Fuel Flow Discussion, Moved from Float Mounting

[Nev]

Cessna identified another cause many years ago - internal fuel line pressure differences coming from the slight variations in fuel line plumbing (different radiused bends etc). These sometimes led to vapor locks on one side, and caused an uneven fuel feed rate. So while closely monitoring your tanks is always a great idea, in some cases it can be treating a symptom without realizing there’s a cause.

While Cessna were unable to replicate the exact problem consistently, they did find that tank cross-venting largely eliminated this particular issue. They also speculated that very specific atmospheric conditions were playing a part in the vapor-loc scenario, meaning you could fly without incident for a long time while always having the potential issue lurking.

[JimParker256]

Note that your "herd" (Continental engine power) also designed the fuel return to always go to a single tank, thus saving money over a full-duplexed fuel valve and additional return line to the other tank. I think that fact alone would argue for cross-venting the tanks. BWTHDIK?

[Mark Goldberg]

If you fly along in a slight slip or skid - one tank will drain first. Very easy to do. My point is that it is not different pressures in the tanks that might make one drain faster than the other - but how we are not flying wings level. Mark

[svyolo]

I think running a tank dry and killing the engine while the other is full is primarily a fuel management error, even the there is also a fuel flow problem. Two problems simultaneously.

Sometimes I will try new stuff, even if it is different than convention. Sometimes it works, sometimes not. Sometimes I actively seek out wisdom from the "herd". Despite being a pilot for work for over 30 years, I am one of the very least experienced here with light aircraft, and their systems.

Sometimes logic is wonderful, but sometimes it can't identify threats or the risk of threats because of external "unknowns". That is where the "herd" is king. Experience in the real world. That includes external threats, and the experience of how often they happen.

Since I chose EFI and that is a choice of the vast minority (going against the herd) I kind of did of a deep dive into fuel systems in light aircraft. Not theoretically based, but I wanted to see what the "herd" did. Luckily Cessna built thousands of high wing aircraft with injected Continentals for me to see what they did, including their change and development over the decades. Piper built more than a couple but I think they were mostly carburated.

Bob's stock fuel system is perfectly acceptable. It is redundant. You have 2 sides. If you run one dry, whether or not one stops feeding, you failed to monitor your fuel state properly. If it results in a mishap, that fact will become at least a "contributing factor" if not a "causal factor".

More redundant is usually better. Not always, especially if it increases complexity or workload. It also may not help long-term if it makes the pilot complacent.

The "herd" went with a cross-vent in high wing aircraft with a fuel valve "both" setting. It is also specified in rules of thumb in a bunch of fuel system textual descriptions for the same application. In this case, I am going with the herd.

[Bcone1381]

What questions should be asked to lead this towards a scientific based solution. Does this issue need a leader, spokesperson, authority, or a facilitator? Who should that person be? Has anyone plumbed altimeters into upper sight gage plumbing to measure data? Does anyone have a aircraft/circumstances that can make this a e repeatable event?

[zkelley2]

The Rotax powered RANS S-6ES I'm currently flying was designed with two wing tanks (each with a sight gauge and a set of fore and aft fuel pickups) plumbed to a single "on/off" fuel valve, then to a gascolator, and finally to the engine-driven fuel pump. It's functionally very similar to Bob's design, if you were feeding from "BOTH" tanks all the time.

It is very common to hear reports about one wing tank draining much quicker than the other tank – sometimes to the point where there is no longer any fuel visible in the sight gauge on one side while the other is indicating nearly full. The owners that report this issue say that it's always the same tank that "empties" first – which would seem to me a clear indication that their wing tank vents are producing different "pressures." The designer's recommendation was to check the length of the vent and to compare the angle of the cutoff. (The vent line starts at the top of the tank, and runs back and down, ending in a tube that sticks out below the wing, near the jury strut, with a 45º forward-facing cut-off. If the vent is closer / farther from the jury strut, or if the extension of the tube below the wing surface is different, or if the cutoff angle was different, then the "pressure" due to air flow might also be different.)

However, there have been NO actual reports of fuel starvation resulting from this setup. The "going assumption" is that the pressure head from the fuel in the "mostly full" tank is apparently enough higher than the pressure head from the "air" in the lines and tank on the other side that the engine-driven fuel pump does not "suck air" in those situations. Even those who say one of their tanks "always empties" well before the other say they have no issues utilizing the entire useful fuel load in flight.

I did make a change to my S-6's fuel system design, however, in keeping with RANS' latest design changes to the S-20 and S-21. I added a small (2.75 gallon) header tank behind the seats, and redirected the wing tank outlet lines to flow into the header tank. The header tank vents only through the wing fuel lines. There's a "float switch" (connected to a low fuel warning light on the panel) that triggers when there is 2.5 gallons remaining (roughly 20-30 minutes) of fuel remaining – that's my "Final warning, you idiot! Get this plane on the ground NOW!" light...) The header tank's single outlet flows to the fuel cutoff valve (On/Off), the gascolator, and the engine. At some point, I'll probably add an electrical fuel pump inline between the header and fuel valve, just in case the engine-driven pump ever fails. (Rare, but it can happen.)

With this change, I see very even fuel burn from my two wing tanks – I rarely see anything more than a 1/2 gallon difference in the sight gauges. I'm happy with the setup.

That's how my rotax powered airplane was plumbed to. I would suggest getting that electric backup in there asap. The fuel pump on the rotax has gone through something like 4 service bulletins "recommending" replacement of the earlier design. They are not the strong point and the engine quits without fuel pressure despite being carb.

[JimParker256]

The Rotax powered RANS S-6ES I'm currently flying was designed with two wing tanks (each with a sight gauge and a set of fore and aft fuel pickups) plumbed to a single "on/off" fuel valve, then to a gascolator, and finally to the engine-driven fuel pump. It's functionally very similar to Bob's design, if you were feeding from "BOTH" tanks all the time.

It is very common to hear reports about one wing tank draining much quicker than the other tank – sometimes to the point where there is no longer any fuel visible in the sight gauge on one side while the other is indicating nearly full. The owners that report this issue say that it's always the same tank that "empties" first – which would seem to me a clear indication that their wing tank vents are producing different "pressures." The designer's recommendation was to check the length of the vent and to compare the angle of the cutoff. (The vent line starts at the top of the tank, and runs back and down, ending in a tube that sticks out below the wing, near the jury strut, with a 45º forward-facing cut-off. If the vent is closer / farther from the jury strut, or if the extension of the tube below the wing surface is different, or if the cutoff angle was different, then the "pressure" due to air flow might also be different.)

However, there have been NO actual reports of fuel starvation resulting from this setup. The "going assumption" is that the pressure head from the fuel in the "mostly full" tank is apparently enough higher than the pressure head from the "air" in the lines and tank on the other side that the engine-driven fuel pump does not "suck air" in those situations. Even those who say one of their tanks "always empties" well before the other say they have no issues utilizing the entire useful fuel load in flight.

I did make a change to my S-6's fuel system design, however, in keeping with RANS' latest design changes to the S-20 and S-21. I added a small (2.75 gallon) header tank behind the seats, and redirected the wing tank outlet lines to flow into the header tank. The header tank vents only through the wing fuel lines. There's a "float switch" (connected to a low fuel warning light on the panel) that triggers when there is 2.5 gallons remaining (roughly 20-30 minutes) of fuel remaining – that's my "Final warning, you idiot! Get this plane on the ground NOW!" light...) The header tank's single outlet flows to the fuel cutoff valve (On/Off), the gascolator, and the engine. At some point, I'll probably add an electrical fuel pump inline between the header and fuel valve, just in case the engine-driven pump ever fails. (Rare, but it can happen.)

With this change, I see very even fuel burn from my two wing tanks – I rarely see anything more than a 1/2 gallon difference in the sight gauges. I'm happy with the setup.

[Battson]

short story; Running on "Both" without having interconnecting tanks. Pressure differential between the tanks caused fuel flow stoppage.

I have gone full circle on this and I'm not convinced that interconnecting the tanks will actually prevent this; having the benefit of clarity via Bob.
I am really sorry to "flip-flop" on this.
Having had the event at our home airfield I got very interested and I cannot understand some of the logic.

The facts are:

• This issue seems to only affect engines with a fuel pump.
• Any fuel pump will rather suck air than fuel, if air is available.
• Low fuel in one tank is universal in the incidents discussed.
• If the engine sucks for air long enough it will stop, whether the selector is on both or L/R

Occam's razor: It seems much more likely that sucking air bubbles due to prolonged unporting or running a tank dry caused these incidents. A cross vent will not prevent that.

The whole mysterious scenario of one tank robbing fuel from the other just seems unlikely. Sorry to the supporters of this idea.... but nobody has offered an adequate explanation, the necessary pressure difference is too great, and the experts don't support it.

Also the hypothesis of any plane with a "Both" selector needing a cross vent, we haven't provided evidence. I THINK the reason for the cross vent is probably as a backup in case one of the external vent blocks. That way, you can leave the selector on both and if there is fuel you're likely to get it, even if a vent blocks. That was the whole point of the both selector, avoiding accidents due to fuel mismanagement.

I understand Bob is considering providing some additional guidance about fuel system design.

[Bcone1381]

The Patrol Book specifies a four way L-Both-R-Off fuel valve.

[Battson]

I think it's worth remembering that Bob designed and recommends a gravity feed system to a carb engine without any fuel pumps.
In Bob's fuel system, operating with both tanks selected in any fuel configuration will be safe at all times.

Introducing fuel pumps and injection brings some benefits, as well as many new design considerations and operational considerations. As a community, I think we are still learning how to manage those risks, there is no one size fits all.

[zkelley2]

I think you can make a good arguement he designed it only fot L/R even if he didnt know it, considering there isn't a cross vent. And I think that argument has been made.

[Bcone1381]

We are thinking now! That is good.

Bob designed the fuel system for a R-Both-L selector. If I fear moving it we have a problem that needs to be vetted out.

[Battson]

I would like to elaborate my post #114. It seems to me that fuel cross-flowing through the fuel selector from one tank to another reduces the amount of fuel flow available for the engine. So therefore, I am wondering if this safety notice or limitation might not be prudent....

"When a significant fuel imbalance exists do not place the fuel valve to BOTH, rather place the selector to either the L or R tank as appropriate."

The trouble is, you need to know why the imbalance occurred, if you select the fullest tank - you are selecting the tank most likely to have a venting issue or negative pressure in flight, assuming the imbalance occurred unintentionally. Technically you should select the emptier tank and refuel as soon as possible.

****

Statistically speaking, playing with the fuel selector gets more pilots killed than tanks without a cross-vent. I feel it's important to remember that fact, we are only human.

[Battson]

I confess to having done exactly the same thing, and I agree the system *MADE THE WAY BOB DESIGNED IT* is very tolerant to issues. I must disagree with zkelley2 above, as I always fly on Both and this was no exception.
We flew for a long time without one fuel cap and I hardly lost a drop of fuel overboard. I was able to tape over the open tank and fly back and get the missing cap (it was a short flight and the closest option).

[svyolo]

There is more head pressure to feed to carb or FI pump, then to cross-feed to the other tank. The head between the two tanks is only the difference between the fuel levels, 1-8 inches. The head to the carb or pump is 4 feet.

To my mind, I think the flow goes to the engine before it goes all the way up the other fuel line to the opposite tank.