Gday,
after flying my Viper today I noticed that my UAT (Hanson UAT) was only 2/3 full. The main tanks had about 1/3 remaining.

It is 2 main tanks feeding into the UAT via a Festo Y to the UAT. Engine is a Spark T12 120 nm .

I think it's the first time Ive noticed this. It should no be this way.
What would cause it? I thought a restriction in the Y, but both tanks seem to be emptying ok and flowing ok.

I pumped fuel both in and out with refuel line and it stayed full.

Just some simple advice is being sought, before I pull the fuel system out for a close inspection.

Cheers

Matt

There is air being sucked through the system from somewhere. Make sure your tubes that go through the Y festo are cut at 90 degrees flat. Check the clunk when there is 1/3 fuel left in the tanks to make sure its all the way on the bottom of the tanks. Are the tanks flat on the bottom or are they curved like a basin? If they are flat it's possible when there is 1/3 file left inside them and you are landing with nose slightly down attitude then most of that fuel is flowing forward into the tank and some air is being pulled into the UAT. This happened to my Ultra Flash tank. The fix was I tilted the tank 5mm or so down at the back of the tank where the clunk was and that fixed the problem. You can test this theory by flying half the time you normally fly and land where you have 1/2 tanks full. If the UAT is full then its clunks not seeing enough fuel even when 1/3 fuel. If this is not the problem then it's sucking air from else where. Also check your fill line on the UAT. It may be the plug on the fill line you use.

Don't use the Festo on suction the don't like negative pressure and check the fill plug I hope it's not a skrew or some festo plug

Hi Matt,

Try filling your mains to say 1/4 full and give the engine a full throttle run on the ground.

Paul.

Had a problem with a friend's model, same symptoms.

Problem traced to a crack in the CAT fibreglass UAT. Very little fuel leakage but it would
suck air leading to it being half full & the engine stoppage.

The Hanson UAT has a screw on lid, is it tight & sealed properly?
All the fitting into the UAT sealed properly?

John.

Not true. Festo fittings are perfectly happy operating between 14 bar and -.95 bar, their design spec.

I rather don't use festo ,basics plastick or aluminum Y or T connectors works much better than festo specially on the Tygon fuel lines ,the only place where I will use the festo it's on the pressure site after the pump and with 4mm and 3 mm lines

replumb your tank to empty in series then see what happens. this will remove festo fitting "y" them check to uat for air leaks remove it and pressure test under water any leaks will show up for sure

+1 Use a pair of pliers if necessary to make sure the lid is absolutely tight. They will loosen up over time and even a quarter turn or so will let air in.

Craig

+1

I had a Festo on the suction side once and it sucked air. Removed the Festo and all was good.

I agree that the tanks should be run in series.

I also no longer use a UAT. I just use a pleated paper filter as a clunk and this has eliminated several potential leak areas from the UAT. I recently flew to long and when I landed the clunk was only submerged half way in the fuel, but the engine never missed a beat.

I had to use Teflon tape on the neck of my Hansen UAT to get it to seal. I also had a frustrating time trying to tighten the fittings on the bottle. Due to this experience I am contemplating the use of CAT's in both of my jets and future builds. I'm thinking there may be less trouble in the long run!

I never heaved problems with Hansen uat in other hand I have two bad cat uat the air was succeed thru the fiberglass structure I did send them with 600 and painted over with finishing epoxy .

Mate, you get the gold star!.
First thing I checked and indeed the F%^%&ing screw top lid was loose bt 1/8th of a turn.
Done a full power simulation with the pump test function. Perfect at all fuel levels. I then unscrewed the lid again a bit, and it occurred again. So I made sure it's nipped up nice and tight.

Cheers
everyone for ya help.

Matt

Glad to help Matt, happy flying! - John.