Fuel Flow Discussion, Moved from Float Mounting

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For technical correctness, 14 CFR 23.975 has been superseded by 14 CFR 23.2430: https://www.law.cornell.edu/cfr/text/14/23.2430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

[whee]

The TCM fuel injection system does return a significant amount of fuel through the "vapor return". As much as 50% of the pumped fuel could be returned based on power settings. The amount of fuel pumped is determined wholly by engine RPM. The amount of fuel that reaches the fuel spider is determined by throttle and mixture settings.

None of this had anything to do with the failure which we are discussing here.

[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.

[zkelley2]

Matt, how close are you to Campinas?

[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.

[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.

[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...

[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.