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-   -   Not drawing from the right hand tank (https://vansairforce.net/community/showthread.php?t=136164)

Saville 03-27-2016 02:47 PM

Not drawing from the right hand tank
 
Well so I check the fuel gauges and I'm astonished to see the left tank much lower than the right....

My fuel selector (Andair I believe) :



Is normally on BOTH. IT was OFF for the annual in January. Then back to BOTH

I fill up often so I didn't notice anything unusual until yesterday's flight. The left tank gauge was going down and was very low while the right tank hadn't budged.

Selector on BOTH. This is not what usually happens. They don't drain precisely at the same rate but it's never been like this.

I landed immediately as I didn't have a whole lot left in the tank.

So now I have a problem to diagnose...why is it not drawing from the right hand tank?

Broken fuel selector?

Plugged line?

Plugged air vent?

My only thought in terms of diagnosis is to fill the left tank, fly around burn down a few gallons with the selector set on RIGHT and see if the right tank level drops.

Or fly around on BOTH.

But this is no diagnosis and I'm hesitant to fill the tank if I now have to dissemble the fuel system.

Any suggestions as what tests to run to determine where the problem is would be appreciated.

BobTurner 03-27-2016 03:22 PM

I gather you are not the builder.
THIS IS A POTENTIALLY DANGEROUS SITUATION. RV's are not designed to use a "Both" position. If you draw one tank down while on both, the pump will suck air rather than pull gas from the other, full tank. (Unless the airplane has been modified to have a gravity fed sump tank below the valve, rather unusual).
Are you sure "both" does not actually correspond to "right" (e.g. wrong faceplate lettering)?
Check for blockage (mud dauber nest?) in the right vent line (just open the tank cap, and see if you hear bubbles when you blow in the vent).

BobTurner 03-27-2016 03:46 PM

Quote:

Originally Posted by BillL (Post 1065726)

Does the POH recommend both?

As an EAB airplane there is no official POH. The original builder may or may not have written one, and if so there are no checks on what he wrote is correct or not. Use caution.

rvbuilder2002 03-27-2016 03:50 PM

As Bob already said, low wing airplanes do not typically have a both position on the fuel selector (there is a lot of info regarding why, in previous discussions that you can find with the search function).

If your RV-8 does have a both position (cant tell anything from the photo because it is so out of focus) you have a not per plans fuel system which could cause you trouble at some point in the future.

Kahuna 03-27-2016 04:29 PM

Quote:

Originally Posted by Saville (Post 1065705)

Any suggestions as what tests to run to determine where the problem is would be appreciated.

It wont be necessary to disassemble the system. First, the fuel selector indicator may have NO relevance to what is actually happening. So you need to find out what IS happening with the valve. This would be pretty easy to do.
1. Firewall forward, remove a fuel supply line route it into a container.
2. Take a wooden dowel, stick the tanks, mark the level. Maintain log of levels
3. Turn on your pump, pump out some fuel into a dispenser with the valve in various positions. Make a determination on what the positions are ACTUALLY doing.

Correct indicator as necessary. Get help from a qualified mechanic or RV builder for help. Both is bad.
Thank goodness you have the option to deal with this on the ground. What you will probably find is that both is actually both. And you should not only not use it, but it should NOT be an option on the indicator. Also determining that both is both, can be tricky. What you might have to do is remove the wing root fairings, and remove the fuel supply line from the tank, and cap it. One at a time, to ensure you know what the valve is actually doing.

cfiidon 03-27-2016 04:41 PM

Gauges
 
All of the above is good advice. However, in the original post you mentioned "the gauges". Did you confirm visually an uneven fuel flow by looking in the tanks? Gauges can lie.

Saville 03-27-2016 05:28 PM

Quote:

Originally Posted by cfiidon (Post 1065760)
All of the above is good advice. However, in the original post you mentioned "the gauges". Did you confirm visually an uneven fuel flow by looking in the tanks? Gauges can lie.

Yes.

Thanks everyone for the advice.

I have no intention of flying to figure this out...that's why I'm looking for diagnostic tests to perform.

The vents are faired and screened so that's the first place to look.



Thanks Kahuna - lots of good ideas there.

RV8iator 03-27-2016 06:43 PM

I think you need to actually understand that BOTH is a bad idea with a low wing aircraft. If you have a BOTH position, don't use it. Feed from either LEFT, or RIGHT only.

Kevin Horton 03-27-2016 07:16 PM

Quote:

Originally Posted by RV8iator (Post 1065790)
I think you need to actually understand that BOTH is a bad idea with a low wing aircraft. If you have a BOTH position, don't use it. Feed from either LEFT, or RIGHT only.

If you don't understand this, conduct a practical test - get some clear, flexible tubing and a T connector. Cut three lengths of tubing and put one piece on each leg of the T. Take two glasses with water - one mostly full, and one mostly empty. Put one piece of tubing in each glass, then suck on the third piece. Watch what happens when the first glass runs dry - suddenly you are sucking air, not water. Now, imagine that each glass is a fuel tank, and replace the water with fuel - your engine will stop once the first tank runs dry, no matter how much fuel is in the second tank.

A Both position works on a high wing aircraft because gravity will cause the tank with fuel to feed even if the other tank is empty. Both positions are bad, bad news on low wing aircraft, unless there is a header tank that is fed by gravity from both wing tanks.

rightrudder 03-27-2016 09:16 PM

Quote:

Originally Posted by Kevin Horton (Post 1065802)
If you don't understand this, conduct a practical test - get some clear, flexible tubing and a T connector. Cut three lengths of tubing and put one piece on each leg of the T. Take two glasses with water - one mostly full, and one mostly empty. Put one piece of tubing in each glass, then suck on the third piece. Watch what happens when the first glass runs dry - suddenly you are sucking air, not water. Now, imagine that each glass is a fuel tank, and replace the water with fuel - your engine will stop once the first tank runs dry, no matter how much fuel is in the second tank.

Here's an alternate test rig. I recommend Stella Artois for test fluid, but this guy looks like a Bud man:



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