would prevent fuel from transferring so it seems like it would only be able to go to the engine where it belongs. I am not planning to add a cross over vent line because I park on slopes frequently requiring me to select L or R tank to prevent fuel from cross flowing and overflowing the downhill tank. I'm pretty sure if I had a cross vent the fuel would transfer through that in that situation.

[JimParker256]

Rollie, I think if you put the cross-over vent at the highest point of the tank, and as far outboard as you can get it, you could avoid the "slope-transfer" effect. The fuel would naturally gravitate toward the inboard side of the uphill tank, thus away from the uphill vent. On the other hand, using a vent on the inboard side would have the opposite effect.
The Commander 114 that I used to have was a fuel-transferring demon... If you didn't place the fuel valve in the OFF position, it would transfer fuel until the tanks found a "level" point. Unfortunately, the nice, squishy trailing-link landing gear (which made for fantastic landings) allowed the now-heavier wing to sag even further as the fuel transferred. If left unattended on a slope with the fuel valve anywhere but OFF, it would transfer almost the entire 35 gallons in the uphill wing to the lower wing, which would then run out through the overflow vent. Many new Commander owners came back to their birds to find their newly-topped-off aircraft sitting in a puddle of 35 gallons of AvGas. An expensive mistake you only make once!

[Archer39J]

It's looking to me like in a property constructed gravity-fed Bob system a cross vent is unnecessary because the worst-case pressure differential isn't enough to be an issue. For engine stoppage at least, you may think it's a problem it's possible to dump fuel like this. Assuming a 100kt ram air pressure of about 0.23 psi, that means at most the pressure differential between the two tanks would be on the order of just under 0.5psi.

The fuel pressure head of a minimum fuel tank is on the order of 1.2psi (aft tank outlet to selector valve). So while fuel will be pushed from the tank with positive pressure to the one with the vent issue (and you'll dump fuel overboard if your tanks are full) the air won't reach the selector valve, and this is worst case when your tanks are effectively empty. Now in systems that don't have that pressure head this can absolutely cause a problem, but for us it doesn't look like this is too much of an issue. Even less so with more fuel and slower speeds.

This is of course for a gravity-fed system, if your moving massive quantities of fuel and your pump is sucking then I wish you well with your analysis.

[Archer39J]

Oh and to add, an additional vent on each tank would help mitigate this possibility, but I'm not going to be installing one. I'll be going with a check valve to prevent fuel stoppage if the vented caps get plugged, ice being my concern, McMaster has some I'm looking into. The check valve won't open at the pressures we've been discussing.