Fuel Flow Discussion, Moved from Float Mounting

[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

[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

[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]

Matt, thanks for telling us about this so we can learn from it. I’m still trying to get my head around the specifics.
I came across a C172 AD that may or may not be addressing a similar issue, from 1972. Not sure if it applies as it may only be applicable to normally aspirated engines, but it discusses certain combinations of altitude, humidity, temperature etc that allow vapor formation in the fuel lines, and makes a recommendation to run off individual tanks above 5000ft msl.

Point 1 of the summary mentions: The original fuel system has been highly reliable with no reports of any kind received for the first several years of operation.

https://support.cessna.com/custsupt/...df?as_id=36949

[Nev]

Posted twice in error.....

[svyolo]

I have re-read most of this thread at least once. I am still trying to get my head around the failure mode being described. I can understand if one tank vent gets block, it won't feed fuel anymore. I can understand running in BOTH, if you then unport the opposite tank that is feeding, you won't get any fuel to the engine. But flying along, straight and level, coordinated, running in both, I don't understand how the tank with the blocked port would suck fuel from the other side. I can't quite get my head around it.

To my thinking, running in L or R is the same as running in BOTH with one blocked tank vent. Only one side feeds, and uncoordinated flight could unport the one tank that is feeding.

One complicating factor, like Battson said, is FI, or a fuel pump of any kind. The BH fuel system is designed to be a gravity fed system. As long as you feed the inlet of the pump by gravity alone, your good. The system is not designed to be suction drawn. Even running in L or R, each side has a fore and aft fuel line, and one will be unported in a climb or descend unless the tanks are near full. If you suck long enough eventually you could end up sucking air out of the un-ported line. There is a reserve of fuel in the length of each fore and aft line, but when that is exhausted I believe you will draw in air if the pump is being suction fed.

The stock BH fuel system seems to have a good track record of gravity feeding 180-260 hp fuel flow. Based on the fuel flow tests I have read on here, it doesn't seem like there is much headroom above that. A few of the tests seem a little marginal for the higher hp engines.

I have seen numerous diagrams of TCM FI including from the manufacturer. They show a fuel return line from the servo back to the pump inlet, and another "vapor" return from the pump to the tank, or header tank. They specifically refer to the return from the pump as "vapor", so I believe it is mostly a bleed line, although it could also return fuel occasionally (or more?). So to me, I think the TCM FI is only drawing engine feed fuel, not a large amount like EFI fuel pumps, and then returning the excess.

Matt: Where did the vapor return line terminate? One fuel tank only? Could this be a factor? You didn't mention a header tank.

[Battson]

The way I understand this is that if you are running on both without a crossover vent and the pressure differential between the tanks exceeds the pressure differential between the tank and the pump inlet or the carb inlet if you are gravity fed fuel can flow back up the line to the tank with lower pressure instead of flowing to the motor. So anytime valves to both tanks were open this could happen regardless of the type of valve you are using. That is my understanding feel free to tell me I'm wrong if you disagree.

I think you got it, assuming I got it. Still learning.

It appears one tank must be close to empty before it can happen, then you also need a significant air pressure imbalance on each tank vent. The issue seems extremely rare / hard to provoke.
The Bearhawk design has cumulatively flown many thousands of hours without cross vents across the fleet, this is the first time it's really come to the forefront as far as I know.

The fuel injection systems need higher fuel pressure to operate, so are probably more vulnerable than a carb.

You can easily remove this risk by installing a cross vent. This does introduce the issue of full tanks cross feeding while parked on a slope, and draining out the vent.

Really good thread in my opinion. I am glad for it.

[Startin22]

The way I understand this is that if you are running on both without a crossover vent and the pressure differential between the tanks exceeds the pressure differential between the tank and the pump inlet or the carb inlet if you are gravity fed fuel can flow back up the line to the tank with lower pressure instead of flowing to the motor. So anytime valves to both tanks were open this could happen regardless of the type of valve you are using. That is my understanding feel free to tell me I'm wrong if you disagree.

[whee]

Great explination Matt! Thanks for sharing your experience and for putting in words what I couldn't.

I would love to come down, help out and explore the Amazon. Too bad I suck at banging rivets😁

[Battson]

Thanks for sharing your wisdom here, Jon, and to Matt for sharing his experience.

​​​​​​​I got educated so I am real happy.

[JJohnston]

I still don't understand what happened here, but (or maybe "therefore") a question occurs to me: if you have the simplest valving of all, just "on" and "off", would your system be subject to this problem? The lines would tee together just upstream of the valve, which sounds to me the same as a "both" position, with the tee inside the valve.

[whee]

I've never experience this failure on my plane; I don't have a both position on my fuel valve. Both times it happened to me was in an O360 powered BH. It also happened to another guy in that plane after he didn't believe my warning or explination. The first time it happened to me was due to uncoordinated flight. Second time was due to a bad gasket on the fuel cap (Cub style). After I started not using both on that plane I never had the issue and neither did the next guy.

I'm not surprised Bob didn't consider it because it is such a remote possiblity for the failure to occur, the reg is so obsure few even know about it and fewer understand the reasoning behind it. Bob may not even know why this may be a concern or even that the reg exists. The only reason I found it is because I am a Luscombe guy. Luscombes have no Both position. Their fuel valve are of poor design so forever ago a guy developed an installation that put an on/off valve for each tank on both side of the cockpit and eliminated the factory valve. A placard is required prohibiting opening both valves at the same time. I had a discussion about the reasoning with some Luscombe gurus which lead me down this path of discovery. None, literally zero, of the A&Ps I've asked about this had any sort of clue about it. I really am looking forward to talking with the FAA inspector about it.

Maybe worth noting, the C206s have no BOTH position and no cross tank vent.

[Battson]

Mark, I am not sure if the FI system makes much difference in the extreme case.

[Battson]

I needed more explanation than this to get my head around it, page 3 helped me the most.

[Battson]

I have really appreciated this forum over the years as sometimes I feel "alone" down here. My goal is to provide accurate information to the group. I am not trying to convince anyone of what happened.

Until September 29, 2019 I flew with the assumption that in a high wing plane flying in BOTH was the safest mode....similiar to what others have said here. I believe I did NOT unport a tank. I would explain it as this; flying uncoordinated caused a pressure differential between the two tanks. Instead of fuel flowing (at the selector) forward through the firewall to the engine driven pump, the pressure differential was enough to cause the fuel to flow back up to the other tank instead. Basically it overcame gravity and starved the engine. Jon told me he was able to replicate this twice. You will not be able to replicate this on the ground unless you have some way to cause a pressure difference in the tanks. Additionally this will not occur just because one tank is dry as air will not flow downhill when there is fuel in the other tank that will push it back up (when running on BOTH).

When I purchased the plane the fuel selector only had "R", "L" and "closed". I accept full responsiblity because I made a modification (installed a selector with BOTH) without doing my homework to see how this modfication would affect everything (again, I was trying to make the plane safer and made a few posts about this modification on the forum trying to make the correct decision). To me this is the rule I did not follow and paid for it:
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.

Thankfully I was the only one in the plane. I was not hurt. A small seatbelt mark on my neck (install shoulder harnesses!). My daughter asked my how I felt soon after the accident. I replied "Blessed and pissed!". We are in the process of repairing the plane. If anyone would like to volunteer to come down, I am looking for some good rivet bangers. I'll throw in a free flight over the Amazon rainforest in a RV6!

OK - unless I get this wrong, the "BOTH" position can only cause this if sufficient pressure difference exists between the tanks that overpowers the suction of the pump, to such a degree that the pump cannot provide the operating pressure required by the fuel servo. Happy to be corrected here if I missed something.

The TCM FI system may be a distinguishing factor here? Or not, given the cross venting was introduced on non-FI equipped aircraft.

The Bendix (on Lycoming) system needs only a few PSI to operate correctly (cruise power), and the pump provides about 25 to 30 psi of head.

So on a Bendix system, the imbalance in venting would require over 20 psi of difference between the tanks. Perhaps over 30 psi. I am sure it's possible to get 30 psi in a fuel tank and zero pressure in the other at 130kts, with the right (wrong?) fuel venting design or with the vents installed incorrectly. You can always get a mechanical system to fail, through bad design or operation. This venting issue could equally have affected the tank you were drawing from if you were drawing L or R only...?


Noting my respect for other opinions here, I still believe that Both is safer than L or R in a Bearhawk, and I think it's the safest choice when flying, all things considered. Let's not lose sight of the other risks of fuel starvation, which are more common. I always fly on Both with no cross tank vent, and have never encountered any issue yet. I am sure there are countless other planes doing the same. So it's not something which is easy to induce.

• This issue discussed here is a remote risk which requires other things to go wrong as well. I understand that running a tank dry by accident is a much more common cause of crash landings.
• If you have a venting issue causing a lower pressure in one tank, then you are going to have to select a different tank to fix the issue, and whether you are selecting from Both or from L to R makes no difference. You still have to change tanks when both tanks still show fuel inside. This is not an intuitive thing to do.

Notwithstanding the above, is there some literature in the Bearhawk manuals about this risk? I believe the system calls for a Both selector?

I am happy to admit I wasn't aware of this risk. Builders installing fuel systems with a 'Both' selector need to be made aware they need to cross vent to remove the risk.