Possible Vapor lock?

Roger Lee wrote:

We use E10 in Arizona with OAT's at  100F - 110F. Because the 912iS has such a high fuel return rate compared to the 912ULS I have never heard or seen any vapor lock issues with the 912iS.

Under the right conditions the 912iS fuel system can vaporize the fuel on the suction side of the pump where the pressure is low.  The high volume return flow actually contributes to this because it increases the demand on the suction side. When the fuel begins to vaporize this causes pump cavitation and the fuel pressure can drop far enough the engine loses power.  A dirty coarse filter (in front of the pump) can contribute to this because it will further lower the pressure at the inlet side of the pump. Fuel injected cars have overcome this problem by locating the pump inside the fuel tank. 

Regarding exactly when winter fuel is switched to summer fuel at a given station, I don’t leave this to chance, I test the fuel myself.  I buy fuel 30 gallons at a time (six 5-gallon cans) and during the change over time period (April-May in California), I test the fuel batches to know what I have. Making a home test kit is easy with hardware store parts, and the test only takes about 10 minutes.  This may seem extreme, but I know exactly what I’m putting in the tank.  If this interests you, send me a PM and I’ll send you details.  I’m looking forward to the day all airports have lead and ethanol free aviation fuel available.  

"The high volume return flow actually contributes to this because it increases the demand on the suction side. When the fuel begins to vaporize this causes pump cavitation and the fuel pressure can drop far enough the engine loses power."

But this means that the high return fuel moves through the hose before it can attain enough heat and cooler fuel is moving through before that can happen. 

Gasoline can vaporize anywhere from 100F -400F depending on the blend in the world and its additives.It burns in the cylinder at temps as high as 2500F. Ethanol vaporizes at approximately 172F. Blending the two may also increase the temps. Altitude (less pressure) and temperature both play a part. That said the fuel in a wing tank with a high return volume (912iS) shouldn't reach these temps. In a 912ULS with a slower return and no fuel hose insulation could possibly get there as some have seen in combination with the altitude.

 E10 -E15 using autos don't have the problem because they have pressurized systems.

Arizona with summer temps around 100F - 115F and multiple types of aircraft has shown that different fuels can be used. Aircraft cowled engines by different Mfg's can make a difference with air flow too. If this was a serious issue most all aircraft that use a Rotax engine and the majority using E10 fuel would all be grounded.

Roger

I agree with all that you say, however when I’ve experienced and observed fuel vaporization on a 912iS aircraft it has typically been after the aircraft that has been sitting in the sun warming the fuel tanks. In this case the fuel load is already hot, then the aircraft takes off and climbs to an altitude where the warm fuel will vaporize at the fuel inlet. The fuel can have significant thermal mass, so this can happen before the cooler temps at altitude have had a chance to cool the fuel in the tanks.  In this situation the fuel being recirculated does not help much since it runs through the engine compartment and picks up heat. 

In my aircraft, if I use a high RVP fuel and leave the plane sitting in the sun for a couple hours, the pumps are much louder because they are starting to cavitate - even sitting on the ground.  My fuel lines are sized correctly and the pumps are low in a compartment below the wing, so it’s not a design issue.  I’ve learned that this change in the sound of the pumps is a clear no-go warning.  

I did an experiment with 100LL in one tank and winter fuel in the other.  I let the plane sit in the sun for an hour then compared the sound of the pumps on each fuel. The Avgas kept the pumps sounding normal and the winter fuel caused the loud scream of cavitation.  The difference was consistent and almost instant when swapping tanks.  No problems with summer fuel.  

"when I’ve experienced and observed fuel vaporization on a 912iS aircraft it has typically been after the aircraft that has been sitting in the sun"

I've seen vapor lock in engine's that have been flying and then parked for an hour or so. Then all the hoses and everything under the cowl heat soak because there is no air flow and most don't have vents on the top to allow hot air to escape which would then also bring in cooler air from below.

I personally have never seen or heard of one just sitting  on the ground and not have been run getting that hot. That would make the skin unbelievable hot. How hot would the OAT have to be to get fuel inside a wing or even more interior that hot. Plus the fuselage would act as a shield and insulator to the direct affects of the sun. You'd bake that plane..

You might see 150F on the skin on a hot day, but it would take all day to get the interior fuel tank and any quantity of fuel that hot to cause a vapor lock. Might be a good reason to have a light colored skin vs a very dark colored one.

I feel like there are some really arguments as to why it is NOT vapor lock. What the heck else could it have been? I feel like under the circumstances the plane was in, vapor lock is the most logical explanation? 

Is there some sort of electrical issue that could explain the event despite NOT throwing any Lane faults?