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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.
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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.Comment
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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.
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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.
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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.
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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.
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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.Originally posted by Battson
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.
Scratch Built 4-place Bearhawk. Continental IO-360, 88" C203 McCauley prop.Comment
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Which post did you attempt to explain it in, Jon? I can't see it for looking.Originally posted by whee View Post
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...Last edited by Battson; 01-26-2020, 05:51 PM.Comment
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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, were those experiences in your own Bearhawk? I'm thinking like Battson on this. His revamped experiment with the glass over head, straws on the bottom. This is a very interesting subject, as Im running EFI with returns to the wing tanks and no header tank. There are many different scenarios in these posts with few spelled out. The only things I can see that would cause more fuel in one vented tank is plugged vent, flying uncoordinated and possibly the differences in pressure at the vents due to the prop wash. A long prop like yours might cause less/low pressure on one side for your tank vent. A crossover tank vent would equalize both tanks. Just thinking, and considering here.
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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.Scratch Built 4-place Bearhawk. Continental IO-360, 88" C203 McCauley prop.Comment
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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!Comment
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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.Originally posted by MattS View Post
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.Last edited by Battson; 01-26-2020, 06:08 PM.Comment
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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.Scratch Built 4-place Bearhawk. Continental IO-360, 88" C203 McCauley prop.Comment
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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.Comment
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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.Comment
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I think you got it, assuming I got it. Still learning.Originally posted by Startin22 View Post
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.Comment
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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.Comment
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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.
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Posted twice in error.....Last edited by Nev; 01-27-2020, 02:56 AM.Nev Bailey
Christchurch, NZ
BearhawkBlog.com - Safety & Maintenance Notes
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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 Bailey
Christchurch, NZ
BearhawkBlog.com - Safety & Maintenance Notes
YouTube - Build and flying channel
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