Fuel on Both?

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CerroTorre

Pre-takeoff checklist
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CerroTorre
In a 172, if you are noticing that there is a significant fuel imbalance, it makes sense to select R or L for a period of time to balance things back out. It's rare that this has happened to me over the time I've been flying this aircraft and it didn't cause any real concern, but it did raise a couple questions.

What might be the cause? I've flown this plane a fair bit of time and never noticed it previously. This developed over the course of two flights - one a straight and level 1.5 hour cruise (in trim, no 'wing low' flying) and a local, no significant maneuvering, scenic flight.

If I had continued to run on "Both", and one tank ran dry while the other had a significant amount left, would there be a risk of fuel starvation? It seems like it's possible. After all, the aircraft appeared to be getting the significant majority of it's fuel from the Left tank even while Both was selected...so it might get a little ... "bothered" ... if I left things as they were...?

Any thoughts?
 
Always the same tank? Start from full? The left tank is vented to atmosphere and the filler caps are a backup vents. But play around with blocking one of those items and see if something makes sense.

Flying out of trim? Extended turning time?
 
My plane will drain one tank (pilot side) faster as well, but as that tank gets lower, it seems to even out and start draining from the other tank at some point, but it has caught my eye a few times. I have read (likely on here) that flying uncoordinated could cause this, but I am on parroting what someone else said...somewhere on the internet so take it for what it is worth. I think bflynn is also correct that it also has something to do with the venting as well. Positive air pressure on the pilot side tank forcing it to feed faster. Seems like that aligns with my left (pilot side) tank always draining faster. Just how i have justified it based on reading the internet opinions. If i am on a long X/C i do keep a close eye on it though and if the non draining side doesnt start...over or near an airport i will switch to that tank to make sure it isnt clogged up or something and run it off that for a while.
 
The reason for the uneven burn can be flying with wings not exactly level, fuel tank venting differences, and differences in resistance of the fuel flowing to the selector.

The POH says both for takeoff and landing. In cruise the setting is up to you.
 
In or 57 the POH says fuel on both unless above 5000 feet then you do the switchy thing. I rarely get above a few thousand feet so not usually an issue.

I think most of the issue is venting related. Ours usually is within a gallon or two one side or the other.
 
My c170b sucks a little more out of one side. I’ve also seen it in a c172m. On a longer flight I’ll move the fuel selector over to the fuller tank. No biggie. GUMPS check before landing like we do every time, right?

cessna says fuel on both for takeoff or land. Those phases of flight include significant pitch and bank changes. I trust that if one went dry then ‘both’ would feed my engine. That’s what the fuel system diagram shows and what Poppa Cezna tells me to do.
 
Here's the catch-22. Usually in Cessna's, the feed is unbalanced because of uneven venting. Usually a non issue, and will even itself out eventually. But what if, unbeknownst to the pilot, the uneven fuel flow is caused by a blockage of some type. You may run down the one good tank thinking it will start drawing on the full tank, but it never does.

That is one reason its good to at least check the fuller tank can supply the engine. Not to mention, if the imbalance gets significant, the airplane may start to turn towards the heavy wing requiring constant input.
 
midwestpa24 said:
Here's the catch-22. Usually in Cessna's, the feed is unbalanced because of uneven venting. Usually a non issue, and will even itself out eventually. But what if, unbeknownst to the pilot, the uneven fuel flow is caused by a blockage of some type. You may run down the one good tank thinking it will start drawing on the full tank, but it never does.

That is one reason it's good to at least check the fuller tank can supply the engine. Not to mention, if the imbalance gets significant, the airplane may start to turn towards the heavy wing requiring constant input.
Click to expand...
If it's really draining one tank a lot more than the other, something is probably wrong and I agree you should check it. It would take a lot to get me to switch to one tank in flight in a Cessna with a 'both' option though.

Most Cessna pilots never switch tanks. I always check to make sure the tank is on both before landing, even though I know I never switched it. I had two friends who bought a 172 together. One was experienced, one was a student. The student kept switching the tanks during a flight, but the other guy never saw him do it. When they were landing, the engine quit and they came up short. The student wasn't PIC that day and should never have touched the fuel selector, but the PIC admitted he should have checked to make sure it was on both. He assumed it was because he never moved it.
 
Does it matter if the 172 is injected? Where does the excess fuel dump into?
 
There’s tons of information (and misinformation) on the internet about unequal fuel use in the “both” position. It’s a venting thing. My Cessna has an under-wing vent on both sides. Some only have one on the left. There’s a vent between the tanks up high at the forward spar carry-through. Figure out which vent scheme you have and do the Google thing.
 
Stewartb said:
https://www.azflyers.org/resources/003tech-Fuel-Flow.pdf
Click to expand...
Thanks for that article. Very informative. And thanks all for the discussion above.

At least one takeaway is that uneven fuel feeding should not result in any fuel starvation even if the left tank goes dry. It would be a tad bit disconcerting to see that indicator go to zero, but it sounds like adequate fuel pressure would still be present in the right tank to ensure continuing flow.

I'm gathering that, like most Cessna pilots, we almost never switch off of BOTH. This was actually the first time I have. But then I generally take off with full tanks so this issue has never been all that apparent. It only was apparent this time because I started the flight with 17 gal on board. 6 left, 11 right. Lower than usual for me at the start of a flight - and low enough that seeing it continue to draw mostly from the left (on the gauges) was disconcerting, a little more so because we were over mountainous terrain.

Either way, sounds like the G in CGUMPS will become more noteworthy for me in the future as I do more and more long flights in the Cessna. =)
 
So that header tank (fuel reservoir tank) is always full and so it’s going to go up the vent and end up in the shortest route to a wide spot in the line which is the right tank?
 
midwestpa24 said:
Here's the catch-22. Usually in Cessna's, the feed is unbalanced because of uneven venting. Usually a non issue, and will even itself out eventually. But what if, unbeknownst to the pilot, the uneven fuel flow is caused by a blockage of some type. You may run down the one good tank thinking it will start drawing on the full tank, but it never does.

That is one reason its good to at least check the fuller tank can supply the engine. Not to mention, if the imbalance gets significant, the airplane may start to turn towards the heavy wing requiring constant input.
Click to expand...

Yup. I spend a few minutes on each tank to make sure. The time it takes for start up, taxi and run up gives enough time to ensure there is at least not a full blockage.
 
CerroTorre said:
In a 172, if you are noticing that there is a significant fuel imbalance, it makes sense to select R or L for a period of time to balance things back out. It's rare that this has happened to me over the time I've been flying this aircraft and it didn't cause any real concern, but it did raise a couple questions.

What might be the cause? I've flown this plane a fair bit of time and never noticed it previously. This developed over the course of two flights - one a straight and level 1.5 hour cruise (in trim, no 'wing low' flying) and a local, no significant maneuvering, scenic flight.

If I had continued to run on "Both", and one tank ran dry while the other had a significant amount left, would there be a risk of fuel starvation? It seems like it's possible. After all, the aircraft appeared to be getting the significant majority of it's fuel from the Left tank even while Both was selected...so it might get a little ... "bothered" ... if I left things as they were...?

Any thoughts?
Click to expand...
I've thought about this often. I have no idea if true but the reason I was given was the different vent design in the 172, the left tank gets more static pressure. If they're both vented the same it must just be from not flying exactly level.

I doubt you would run dry, the fuel from both tanks flows down into the selector manifold, as long as it is on "both" and one has fuel in it I don't see how the selector manifold could actually get "empty"

In a 182 recently we were doing some maneuvers as part of a checkout. At one point the left tank (fully updated cockpit, digital gauge) was reading 0 gallons. It was odd. We were on both. Engine was fine and happy. After returning to level flight it magically went back to 17..
 
Stewartb said:
https://www.azflyers.org/resources/003tech-Fuel-Flow.pdf
Click to expand...
That's a really good rundown on the causes. One thing they didn't mention was the formation of bubbles in the vertical sections of the fuel lines in the 172s. Those lines leave the tank and go aft long the upper doorsill and then go down the aft doorpost and under the floor to the fuel elector. With fuel sloshing around in the tanks, a bit of air can get into the line and if the fuel flow is just about right, that bubble gets into the vertical section and stops there, restricting the flow. It acts like a dam. It rises in the fuel at the same rate the fuel is descending, so there it's stuck. This can cause pretty uneven feeding, as fuel has to squeeze around it, and if it happens on both sides at the same time you have power loss. So an AD came out to placard the selector with the note to "Switch to single-tank operation above 5000 feet." Altitude played a role here somehow as the head pressure on the fuel decreased. The single-tank operation meant that the fuel flowed twice as fast in the line and took the bubble through the system to the carb's float bowl, where it was vented to atmosphere.

Cessna came out with a couple of Service Kits to modify the system to add a line from the horizontal section of the line, forward to the vent crossover, to remove any bubbles that entered the line. Later models had this installed at the factory.

https://support.cessna.com/custsupt/contacts/pubs/ourpdf.pdf?as_id=36949
https://support.cessna.com/custsupt/contacts/pubs/ourpdf.pdf?as_id=30276
 
^interesting. Learn something new every day here
 
455 Bravo Uniform said:
So that header tank (fuel reservoir tank) is always full and so it’s going to go up the vent and end up in the shortest route to a wide spot in the line which is the right tank?
Click to expand...
Why would it go back up to the main tank? Fuel return back to the reservoir is always going to be less than the outflow.
 
Tantalum said:
I've thought about this often. I have no idea if true but the reason I was given was the different vent design in the 172, the left tank gets more static pressure. If they're both vented the same it must just be from not flying exactly level.

I doubt you would run dry, the fuel from both tanks flows down into the selector manifold, as long as it is on "both" and one has fuel in it I don't see how the selector manifold could actually get "empty"

In a 182 recently we were doing some maneuvers as part of a checkout. At one point the left tank (fully updated cockpit, digital gauge) was reading 0 gallons. It was odd. We were on both. Engine was fine and happy. After returning to level flight it magically went back to 17..
Click to expand...

In a closed system, pressures are equal everywhere. In the Cessna system, if there is no air movement caused by leaking fuel caps, and if the crossover vent isn't full of fuel, the left tank will be at the same pressure as the right. Pascal's Law. But there are other factors, like leaking caps, small differences in fuel line length, bend radii, number of connecting fittings, fuel selector deterioration (from lack of maintenance and failure to remove the drain plug in the bottom of the selector and let out the accumulated crud at every annual) that can cause uneven flows.

And you are right: if the system is OK you will have fuel going through the selector until the tanks are empty if the selector is on Both. There's enough gravity-related pressure to do it. But having one fuel cap badly leaking? Especially the right cap? The fuel from the left side will flow right through the system--through the left lines to the selector, through the selector, and up the right lines into that tank---and get sucked overboard. This will go on until the left tank is empty, and the air that follows it will keep the fuel from the right tank from reaching the engine.

Gauges dropping to zero and back again are common. The original senders are wirewound rheostats, and the float moves a lever that moves a runner across the coil of resistance wire to change the resistance. That runner wears the wire and itself and gets intermittent. And if the airplane is tied down outside, the wind rocks it and the fuel moves a bit and moves that float and runner and the wear goes on even if the airplane is never flown. McFarlane has PMA'd senders that use a carbon track instead of a coil of wire, but that will eventually have its problems, too. The 182 you experienced that in would have carbon-track senders. There are some others available that use a magnetic sensor that eliminates the mechanical wear factor. IIRC they're part of a digital engine/fuel instrument upgrade. EI or JPI maybe. Can't remember.
 
EdFred said:
In a Lycoming, as far as I know, nowhere. I don't know if injected 172s were L or C.
Click to expand...
Injected 172s were and are Lycoming IO-360L2A engines. The earlier ones didn't have a fuel return system, and a few vapor-lock events occurred. A kit was made available to add a return line and check valve to send some fuel back to the header tank under the copilot's floor, and the factory started installing that system in new airplanes. It removes any air from the fuel pump output.
 
Last edited:
asicer said:
Why would it go back up to the main tank? Fuel return back to the reservoir is always going to be less than the outflow.
Click to expand...
Yup. It goes to the header tank (reservoir) which is vented to the right main. Any air in the returned fuel escapes through that vent line.

Returning the fuel to the right main would cause problems when running on the left tank. The right main would get full and the left would get empty faster.
 
I run off of one tank when I am getting low. I run the lower tank down to where the gauge says empty, then switch to the fuller tank. My fuel computer said I had about 11 gallons remaining, since I had run the right tank down to "empty", Ianded with left tank only. Made a turn on the taxiway and the engine coughed like it was out of fuel. It turns out I DID have 10 gallons remaining, with only 1 or 2 in the "empty" tank. But, fuel on BOTH is going to pick up all the fuel from the nearly empty tank as it sloshes.
I will never land it with one tank selected again.
 
Dan Thomas said:
Injected 172s were and are IO-360L2A engine. The earlier ones didn't have a fuel return system, and a few vapor-lock events occurred. A kit was made available to add a return line and check valve to send some fuel back to the header tank under the copilot's floor, and the factory started installing that system in new airplanes. It removes any air from the fuel pump output.
Click to expand...

Good to know. At least it wasn't sent up like the Conti system on the Bonanzas. Good lord!
 
It happened in the Maule too and the dealer showed us this "mod" where he put a short piece of rubber hose cut at an angle on the vent tube under the wing. I guess the idea was that the angled cut would provide a bit of a ram air effect and slightly increase the vent head pressure. It seemed to work but I took it off thinking that if I ever crashed the NTSB guy would walk up to the wreck and immediately point to that little chunk of hose and say "Hey Fred, come take a look at this" and I'd have some splainin' to do.
 
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