Fuel Flow Discussion, Moved from Float Mounting

[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.

[MattS]

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!

[whee]

It has been a while since I tried to explain it. I did not make an attempt in this thread. It is not a simple thing to replicate. Simply running one tank dry will not simulate the failure, you have to also induce a unequal vent pressure in the tanks of sufficient difference.

I have have experience fuel starvation twice in a BH with fuel in both tanks. (Aprox 3 gal in one tank and 10 in the other.)

The thing about this that I find most incredible is that people think it isn't a thing. Like the FAA got it wrong and created some pointless rule for no reason back in the 1930s and has kept that rule for almost a 100 years.

[whee]

Some info about Continental Fuel injection that may be of interest during this conversation.

Restart after running a tank dry is a non event. Almost immediately after selecting a tank with fuel in it the engine restarts.

The primary fuel pump is a positive displacement pump that is driven by the engine and has a reputation of almost never failing. As long as the engine is turning and there is some fuel present the engine will get fuel.

I purposely came up with a scenario that would unport a fuel line cause the pump to suck air. Because of how TCM FI works this sucking of air causes surging of the engine. Gravity is still supplying some fuel so the pump has to build up enough fuel pressure to open the cut off valve located in the fuel spider. So the valve opens, the engine revs up, the fuel pressure drops, valve closes, engine dies. This repeats until there is no more fuel or a steady flow of fuel is restored.

[Battson]

The point I'm trying to make is that "Both" is an unsafe position in a Bearhawk because it does not have a cross tank vent. This is the FAAs position (strict prohibition of a Both position when tank airspaces are not vented together) and I agree with them. It presents a failure that can cause fuel starvation when there is still fuel in both tanks. Specific conditions must exist which is why it rarely happens. I experienced it twice in a Bearhawk and it is my opinion that this is what caused fuel starvation on Matt's plane.

IMO, "unporting " is not what caused Matt's failure. Unporting would have caused a partial power loss but not a total shutdown.

I have a meeting with the head maintenance inspector at my local FSDO office in a couple weeks. After we finish the repairman's certificate interview I'm going to ask him about this issue in hopes that he has a way to explain it that others will understand. Obviously I'm not capable explaining it clearly.

Which post did you attempt to explain it in, Jon? I can't see it for looking.

Point taken about "unporting", I should be clearer. We are talking about a lack of adequate venting.

I am fairly sure the high risk situation relates to low wing aircraft. But I cannot imagine a situation where fuel exists in both tanks and you can't get any, with the selector on both... Unless you have two blocked vents.
EDIT - thanks for the explanations Jon and Matt, I get it now. See response below.

I would be happy to run one tank dry on the ground and see if the plane keeps running on both. Easy to test this...

[whee]

For most Bearhawk pilots BOTH is still going to be the safest option.
___________________

From what I have read, there a working assumption that 'sucking air' or 'unporting a tank' was the cause, due to some venting issue. How was this determined?

The point I'm trying to make is that "Both" is an unsafe position in a Bearhawk because it does not have a cross tank vent. This is the FAAs position (strict prohibition of a Both position when tank airspaces are not vented together) and I agree with them. It presents a failure that can cause fuel starvation when there is still fuel in both tanks. Specific conditions must exist which is why it rarely happens. I experienced it twice in a Bearhawk and it is my opinion that this is what caused fuel starvation on Matt's plane.

IMO, "unporting " is not what caused Matt's failure. Unporting would have caused a partial power loss but not a total shutdown.

I have a meeting with the head maintenance inspector at my local FSDO office in a couple weeks. After we finish the repairman's certificate interview I'm going to ask him about this issue in hopes that he has a way to explain it that others will understand. Obviously I'm not capable explaining it clearly.

[JimParker256]

I am fairly certain that aircraft certified under Part 23 have to meet a certification requirement that if one tank is run dry to the point that the engine quits, the engine must restart within 5-7 seconds (time could be slightly off) – with no other action required by the pilot other than turning on the fuel boost pump and switching the fuel selector to a tank containing usable fuel. I don't know if this requirement was ever part of CAR3 or CAR4. I've tested this in my previous Commander 114, while flying with an experienced type-specific CFI. We remained well within gliding distance of the airport at all times. We made two separate flights in order to verify both tank's usable fuel quantities.

I would not, however, intentionally run the tanks dry as part of a "routine" operational procedure.

By the way - my Commander 114's tanks held about 1.5 gallons MORE than the "official" usable fuel capacity – which is quite unusual. Often – especially with bladder-type tanks – the actual capacity can be significantly less than the officially reported fuel capacities, due to "bunching" or "wrinkling" of the fuel bladder.

[Battson]

Sorry to see this crash, I know it's very disappointing and upsetting event. I hope everyone walked away unharmed. Fortunately the aircraft looks repairable.


___________________

I am concerned that this discussion could confuse builders / pilots about fundamental fuel system design principles.

For most high wing aircraft, BOTH is the safest position to fly in, because if there is fuel in the tanks you have the best chance that your engine will keep running*.

*no fuel system is foolproof.

More complex fuel systems are more hazardous, statistically speaking. Multi tank, multi pump, and fuel return systems increase the risk of fuel starvation events.

For most Bearhawk pilots BOTH is still going to be the safest option.
___________________



We need to understand the fuel system in question before making determinations about your own fuel system design.

From what I have read, there a working assumption that 'sucking air' or 'unporting a tank' was the cause, due to some venting issue. How was this determined?

A few footnotes:

• It seems a lot of content in this thread has been re-posted from this unofficial source: https://aviation.stackexchange.com/q...-fuel-selector
• Someone made the comment about cross-vented tanks preventing this issue. Remember that many Cessna first added the cross vent to address venting issues with the 140 and 150 fuel systems, I am not sure if this was related to fuel injection.... I don't believe those aircraft have FI.... this was before the AD requiring vented tank caps, too.
• If a pilot does starve an injected engine of fuel (even just for a short moment by unporting a tank during a turn), the pilot may need to select throttle to mid range (50% power) otherwise the engine may not restart when fuel becomes available again, depending on the fuel system.
• Lots of guys talk about running tanks dry as part of normal ops, this is not safe practice unless you know your fuel system. Some fuel system designs will not restart if a tank unports. Others will restart just fine and very reliably.

[Battson]

Try that experiment again with the glasses above your head and straws out the bottom of the cup...

This would be an issue with a low wing, which typically don't have a both position on the tank selector. Whereas most high wings do.

See Jon's post below.