What I learned today about flying in a friends 750 STOL  Don’t run a tank dry with fuel selector valve on both.  Switch to the more full tank early and use all available fuel before switching back to the less full tank.  

Here’s what happened, my airport’s fuel pumps are being serviced so I elected to go to an airport about 18nm away.  I had 8 gallons in my right tank and maybe one gallon in my left tank when I took off with the fuel selector on both.  This engine normally uses 6 gph.  I flew direct to the nearest airport, landed and was taxiing to the pump when my engine died of what sounded like fuel starvation about 100 yards from the pump.  Restart unable so I towed it to the pump and refueled.

After refueling I determined that I had four gallons left in the right tank.   The engine still wouldn’t start.  I disassembled the fuel line before the gascolator to check fuel flow and found very little flow in both, left, or right positions of the fuel valve.  My friend Paul showed up and started sumping fuel from the lowest sump on the left in the floor behind the pilot’s seat and instantly starting getting fuel to the open fuel line in all three positions.  

So what happened?  I’m still not sure and Paul and I think that maybe there was a air lock, kind of like an air embolism caused by running the left tank dry that allowed air to get past the fuel selector valve and effectively blocking the system.  Was this really the cause?  I’m not sure and would love to hear from wiser, more experienced pilots than myself.

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Venting both tanks is very important....I'd check to see if vents are blocked....Gravity will cause any fuel to flow down unrestricted lines to low point. Gas is heavier than air...it will displace any air normally in the lines...

When I did fuel flow tests, I used 1 gal in left tank, (right tank dry) let it run out, then 1 gal in right tank, (left tank dry)  let it flow out...all seemed OK.

If that all proves OK, then I'm at a loss and would love to hear a reason as it also would affect my system.

Hi David,  Mark is correct about the venting, double check those, hopefully it is that simple.  If not, read through this discussion: 


I posted a link to a youtube video that discusses airlock in water lines.  Another builder posted a document that explains the same phenomenon.  Variation in fuel line routing and components can affect your susceptibility to this issue.  Some builders have no issues, yet others do. Not sure why but I suspect build variation. These low head pressure gravity flow systems are anything but simple.  Hope this helps, good luck and fly safe!


See "Air in fuel system" in the STOL CH750 forum.  Great thread on the subject.  I was doing unusable fuel check; had 6 gallons initially in both tanks, however, all lines were filled with air first.  Got suspicious results from first tank tested (left) but did get flow.  When the flow dropped below the minimum rate required for 100 HP, I tested the right tank and got good results - about half a gallon unusable.  I drained all fuel possible and started over with other tank....no fuel flow.  Had to blow into tank to start flow again.  Must have had an air lock having drained tank and all lines thus introducing air into line.

My results seem to verify what happened to both of us.  I plan to placard aircraft "Do not take off with either tank less than 1/4 full" .  Many Cessnas have similar fuel selectors and gravity feed systems and have these placards.  That must be why they have such placards. 


Thanks for sharing.  Glad you’re OK. Do you not have an electric boost pump mounted as close as possible to the fuel tanks that pushes fuel forward to the engine driven pump?

I have the problem that my left tank empties first to 1.5 Gal rest and then the right tank begins to empty. I have no heavy wing. Doesn't make me feel comfortable because there is a risk that in a turn the left tank port gets unsupported and brings lot of air in the system.

I am investigating this problem already quite a while. My system is easy. Left and right join at lowest point behind pilots seat, then gascolator and fuel pump (also lowest point), then up to selector both/off and further up to Rotax mechanical pump. Air should be forced out because its lighter than fuel. Caps have snorkels.

I think the problem may be caused by the bend of the fuel lines where they pass through the rear wing spar, which is actually the only *bad" bend in my system. I assume that whenever air is trapped in that bend the increasing pressure in the right tank forces it out to the left line at one point and then fuel flow starts from right. If this is the cause I wonder why other builders do not have problems with this bend because thats the normal routing of the lines 

Maybe the bends caught air in your case and the remaining fuel in one tank could not force it out.

(BTW: Flow tests on ground are always good)

Michel the way your aircraft  burns fuel doesn’t make sense, it’s very simple and gravity should ensure a fairly even flow. Bends in lines or snorkel vents shouldn’t make a significant difference.  Is there any chance your left wing dihedral is more than the right wing, and gravity is flowing the left tank fuel into the right in flight?

Well, it's really strange. The dihedral is about the same and incident angle too. Inflight the plane is level according to Skyview (And the ADAHRS is also level on ground). The left tank empties to indicated 0 so for that the angles need to be very different and they are not. Fuel flow test on ground for each line showed no difference. Once back on the ground with the plane, the tanks level out pretty fast.

Maybe it has to do with positive pressure on the snorkels. They face directly in the wind because I couldn't bend them more down. I tried the Zenith's original caps and with them it seems slightly better.

Or maybe it has to do with the Rotax fuel return line which in my case goes into the top of gascolator. But this setup is also tested by other Rotax owners.

It stays strange. If that bend in the rear spar is not causing it then I am a little bit lost.

Yes strange indeed.  I use the Zenith caps and have no significant fuel burn difference between tanks. It’s time to change something then.  Perhaps remove or isolate the fuel selector valve from the system, or block off the fuel return line temporarily, or reroute the fuel return line to a fuel tank, and see if that changes anything.

Yes, I will try to bend the snorkels more. Maybe positive pressure doesn't help to let vapor/air to vent easily. Afterwards I block the return line, Maybe there is air/vapor coming in the system and it travels up to the right tank and hinders fuel flow from there. Unfortunaltely I do not know though if it's better in colder condition. That could point to the problem with the return. 

Rerouting the return line would not be fun. Have to add an access panel where the port is in the tank which I blocked :-( Well, after the first built one is always smarter. Did not route the line back to the tank because in earlier days Rotax didn't ask for a return line....


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