fuel pumps

Hi Mark, I am having the same issue with my pumps. It gets worse in hot weather. I generally keep mogas in one tank and 100LL in the other. when it was hot, 90ish, the engine would start to die when running on either tank. Have you found a solution yet? I am not sure I want to change the wires to the switches. Mine is a Sling Tsi with a 915.  Ben Taves

Ben

I’ve observed a few TSI builds online and I know individual builders approach the fuel system differently. I see some with rubber tubing on the supply side, others with aluminum. The placement of the boost pumps also seems to vary.  Is this something that has happened from day one, or something new?

 A free flow of fuel to the inlet of the Rotax pumps is critical, and the minimum supply line size is 3/8 (AN-6).  If you have a boost pump between the selector valve and Rotax pumps, is the problem eliminated with the pump running?  And if your aircraft has rubber tubing on the supply side, it’s possible a piece of rubber got shaved off by a barbed fitting and is inside the tubing causing problems. And obviously, check the coarse filter (gascolator) that is in front of the Rotax pumps.  

Jeff, there is a boost pump between the selector valve and the Rotax pumps, and all of the tubing is 3/8 with AN-6 fittings as far as I can tell. The fuel pressure has always seemed to be a little low, and when brand new the yellow light would nearly always come on when turning off pump 2. I lowered the pressure setting that the yellow caution lite came on which helped but the pressure would still hover just above the point where the engine would stumble.

A few days ago the temperature was quite hot, around 90, and I couldn't turn off pump 2 without the engine completely stopping. I always watch the pressure carefully when I turn off the second pump but this time I had to turn it back on immediately. Yesterday the OAT was not as hot and I could turn off pump 2 with just a slight stumble. I am quite sure that temperature has a big impact. I think that the boost pump just after the selector valve turns on only with pump 2 since I can hear it clicking just before start up if that pump is on, but I can't hear it when the engine is running. I will try turning off pump 1 instead of pump 2. If I were building another plane I would use 1/2" supply tube and locate the pumps as low as possible. Ben

Ben,

I agree on the larger feed line and lowest possible pump location. If I were building an iS powered aircraft I would use 1/2” tubing on the supply side.

Is your boost pump in direct line with the fuel supply line or is it offset into a parallel circuit with a check valve in the main line?  I have not seen a fuel diagram by Rotax that includes a boost pump in front of the main pumps. The Rotax iS fuel system circulates a lot of fuel and I have to wonder if the restriction from the boost pump and/or check valve can be problematic when the boost pump is not running.  I see at least one company is selling a dual boost pump controller (one pump per tank) for the TSI, which makes me curious about what problem they are trying to solve.  

Also, did you (or the builder) install a vapor bypass line and proper restrictor? 

The vapor bypass line is more important than most folks think.  If its not installed, and you get vapor in the line (like running a tank dry) the fuel pressure regulator will see the pressure drop and close off the return fuel line to try to get pressure. At that point the vapors only escape route is through the injectors, which can take a while (and seem like an eternity in flight).  

Jeff, I am not sure how the boost pump is plumbed. I flew yesterday and I can see that the boost pump is wired to the number 2 pump and it's intermittent as the amperage would be 0 then up to 3-5 amps for a second or two then back to 0. It looks like it's a pulse or an intermittent type of pump. I am guessing that using the number 1 pump as a backup might be better. I assume that the way it's plumbed the number 1 pump would have to draw fuel through the boost pump reducing supply and head pressure, and the number 2 pump would always have the boost pump working. The fuel tank selector is inline before the boost pump and after that goes to the Rotax pumps on the firewall. I am not completely sure about all of these details. I don't know about a vapor bypass line or restrictor. I have thought about running larger lines from the tanks but that's a huge job and the fittings are all sized for 3/8" at the tank and probably everywhere. I am sure that larger lines would help somewhat but the fuel would still need to go through smaller fittings. 

Yes, there is a company, Midwest Panel Builders, that developed and sells wing root pumps. They might sell them under a different name. The purpose is to slightly pressurize the supply lines so that the fuel doesn't vaporize in warm temps and high altitudes when using mogas. Mogas has a higher RVR so there have been vaporization problems. I talked to them about my issues and he said that he didn't think the wing root bumps would help. I don't necessarily agree with him, I actually think they would help. If you raise the supply pressure you also raise the head pressure, and completely eliminate any vaporization.