Fuel Flow Discussion, Moved from Float Mounting

[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.

[zkelley2]

You could also plumb the interconnect off the sight gauges.

[zkelley2]

All the Maule's that have a both, have an interconnect. I think some of the early M4's didn't have a both. The pacer did not, but there's an STC to add a both.

[zkelley2]

It depends on the fuel selector you install and how you build the fuel system, if it has an interconnect and if it has a pump or gravity feed. A problem I already see is most fuel selectors in the off position allow fuel to transfer between tanks, just not out the outlet port. So for fueling if you're worried about transfer, you would want the selector in R or L, not both or off. This applies to parking on an incline as well.

[jim.mclaughlin924]

I do not have a both position on my yet to fly patrol. I did weld in a couple bungs for a interconnecting vent between the two tanks just to have redundancy in case a cap vent got plugged, which I have had happen. It probably is not needed, but easy to do.

[rodsmith]

I don't know if my Maule had an interconnecting vent between the main tanks. I very rarely used the both position. With all tanks full I would run 1/2 hour on one side, 1/2 on the other, then start pumping the aux into the tank I was running on, swap sides again in another 1/2 etc. That way I would never overfill a main tank. On a long flight I would run one side dry but never landed with less than a quarter tank. This was a carburated engine with mechanical and back up electric fuel pump. Figure I will use the same scheme with the Bearhawk, only difference is AP fuel injection, and AP says don't run a tank dry.

[robcaldwell]

Eric Newton's Sample POH only mentions the following about fuel selector positions in section 2:

• To ensure maximum fuel capacity when refueling, place the fuel selector valve in either LEFT, RIGHT or OFF position to prevent cross-feeding.
• Takeoff and land with the fuel selector valve handle in the BOTH position.

Do we need to amend this? I've always flown in BOTH on most of the high wings I've flown and not thought much else about it. I've only switched to LEFT or RIGHT as MattS says he did on a long cross country to level the tanks.

What can we take away from this to better understand fuel selector operation? Not saying he did anything wrong. Just trying to glean a "learning moment" from this incident.

[zkelley2]

Thanks for this... It seems to apply especially in a low-wing situation. But whether our system is entirely gravity or not, it is still partially gravity-fed. If the pump is forward of the fuel valve, how can it be possible for one straw to be in the air? Wouldn't tank gravity pressure provide flow through the valve and up to the pump? Or is the issue that the pump is pulling more than the gravity pressure provides? Not trying to argue, but rather just to understand how this could happen in our case.

Yes, it's a much bigger issue in a low wing, but can still happen on a high wing with a pump, hence the FAR. I'd think you'd need to be in a quite high pitch attitude in our case. Also why low wing pipers don't have a both.

You can also get the pressure imbalance like John is talking about, gravity or not, and so if you're going to run both, you NEED to have a vent line between the tanks. To atmosphere does not count.
All of the Cessnas with both have vents between the tanks, but also have tank vents to atmosphere. They started doing this in the C140's. The early 140s have a L/R and no vent between them. When you get to the later 140s with a both, there's a vent. I'ts much easier to make the vent in a high wing than a low wing.
You'll also see, I think starting with the 170 POH's and I think it's even placarded on the 172 near the selector that even though they are gravity feed with tanks vented to each other, they recommend cruise flight in L or R, not both.