Zenith Aircraft Builders and Flyers
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Fuel Design: Risk of sucking air from one tank while in a steep turn
There are several solutions to prevent a fuel system from sucking air from a fuel tank while in a steep turn when the fuel level is low. This has been a hot debate on other forums.
I am curious to hear from folks who are actually flying their planes on which design you use to address this risk.
1) Header tanks?
2) Check valves?
3) Limit fuel selector valve to only use R or L tank but not both?
4) None, I use "both" on my selector valve and do not have a header tank and have not had any issues.
Could you you please comment which model you are flying and what engine, and if it is F.I. or carburated and if you have fuel return lines.
Thank you !!!!!
Charlie
Replies to This Discussion
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Permalink Reply by Larry Smith on
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Thanks Ken. I may start with moving back to the simple on off setup.
Fortunately, there was minimal damage to the gear. However, if I hadn't cleared the trees on final, it would have been a very different story.
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Permalink Reply by Ralph Powell on
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Ryan,
The question in my mind is not why have a both selection, but rather why even have a right or left option. Tank selection is important if you fly a fuel injected engine like I have, otherwise you have to play some rather remote what if games to justify right or left. Just ask your friends who fly 172s when they last turned their fuel selector from the both position to right or left was. Never?
I looked very hard at installing a header tank in my Cruzer but instead decided on investigating installing an audio low fuel alarm on my Dynon. After your posting I see the need to also test my usable fuel levels more closely.
Ralph
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Thanks Ralph, I agree. I am just trying to find someone who has a reason for R/L. I can't think of one, unless there is a venting issue in one of the tanks, or possible contamination in one of the tanks (water), you have the option to troubleshoot and remedy the problem safely in flight.
You are right, the only time I use R/L in a 172 is when it is parked to prevent cross feeding.
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Permalink Reply by Bob Pustell on
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To partially answer the question about "why not have a both position on the fuel selector" I will point out that the Federal aircraft rules require that both fuel tanks have the vent system interconnected if there is a Both setting on the fuel selector. Most Zenith planes (assuming built to plans) do not have provisions for a joined fuel vent system, they just use various vent arrangements for the individual tanks. The Cessna 172 mentioned earlier does have an interconnected vent system and one little vent fitting sticking out of the left wing that vents both tanks. Most other light aircraft (Cherokees, Grummans, etc) have individual vents for each tank. Those planes have a Left/Right/Off fuel selector. The Cessna has a Left/Right/Both/Off fuel selector.
We are building experimental planes and could use a both setting with individually vented tanks but I would advise against it. The Federal requirement is there for a reason (actually, several reasons).
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Permalink Reply by Ken Ryan on
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Please don't leave us guessing! What are the several reasons not to use an ON/OFF valve (instead of Left/Right/Off) with individually vented tanks? ON/OFF is, after all, what Zenith plans specify.
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Permalink Reply by Geoff Klestadt on
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Because a clogged vent may render one tanks fuel inaccessible. The C172S (injected) manual states that it has three vents; the ram air one behind the left strut which connects to both tanks as well as vented fuel caps.
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Permalink Reply by Jim and Amelia Isaacs on
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Interesting Bob. Can you please steer us to the FAR (Part 23?) you are citing?
Thanks!
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Permalink Reply by Loren Warner on
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A little late in the conversation, but here is a schematic of our fuel system with Viking 130 engine
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Bob,
I did a little digging for a rule or rules that link fuel tank vents and a fuel valve with a BOTH position. I could not. I did find in FAR 23.975 “Fuel Tank Vents and carburetor vapor vents” in paragraph a)(4) that statement “Airspaces of tanks with interconnected outlets must be interconnected.” And in FAR 23.995 “Fuel valves and controls” there’s nothing I can find relating fuel system valves to vents.
Geoff,
If you’re answering Ken’s question (were you?)if the tanks are individually vented, should one tank vent become clogged, wouldn’t that tank’s fuel be inaccessible regardless of whether Left, Right, or On is selected on the fuel valve? Perhaps the vent design of the C172 you describe would indicate the 172 tank outlets are interconnected, so to comply with 23.975 a)(4) the tank vents are interconnected by the ram air vent behind the left strut.
So after ciphering through all of this I conclude one can have fuel tanks, with interconnected outlets and vents to comply with the rules, but use any type fuel selector valve you want, from a simple On/Off valve to a complex individual or both selector valve. It’s just how you want to plumb it to suit your desires.
But a fuel valve, any fuel valve, is important according to FAR 23.995 (a) which states you can’t have a system like Loren’s, with no means of rapidly shutting off the fuel flow to the engine from the cockpit.
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Jim, it's very possible I misunderstood what you are saying, but I doubt there’s any aircraft out there that can stop the fuel flow as rapidly as we can. For us, it’s a simple matter of turning off a toggle switch on the instrument panel in front of the pilot for one or both fuel pumps. Not a drop of fuel can pass these fuel pumps when there are turned off. Further, because our header tank w/integral fuel pumps are in the aft fuselage, we have no fuel fittings which could leak anywhere within the cabin.
Moreover, while much in the FAR is useful information, it is not a mandate for Experimental Aircraft. Section 23.1(a) of the FAR, under Applicability, states “This part prescribes airworthiness standards for the issue of type certificates, and changes to those certificates, for airplanes in the normal, utility, acrobatic, and commuter categories.” So, unless I missed something, none of Section 23 applies.
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I agree completely Loren, although the fuel system requirements in FAR 23 should be considered in our designs for safety purposes, the FAR does not apply to our Experimental aircraft. I was just researching rules because Bob’s post intrigued me. Also I did not see any shutoff valve or pump shutoff switch or any other type of fuel shutoff device on your fuel system schematic, that’s what was behind my comment. Thanks for the clarification.
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Sorry to not reply promptly, been out of town briefly. You guys got it all right - the requirement to interconnect fuel tank "airspaces" is what I was calling interconnected fuel vents. You found the correct reference. And it is also correct that the rule does not apply to Experimental aircraft. However, if it's good enough for the certified types, it's good enough for me. However, folks are right to say we can set up the fuel system however we want as long as it passes the various function and flow tests once built. And yes, most of the high wing Cessnas do have interconnected tank vent plumbing to go with the "Both" setting on the fuel selector.
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