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Robert

No fuel crossfeed.

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Hi all:

Physics is broken in my hangar.

2008 CTLS. Leveled with a digital level. 14 gallons of fuel in one side and none in the other. Let sit overnight, no change in fuel levels.  (Must be a pinched line right?) So I conduct a fuel flow test. 90 liters per hour left side; 89 liters per hour right side. (No pinched line.) Run fuel, one side at a time, by clamping the opposite side to ensure no air blockage in the lines. Place right wing on ladder so it is 14 inches higher than the left wing, remove fuel cap (just in case it is a pressure issue) and let sit overnight. No change in fuel levels. 

This is not a complicated fuel system, so what am I missing?  There are no valves or other devices between the fuel pickup in the tanks and the T connector behind the instrument panel. Although, my T connector is really a Y connector.

It fails no test in the manual and operates normally, but... that pesky physics thing and I must understand what is going on to safeguard my sanity. Any ideas are appreciated.

If Rod Serling started narrating in the background, I would not be surprised. 

Robert Harington

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I looked at the system diagram in the POH, section 7.10.  I concluded that the laws of physics are actually being violated in your hangar.  Perhaps there is some sort of local gravitational anomaly?  That is my primary thought.  

My only other thought is that maybe the flapper valves are preventing back flow of fuel into the main part of the tank, where you measure the fuel (the part of the tank under the valve cap).  The POH says, "Fuel flows via a flapper valve into the inner section of the fuel tank inboard of the anti-sloshing rib. The flapper does not completely seal the inner tank. It does, however, greatly restrict the return flow of fuel into the outer chamber when one wing is low (sideslip)."

Two possibilities here:  (1) the flapper is sort of stuck and actually is sealing the outer tank against backflow from the inner tank, and thereby allows the air to get pressurized and prevents the fuel from flowing into the inner tank.  Or, (2) some fuel is returning into the inner tank, but not getting over the flapper valve and therefore is not showing up when you dip the stick into the outer tank. 

But, after carefully considering the various possibilities, I am going with local gravitational anomaly.  See if there is some part of the hangar where you can get other weird things to happen.  For instance, maybe you can get paper clips or other small objects to float in that hangar?  That would be would be cool.

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The early 2008 LS planes were shipped without 90 degree elbows at the wing root. The newer planes are using a spring in the gas line  instead of the elbow. Look at and feel the gas line through the sight tube holes for a pinched line. I don’t know the physics of it all, but a pinched line at the wingroot will act like you are discribing.

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How about the inline fuel filter? I recently bought a CTLS from a fight school with dubious maintenance practices. Attached is a photo of the junk I found in the inline filter. Could yours be somewhat clogged such that when fuel tries to flow backwards through the filter some debris obstructs but causes no obstruction with the forward passage of fuel?IMG_1894.thumb.JPG.a1c3d02c92edf096cfd503a49d320fa6.JPG

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I realize the filter is downstream from the T connection but if there was a piece of debris somewhere in the line, perhaps it could act as a one way valve.

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I only fuel one wing most of the time and let it equalize in flight or overnight.  It doesn't take too long but often stops shy of level and sometimes doesn't want to begin.  

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Thanks for the replies. 

Roger, I will raise the wing even further to increase the pressure head. However, the fuel should act as a water level and even with little pressure head will seek its own level, just at a reduced flow. As I am using the sight tubes to measure the fuel in each tank, I can say with a certainty that if any fuel flowed it was less than a pint in 24 hours.

How much flow do you measure on the fuel flow tests you do? The 90 liters per hour I get is above the 35 liters per hour minimum specified in the maintenance manual, but I don’t have any other frame of reference for comparison and maybe both sides are restricted. 

EFB, I am not dipping for my reading. I am using the sight tubes that I have marked in one gallon increments. Also, I checked the flapper valves on this annual and they move freely. 

Although, I did notice that my beer tends to disappear faster on days that the CT isn’t flown, so maybe space time does have the plane trapped in the quantum foam. 

TIP, I do not have the elbow and the line does have to make a 90 degree turn. Which is why I do the flow test after every wing removal and manually check the line for kinks. If current CTs have the spring in the fuel line, then I will not need approval to install it. I have wanted to do this for awhile, but didn’t because it is an SLSA. This will be a good time for that, because I am not going to sign off this annual until I find out why it doesn’t crossfeed like every other CT in the fleet. It was actually everybody on this site talking about their crossfeed issues that caused me to try to measure it. Oh well, off come the wings again. 

MeHenck. Wow. I mean, wow. I pulled the wings this annual because I found some cottonwood fluff about the size of one of your rocks in the inline filter and wanted to ensure the rest of the tree wasn’t in the coarser screens in the tanks. If I found what you show, everything from tank to carb jets would have been replaced after panning that gravel for gold to pay for it of course. 

That being said. I have found some Hylomar residue in the tank on previous checks., so there is that small possibility. 

Again thanks for giving me some ideas. This is not a safety of flight issue. Just dammed strange. 

Robert

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Tip: I could not find any reference to the springs in the fuel lines LOA or otherwise. Can you send me your reference?

thanks

Robert

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They aren't springs that FD puts in, but aluminum tubes which helps keep them from kinking, but a royal PITA to get them in, out and straight. I replace the fuel hose and use springs. Makes life really easy. No fuss or muss trying to keep them straight and inline. The first time I pull a wing on a CTLS with those tubes in place I replace that fuel hose with a spring.

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Physics is fixed. 

The short answer is that air was trapped in the A pillar tubes.

This barely slowed the flow down, but allowed no flow back up to the tank. The solution was to fill the tubes from the bottom by blowing air into the opposite tank and forcing fuel up up into an empty tank; then doing the same on the opposite side. The clue that gave it away was a gurgling sound as I finally ran up without a headset on. My flow increased from 90 liters per hour to 125 liters per hour and I can now fill the aircraft by dumping fuel in just one side and it will level out in a short time. 

Thanks all

Robert

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I have 4 CTLS in the hangar. Two 2008, one 2009 and one 2010. None of the tanks ever equalize. 15 gallons in one tank and 3 in the other and the can sit for weeks and never equalize. I don't understand it but wish I could. If you have somewhat full tanks and jack up one side, gas will flow out the vent. This I understand but why don't the tanks equal out when sitting level?

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Mine equalize in my hangar most of the time but not all of the time.  Mine equalize all the way to equal sometimes and sometimes just gets partially there.

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Al,

I believe it is trapped air in the fuel lines. It flows out just fine, but will not flow back the other way. I believe this is limited to some of the CTLS and not the CTSW or the CTLSi due to the larger risers in the A pillar and the slightly restricted fuel lines at the 90 degree bend at the wing connection.

If it concerns you, then drain both tanks, put a few gallons in side 1 and gently pressurize that tank until side 2 has a couple of gallons in it. Clamp side 2 above the T connector and drain side 1 completely. Remove the clamp and gently pressurize side 2 until both tanks are even. Put five gallons in either side and watch as they now equalize in a short time. 

Then you will have to read all the tips everybody has provided on this site to transfer fuel in flight. (Fuel follows the ball)  Also, don’t pressurize the tanks over about two psi to prevent any damage. I made an adapter that fit over the filler port and blew into it using lung power. 

Robert

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It will flow. The trick is to have ENOUGH differential head pressure from one side to the other. If the pressure differential is marginal it won't transfer.

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Mine never flows from  just sitting but now it is acting different in one on the planes. Went for a long cross country yesterday with 14 gallons in left tank and 15 in right tank. Dipped it after the flight and there is 14 in the left tank and 1 in  the right tank. I have been looking at this for  a couple of weeks now as it has been  reported that  the right tank will completely empty and  the left stays the same. It does seem to start using from the left tank after the right one goes dry but this  is a little unnerving to the pilots. Anyone have an idea of what is going on?

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Nothing's wrong. It even happens on some certified aircraft. It is a function of a flat fuel tank. Plus your instrument panel may not be perfectly square and the instrument may not be perfectly square in the mount. So when you think you are perfectly coordinated you aren't. So from now on fly 1/2 ball out to the right and the fuel will most likely stay equal. If you get real low like this again fly 1 ball out to the right and the fuel will feed the engine an d transfer to the right tank. It is a slow process so it isn't a fast process. It isn't something to seat over and most likely you can't fix it. Most of us have an imbalance in fuel drain we just compensate. 

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It's not so much the imbalance I worry about but the fact that in flight it only uses fuel from  the left tank and completely drains the right tank. Not comfortable with the tank running dry all the time.

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I have been observing and cogitating about this issue for a few years in three high wing models.  My old Glastar, with only a "both/off" fuel selector, would do something similar to yours except it would preferentially use from the left tank . I once performed an experiment where I flew for an hour with two gallons in the left tank and 12 gallons in the right.  I landed with two gallons in the left and 5 gallons in the right. In my experience it never went below two gallons while there was sufficient fuel in the other tank. Many others report similar issues with that type, but there has never been an incident I am aware of in that model of aircraft where there was fuel starvation with sufficient fuel in the other tank. 

The Cessna 152 model I fly in occasionally has a similar issue.  That aircraft has only a "both/off" fuel selector and no way to monitor in flight the status of fuel levels in the wings and when we land, frequently we note quite a difference in fuel levels in the tanks.  Reportedly, there has never been a fuel starvation issue with adequate fuel remaining in one tank in that model as well.  Cessna doesn't seem worried about uneven fuel levels in the wings and I would guess there are a ton more hours flown in that type by sloppy pilots/students than the FDCT type.

It would be my observation that the hydrologic pressure head between the wing level and the "Y" where the two fuel lines join in the mushroom of the CT would be high enough to overcome any differential in the venting pressures.  The issue of a severe skid/slip during flight causing the fuel to unport the full tank's outlet would remain a possibility but a disaster in that setting would require no fuel in the favored tank, and the only fuel in the unfavored tank to slosh outboard long enough for the fuel lines to completely empty.  That situation would require a significant skid/slip and low fuel in the unfavored tank.

It would be my assertion that as long as you can see any fuel in either of the sight tubes, fuel covers the outlet port and therefore supplies fuel to the system. Gravity is in your favor, overcoming any vent differential. If you do fly with extremely low fuel levels, make sure you can see some fuel in at least one of the sight tubes. If you can not see fuel in either of the sight tubes, your exit ports are uncovered and things could get exciting shortly. 

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On 10/31/2018 at 1:35 PM, Robert said:

Physics is fixed. 

The short answer is that air was trapped in the A pillar tubes.

This barely slowed the flow down, but allowed no flow back up to the tank. The solution was to fill the tubes from the bottom by blowing air into the opposite tank and forcing fuel up up into an empty tank; then doing the same on the opposite side. The clue that gave it away was a gurgling sound as I finally ran up without a headset on. My flow increased from 90 liters per hour to 125 liters per hour and I can now fill the aircraft by dumping fuel in just one side and it will level out in a short time. 

Thanks all

Robert

I'm not sure how I had missed this topic, because this is a subject that I know about.

Air and surface tension in lines is well documented. An air obstruction requires more pressure to dislocate than it takes to simply cause fluid flow in a hose.

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