Site Gauges

[Crash]

Wondering if anyone else out there has had this problem.

I finished my new PA-18 in March with Dakota wings and tanks. I started flying it a lot right from the start. One day after visiting a friend at a cabin I took off and flew a couple of miles on the R.H. tank and the engine quit. I switched to the L.H. tank and it restarted. I looked at the site guage and it had about an inch of fuel showing with the ball floating on top (1/4 tank). I rocked the plane and the ball went up and down but the engine quit when on that tank. I wrote it off as a piece of junk in the lower restricter elbow of the site gauge. Because when I landed the gauge went to empty and it seemed to function right when I put gas back in that tank.

A couple of months later I was in the middle of Cook Inlet on the R.H. tank and the engine quit again. I went to the L.H. tank and it started again, but it still showed 1/4 tank of fuel in the gauge and the ball went up and down when I rocked the plane. This was not funny anymore. I figured the only change I had made in the rebuild was the larger Dakota tanks (should not make a difference) and Atlee Dodge pressure fuel caps as required by the 180 hp STC. I figured it must be the pressure caps holding differential pressure between the tanks and forcing fuel to stay in the site gauge.

I removed them and went to stock caps. It still had the same problem on the R.H. tank with the stock fuel caps but the L.H. gauge worked right.

On my last long trip I went back to the pressure caps to see if I was dreaming. The engine now quits with a 1/4 tanks showing on both tanks.

Whats UP????? Any ideas out there? Crash

 

[kase]

Is the bottom of your site gage lower than the tank?

 

[StewartB]

Similar topic from the past.

http://www.supercub.org/phpBB2/viewtopic.php?t=2547&start

SB

 

[StewartB]

Serious question. You have the Cessna style fuel system, right? So you must be selecting a specific tank to get it to quit. Does the sight tube indicate consistently between left, right, and both?
SB

 

[slowgo]

Crash/Stewart:
I have the same problem in my rh Dakota tank, although not as severe as mine shows slightly less than 5 gallons before quitting. I have header tanks and the original configuration for a fuel system. You can rock the tank and tip your wings, and it stays at about 5 gallons while bone dry.

 

[Crash]

Is the bottom of your site gage lower than the tank?

Guys that know me will tell you how annul I am about perfection. I took the stock site gauges to a horizonal head milling machine and lengtened the window openings to match the depth of the Dakota tanks (if I hadn't told you you could not have guessed). Then I used a digital Smart level to match the bottom of the gauge to the bottom of the tank when I put them in. When the ball is at the bottom of the window you can see the top of it and it is setting right on the bottom of the tank. When the ball is at the top, it is at the top of the tank. They should read "dead nuts". Something is holding fuel in the gauge at 1/4 tank even though the tank is bone dry in flight. You land and the fuel runs out of the gauge and it reads empty. Crash

 

[AkPA/18]

Hi Crash

Where is the lower sight gauge outlet in the new Dakota tanks compared to the stock tanks---or---where is the outlet to your sight gauge???


Mark

 

[cubunltd]

What kind of gaskets are in the top and bottom of your guages? they should be cork. if they are rubber, the fuel is swelling them shut.
cubsunlimited@verizon.net.

 

[cubscout]

I hope you get this sorted out, there have been some helpful suggestions here. This is not meant to be critical in any way, just my conservative perspective. I understand you Alaska folks need to work nearer the edge than most of the rest of us. And I don't have the level of expertise of most folks posting here, but hear me out:

When I checked out in Supercubs during the Johnson Administration (NO! Lyndon, not Andrew!, despite what others have said) I was admonished to calculate fuel burn rate against clock/tach time, but to NEVER rely on fuel gauges. Use them as a crosscheck, yes, but DON'T trust them.

While I mistrust Supercub sight gauges less than any other aircraft fuel gauges out there, I still mistrust them.

I ALWAYS dip each tank with one of those nifty calibrated clear soda-straw things, and work off of clock/tach time at a known fuel burn rate. And take the worst case of what I dipped, or the gauge, in case I screwed up and left the cap off.

Thanks. Cubscout.

 

[J5Ron]

I know of a few guys who have taken off the original fuel guage with the ball and replaced it with clear fuel line...I had problems with one of my tanks guages and did this and got a more realistic reading of fuel in my tank...

 

[ground loop]

Crash, I remember telling an old time cub pilot about 6 years ago that I was going to replace my electric fuel gauges for the sight guages. He told me not to trust them because he had them show fuel until the landing impact straightened things out. Since then it has happened to me several times. For me, it seems to happen more in hunting season, (maybe I just watch fuel closer because of living on the edge). It ain't the tanks, it ain't the installation, its probably the nature of the guages. The very wise old cub pilot called it a vapor lock.

 

[S2D]

Guys that know me will tell you how annul I am about perfection. I took the stock site gauges to a horizonal head milling machine and lengtened the window openings to match the depth of the Dakota tanks (if I hadn't told you you could not have guessed). Then I used a digital Smart level to match the bottom of the gauge to the bottom of the tank when I put them in. When the ball is at the bottom of the window you can see the top of it and it is setting right on the bottom of the tank. When the ball is at the top, it is at the top of the tank. They should read "dead nuts". Something is holding fuel in the gauge at 1/4 tank even though the tank is bone dry in flight. You land and the fuel runs out of the gauge and it reads empty. Crash

See what perfection gets you !!

What is the status of fuel in the tank?? is it really empty or do you still have some in it? You said bone dry but is it.

What could cause suction on the top of the ball. is the normal top opening plugged and fuel emptying out the "t" that goes to the front of the tank sucking the ball up and what little is in the line behind it?

Kinda like a leaking cap on cessnas will suck the bladder up making the guage read fuller than is.

I'm not really familiar with the headerless System. Do you have T's on both top guages going to front of tank?


that might tell us whether its the quage problem or feed problem.

 

[kase]

Ive seen several cubs were the bottom outlet of the site gage is lower than the outlet of the tank site gage tube. It acts like a P trap in a sink plumbing system. My engine will quit and still show gas in the gage on the right side. I dont remember the last time I ran the left dry.

 

[Crash]

Hi Crash

Where is the lower sight gauge outlet in the new Dakota tanks compared to the stock tanks---or---where is the outlet to your sight gauge???


Mark

The Dakota site gauge ports are in about the same place as stock tanks.

 

[Crash]

What kind of gaskets are in the top and bottom of your guages? they should be cork. if they are rubber, the fuel is swelling them shut.
cubsunlimited@verizon.net.

I put new Univair cork gaskets in at rebuild. The site gauges worked very well with one stock tank and one Dodge tank for years. Hmmmm??? Crash

 

[diggler]

delete

 

[AkPA/18]

Sheeesh Crash---you go through the above post with the digital level and then you reply with about the same as stock. That just isn't like you! Now this is just my opinion---the only way you will have remaining fuel in your sight gauge when the fuel gets below the tank port is if your sight gauge port is lower than the tank port as KASE mentioned like a P-trap. The curious part is that the ball would climb as you rocked your wings in flight. I don't know if your crossfeed line was suppying fuel to your sight gauge from the other tank or not. Do you have an inline valve for your crossfeed line that you can shut off when you do that (id you don't you might think about that as they are handy on unlevel ground). I suspect that on landing you might have got into some sort of attitude that allowed the sight gauge to drain back into your tank an then read empty. This is not a comment on your landing skills! In any case---I would tend to believe that the bottom of your sight gauge port is lower than your tank port. Let us know.

Edit: also the fact that slowgo above is having the same problem leads me to believe that there might be a slight difference in the location of the lower tank port after install on these new tanks.

Mark

 

[snowblind]

SITE GAUGES

Powerplant failure due to fuel starvation is a serious matter. Especially
when it appears you have sufficient fuel in the fuel tank selected. This exact scenario has happened to three pilots at Lake Hood Strip in AK with Super Cubs. I like this part (sounds like a FED) "probable cause blocked vent on fuel cap on respective fuel tank". This has happened to a few of the fuel tank caps which you see on the 180 HP Super Cub"s. These caps have the upside down "L" tubing fastened on them. Corrective Action ="Replace caps or enlarge existing vent hole. " Ops Check OK. No more problem. This make work even on the regular fuel caps.
Fly Safe

 

[AkPA/18]

Hey Crash--one more observation---if I read your description of where the ball is located on your install post right ---your sight gauge port has to be below the tank port. If the ball is located to the bottom of your tank--the port on the gauge is definitely lower than the tank port.

Mark

 

[Crash]

Ports

The hose routing is straighter then the stock tank. A stock tank would have more "P trap" effect. I talked with Steve at Atlee's today and he said he knows of several other guys experiencing the same thing. I might try putting the restricter elbow on the top instead of the bottom. It might be getting an air bubble in the necked down part. Whats wierd is the ball goes up and down like there is fuel in the tank. When you land and sump the tank, there is nothing in it. It just makes no sense at all. Crash

 

[StewartB]

Since you have the tank out, you have the perfect opportunity to try to duplicate the phenomenon on the bench. Is it tilt? Pressure? Vacuum? Grab a six and have a science night.
SB

 

[moneyburner]

Okay, here's my two cents worth from a dumb Cat mechanic:

From my experience with tanks - and not fuel tanks in airplanes - if the pressure on the upper and lower ports of any sightglass are not exactly the same (be it zero or 3000 psi), then it isn't giving you a level, it's reading pressure (or vacuum). This is how a manometer works to show small pressures such as inches of mercury, etc.

So - if you have fuel in it and there is a very slight difference in pressure - say just a bit of vacuum on top - that ball may just sit there while you contemplate the silence until the bottom port is uncovered and the fuel drains out of the gauge. There isn't a pump on my engine, of course, but there is still two feet or so of suction head pulling on the fuel. It might be enough. Like holding your finger over the end of a drinking straw. After nearly all the fuel is gone from the tank, the engine quits (or stutters) - you switch and after you land, everything equalizes . . . is that what is happening?

A restricted vent might cause this, or a vent that is in an area of lower pressure (say, above a wing, like my 12). In my line of work, a vent needs to be engineered to be big enough to allow no pressure drop above when drawing off fuel, and there are usually at least two. (Caving in a 250,000 gallon tank because the vents were plugged will usually get you run off around here). If the engine is drawing it faster than the vent can compensate, the pressure above the fuel is going to be just a tad lower. Does it just happen during prolonged cruise power settings? Does it fix itself after a while when you switch tanks?


Mike

 

[ground loop]

I went deer hunting out towards Montague. When I was really committed over the ocean, as usual the engine started making all those "wierd noises". When I looked at the sight guage it showed empty. I thought I developed a huge leak since that tank should have had about 20 gals. It turned out I lost a fuel cap. Never lost any fuel, it just showed empty!
Different pressure gradients are present in the atmosphere and they depend on the varient conditions. Isn't it also possible that various pressure gradients can exist in a fuel tank system dependent on ambient conditions. After all gas is volatile and is very sensitive to temperature and altitude (pressure changes), and funneled air through a vent isn't guaranteed to distribute itself equally throughout the tank.
In my opinion, as in my plane, no ball should be present in the sight guage. It is just one more thing that could be misleading!

 

[supercubc37]

What about the balls themselves. Are they the Univair new style cork ones? I talked to Bill at Stoddards when we were in Anc. and he said the last batch of cork ones would swell up in the sight glass. That is the reason for the change to plastic ones. If they are cork they may swell out of round enough to block the gauge and trap fuel and still move with a little pressure change caused by rocking the wings. Just a thought.

Matt

 

[moneyburner]

When I looked at the sight guage it showed empty. I thought I developed a huge leak since that tank should have had about 20 gals. It turned out I lost a fuel cap. Never lost any fuel, it just showed empty!

. . . does having the cap gone pressurize the tank? . . . I would have thought the opposite.

 

[Crash]

Hey Crash--one more observation---if I read your description of where the ball is located on your install post right ---your sight gauge port has to be below the tank port. If the ball is located to the bottom of your tank--the port on the gauge is definitely lower than the tank port.

Mark

Mark: What I meant in my comment about the ports located in about the same place as a stock tank was not refering to the spread of the ports, just the relevent location. I may have misunderstood the question.

I think the balls are plastic. I purchased them last winter from Stoddards. I am thinking about taking them out as well to elimnate the possibility of them acting as a floating stopper. I just want to see how much fuel is roughly in the tanks when it gets low. If the tank is out of fuel there should be none showing in the site gauge. I flew this plane for 11 years before the changes and never had the problem. Crash

 

[AkPA/18]

Heck Crash---that was my best shot at the time and it looks like I swung wildly and missed after rereading. To me this is some of the cool stuff about aviation. There should be no mysteries and if there are you just aren't looking close enough. I am sure you will pinpoint the reason and look fwd to the explaination.

Mark