Fuel imbalance c208 floats

[SarahLovesSeaplanes]

Everyone knows you have to get both fuel selectors shut off at the docks otherwise face a nasty fuel imbalance when you return to the plane. However, yesterday after being off for several months I did some recurrent training and noticed another issue and wondered if this is normal or what. I mentioned it to the AME and he just shrugged it off as normal C208 fuel issues but it seems more dramatic to me than I remember but it's been a long time since I flew the Caravan so maybe I'm remembering wrong.

For reasons unknown fuel wants to drain from the right tank to the left when on water regardless of tank selector settings. I’ve tried every combination possible. Left tank shut off, right tank shut off, even with both tanks shut off the fuel seems to want to flow to the left tank though it does seem to slow down a fair bit. This seems to happen mostly on the water and at the dock but not when on pavement.

Yesterday I made over 12 flights (short training flights) and half way through I just stopped using the right tank and flew completely off the left just to keep the fuel somewhat balanced. It seems in flight everything is fine and the normal procedures for keeping the tanks balanced works ok. But once on the water there seems to be no stopping the fuel from rushing into the left tank. I resorted to circling the plane on the water to get a right side heavy imbalance of 200+ pounds but in the time it takes to taxi from the open water and dock (about 3-5 minutes) I will often end up with a 200+ pound left heavy wing. That’s 400 pounds of fuel going into the left tank in a few short minutes. You would think that with a right heavy wing it would keep the right wing low and prevent further transfer to the left wing but that just doesn’t seem to be the case. And keeping a tank selector shut off doesn't seem to do any good either. The plane has single point fueling if that makes any difference.


Any ideas?

[oldncold]

2/thoughts
a) faulty fuel shut off could be worn they are a rotating type ball valve /if i remember correctly
b) leaking left float the amphib often picks up lot of water in the back 2 compartment due poor seal for the internal float cabling . you mentioned 12 flights so lots of landing and take offs sounds to me aircraft need to beremoved from the water all compartments internally tested and sealed always a good idea before start of season .

[Edo]

Take the AME for a taxi around the lake. Just tell em fuel valves are yours and go taxi around for a few minutes. Sounds like a valve is not closing all the way.

[Redneck_pilot86]

When you say heavy left wing, are you basing it on the gauges or the feel of the airplane? I'd agree that it sounds like a worn selector valve, but I know nothing of the single point refuelling system plumbing.

[xsbank]

Where does the return fuel go? Does this only happen when the engine is running? Lots of fuel supplied to the engine but only a small amount burnt. Boost pump issue? Is it on while taxiing? Pump can move a lot of fuel.

I don't know the Caravan, perhaps that's obvious...

[Just another canuck]

Even with both fuel selectors in the off position, I always found the Van would drain fuel from one side to the other if not perfectly level. Perhaps a leaky float on the one side is the root of your problem. However, the rate of transfer which you describe would likely point to some other problem.

[letsfly]

1) If the fuel selector is leaking close them on the dock and see if the engine continues to run!
2) Depending on fuel levels I have had vent draining occur between the tanks with valve closed.
3) Should there be a valve leaking the slightest wind will lift a wing and cause the transfer to the opposite side very quickly.
4) In-flight the slightest yaw will transfer fuel as well.
On the ramp or hard surface if the aircraft is level fuel cannot transfer unless you have a stiff wind again causing the wing lift. One the imbalance starts the heavy wing will start gaining fuel.

[SarahLovesSeaplanes]

Thanks for all the feedback.

I hadn't thought about closing both selectors off and seeing if the engine continues to run in order to verify a leaky selector exists. How long will the engine normally run if there are no leaks coming from the selector valves? Is there any danger to the engine in doing this?

[letsfly]

A normal shutoff of fuel in low idle is approximately 30 seconds. Depending on the leak it may vary. No risk in the engine shutting down, we do it when the flight is finished anyway.

[trey kule]

Not sure that 30 seconds is exactly correct if you turn the selectors to off.. IIRC, the fuel tanks feed into a resevoir and that resevoir will keep the engine running for a few minutes at crusie power..The time is in the POH,, but if you are flying a legacy instrumented Caravan, there is a red fuel interruption to the fuel resevoir annuciator light...you know...the ones you do a check on before every flight when you turn off both selectors it stops the flow to the resevoir. when you move the condition lever to cut off, it cuts off the fuel to the engine..There is a difference in the time.
BTW,, if you turn off both fuel selectors it is going to get very noisy in the cockpit. Might want to take a moment to review the CB panel and see if you can quiet it down a bit. IIRC correctly, if you pull the CB you will get an annunciator light on as well.

[Redneck_pilot86]

Pulling the start cont breaker will silence the fuel selector off warning horn. IIRC it is 90 seconds for the header tank to drain at cruise power, so I would think it is going to be something more like 5+ minutes for the selectors off to drain the header tank with the engine only running at low idle. There should be no damage to the engine by doing this, as a normal shutdown is accomplished by fuel starvation.

[trey kule]

You dont need to empty the reservoir tank..Again, my memory is a bit lacking, but I think you will get an annuciator light when it drains to about 1/2. At which time you can select one or both of the fuel selectors back to on..No need to have the engine shut down. A restart is another cycle on the engine.
Just a thought though, if this does seem to be an abnormal situation, why is it not just being snagged and left to the maintenance folks, who know these systems backwards and forwards to do the trouble shooting?

[xsbank]

Trey is correct - don't play games with an a/c that is not serviceable - write it in the log.

[ragbagflyer]

Sarah, I've forgotten to shut off the caravan fuel selectors on uneven ground and it didn't result in the speed of fuel transfer that you are talking about; not even close. I don't know how the single point refueling system is plumbed, but it is obviously capable of moving a large volume of fuel in a very short time. I'm guessing that when refueling through that fitting there is some sort of flow divider that directs fuel to each tank through appropriately plumbed fuel lines. If this divergence isn't properly closed after fueling it seems possible that a large amount of fuel could quickly move from one side to the other. Again, that's all speculation because I'm not familiar with that STC, but it may be worth having a look at.

[Nephilim]

Hello Sarah. This phenomenon you describe is usually not noticeable on wheeled aircraft due to level and compact surfaces, on floats however, this mechanical defect is sometimes discovered. Some fuel selector valves don’t seem to work as they were made or intended to work, but leak/transfer fuel between tanks overnight no matter the selector position. “A sticky valve” is usually the problem and not that big of a deal if you are aware of it and know how to work around it.

I would bet $100 that your aircraft does have a “a sticky valve” of some sort. On a float plane the scenario you describe is usually caused by the fact that the left side is where you last step out when departing your aircraft for the night. Assuming no prominent leaky float, you may be on the float tying ropes, unloading, checking oil levels, etc. and that weight imbalance even for just a few moments causes fuel to transfer to the left side.

You might think that the levels would balance again once off the float for the night, but it doesn’t because that little bit of fuel that transferred to the left side (therefore weight) while on the float, has now caused a slight imbalance and fuel will continue to transfer overnight (gravity) depending on whether or not the valve is sticky a bit or a lot. The more amount of fuel that transfers overnight will tell you how much the valve is sticking open, a little or a lot (sometimes this works to the pilot’s advantage because you only have to fill the right wing in the morning to achieve ¾ tanks).

For the same reason as I’ve noted above, it is also most important when straining fuels to strain twice as much from the left side because contamination can float on the bottom of the tank just left of the strain dips when standing on the left float and be missed, especially if you are a big guy like me.

Trying having someone walk down the right wing (or a couple guys standing on the right float) until you have a definite right wing low imbalance and then strain your left tank, you might be surprised at to what you find! Vice versa too!

Once you have trouble shooted the problem while working with it, find out when on the manitenance cycle fuel selector valves are to be checked then snag it at that time if it is just a fuel tranfer sticky valve. However, if you determine through trouble shooting that the selector valve allows fuel to the engine even in the shut off position, should be snagged as soon as possible.

Safe Flying