PA-18 Fuel Gauge Inaccuracy

We have been troubleshooting this problem on our PA-18 for a long time without success. Prior to a recent flight, the R and L gauges both read 5/8 tank (using three point scale). Once flying... and, using the Left Tank only... the R gauge indicates full (can't even see the floating ball). When we lift the tail up on the ground to "level flight"... the R gauge indicates correctly (5/8 tank on the "level flight" scale). Disregarding the fact that the R fuel gauge is inaccurate, the fuel system seems to flow just fine (on either the R or L tank). After studying the fuel system chart, we noted that the only way this discrepancy in the R fuel gauge could occur is if there is "suction" occurring across the top of the R fuel gauge. So, we capped off the vent line that attaches to the top of the R fuel gauge and went for a flight on the L tank only... indeed, the R fuel gauge now indicated correctly... thus confirming? that there must be a significant flow of air through the vent line, and therefore across the top of the R fuel gauge... which creates a slight vacuum that "pulls" the fuel level up in the fuel gauge. We are baffled as to where this air leak? is coming from. In fact, at one point we pressurized the right tank to 2psi, and noted no air leaks. Our next plan is to isolate the R fuel system just before the fuel selector valve... hopefully to determine if the leak is in the header tank system or the fuel selector valve itself. Does anyone have any insight... or ideas? As we are baffled, it is also possible that we are off-base with our "air leak" diagnosis. BTW... on the last flight, with the L fuel gauge indicating about 1/4 tank (level flight)... we had fuel starvation and recovered with a quick swap to the R tank. Needless to say, we are having some trust issues with the fuel system.

Fuel gauges are like a compass, they are not accurate. Both give you information that you need to prove true. Both are just a guide on what you " might " have or where you " might " be going.

Glenn

what kind of fuel caps,this left right no both? if stock caps vent is bad if pressure tubes one is full of something.old balls stick stock has no venting between tanks 180 has cross vent over top

That's funny! Every single PA-18 I have ever been in has fuel gauges that wander around. I either fill them or dip them, then time how long I am on each tank. My Decathlon solved all that; they only put a gauge on one tank. When it says zero I have nine gallons left.

I gave a presentation yesterday at an FAAST meeting in the Twin Cities, based on the thread I initiated after Johnson Creek last summer:

http://www.supercub.org/forum/showt...to-Johnson-Creek&highlight=fuel+johnson+creek

The problem I had was exasperated by the fact that you cannot see the little cork balls if the tanks are full, or empty. The synopsis of the thread I just referenced is that I had personally filled my tanks a few days before launching to JC, the aircraft was in a secure (or so I thought) hangar, and after loading the super cub for flight I looked at the sight gauges, couldn't see the balls thereby confirming that the tanks were full, only to launch and have the engine quit about 50 feet off the ground. Someone had gained entrance to my hangar, had emptied the tanks (broke my ELT antenna off the back of my super cub in so doing), and created a major problem for me. It could have been worse, had they left only enough to get me over a populated area as I left the airport to head west.

I bring this up because had I recalled some of my college physics I would have noticed that since fluids refract light a great deal, and given that air does not, a careful look at my sight gauges would have demonstrated that there was air in the glass column. Someone here pointed this out to me, and I thank whoever that was because that knowledge could save someone's life.

Here is a photo of my sight gauge with the tanks a bit above half full...

Please take note of the fact that the white line in back of the tube is relatively straight across at the 3/4 mark, but significantly deviated at the 1/2 mark in the three point configuration...physics at work.

Here is a photo of the tube with a full tank...


Please note that all lines are bent, consistent with refraction of light.

I trudged across to Darrel Starr's hangar to visit and told him I was preparing for my presentation and described my sight gauges. He took me to his beautiful super cub and much to our chagrin noted that his newer sight gauges were designed such that the metal jacket does not permit one to observe the refraction differences between fuel and air...he will not be able to use this simple trick...


I thought this would be a good place to review these findings, perhaps saving you folks a problem that could have tragic consequences.

Randy

On my CT210 I bought one of those little tubes that you can calibrate and create a graph that I use frequently to determine accurately how much fuel I have in my tanks. The only caveat with this is that the aircraft must be on level ground. If it is on a slope all bets are off.
View attachment 24984View attachment 24985

I have given consideration to using one of these on my super cub, but have decided against this given that it would be much more difficult to make sure I am measuring perpendicular to the wing in a longitudinal direction, and that this would make my measurements less accurate.

I know that a fuel flow meter, if calibrated carefully, can give accurate estimations of fuel flow and hence used fuel. I haven't put one of these on my super cub. I do have one on the CT210.

Randy

Randy,

If I currently owned a cub with an electrical system, it would have a fuel flow computer installed. I discovered those things many years ago and found them to be pure magic, and incredibly accurate. I've flown Cessnas, Beavers, Cubs, Huskys, Scouts and Maules with the things and I am a believer that they are one of the best tools you can install in an aircraft.

That said, a fuel computer isn't going to reveal that someone stole fuel out of your tanks. But, otherwise, they are a great tool.

MTV

WindOnHisNose said:

On my CT210 I bought one of those little tubes that you can calibrate and create a graph that I use frequently to determine accurately how much fuel I have in my tanks. The only caveat with this is that the aircraft must be on level ground. If it is on a slope all bets are off. . Randy

Click to expand...

sounds like a great place for an APP to calibrate and compensate for the tilt of plane. Just set phone on top for a second than calculate/ look up change

manualy you could make a chart by filling an exact amount about half tank and tilt and mark down the readings. Probably not useful if over 3/4 full...

I never go anywhere in a light aircraft without personally fueling it, then checking the caps, or dipping it and then checking the caps. Part of preflight. And my hangar is locked.

A wood dipstick is really easy to make, and easy to duplicate. Way easier to read than that Cessna glass tube.

When ever the ball is out of sight, I verify that the tank is full with the index finger check. If I cannot feel the fuel, then the tank would be empty. For everything in between I correlate with a wooden stick the position of the balls. I also have an Insight G2 with fuel flow, but you can mis manage fuel flow units so the value of a finger or stick check cannot be over emphasized.

With regard to early pre flights. I've seen birds build nests in a matter of hours in cowlings. While it seems like a good idea to preflight the night before, preflight should occur as close as possible to flight.

Fuel caps? Top of fuel gauge blocked with mud dobber?? We have some tanks that have a elevated fuel filler neck and I am beginning to believe that it has an effect of vaccum preventing the fuel flow to the carburator. On the ground it is fine but in the air it flows less than the stock fuel filler neck. IMO.

We had one like that. Necks were not elevated. Cured with CC caps with that giant"Pffft" on top to slightly pressurize tank. There is supposed to be a vent system somewhere, and it is best not in that low pressure area above the wing.

Easier checking Cub fuel vs Tri-Pacer... This was yesterday so had to share the ha ha! :lol: