Possible Vapor lock?

Wesley,

As Roger points out, it would be very hard to have a vapor lock downstream of the pumps where the pressure is high.  And like Roger, I have never seen actual vapor lock in a 912iS aircraft that causes the engine to stop. He and I look to agree on more than we differ.  And to be fair, what I'm suggesting is not technically "vapor lock". 

What I do believe is that fuel vaporization (boiling) in the pump supply line can cause the pump to cavitate and lose partial pressure, resulting in partial power loss until a remedy is applied. Sometimes this can be as simple as turning on the second pump, or lowering altitude. This is exactly what you described in your original post, and what got my attention. 

Since this happened to me a couple years back, I have spoken with three others with a similar experience in Rotax 912iS aircraft.  In all cases, the aircraft have been low wing, where you don't get much (if any) contribution from gravity to bring fuel to the pumps. And in all cases, the pilot was using winter fuel, or a fuel with an unknown vapor pressure, on a warm day.  

With winter fuel having an RVP of 13, the fuel in the tanks does not have to get very hot to get the pumps cavitating. If the fuel was even 85 deg f., then on the low pressure side of the pump the fuel can begin to boil around 9000'. Raise that temperature just 5 degrees to 90 deg f., and now you are looking at the fuel boiling at around 6000'.  Of course not all pump cavitation results in a consequential loss of pressure, but under the right conditions I believe it can.

This is just my opinion of what you likely experienced, and yes I could be wrong, it would not be the first or last time.  But I wanted to share my experience with you in the hope it may help. Of course you should do an inspection of fuel system including the filter on the inlet side of the pump, but I'm guessing you won't find much. 

Happy Flying!

I completely agree, this was far from “vapor lock”. If anything it felt more like a slight feel starvation or weak fuel delivery moment. In fact, as I was describing the scenario to my wife it became clear that nothing catastrophic actually happened. The fuel pressure dropped below mins by about 2 psi for maybe 5-7 seconds. With an accompanied slight Loss in RPM. Maybe from 5200 down to 5000 or slightly below. No engine failure, no stumbling or engine stalling, no need to pitch for best glide and restart. Just a slight throttle reduction and descent back down to lower altitude.

Fuel vaporization is a much better way to describe this. With a small about of vaporizing fuel forming in possibly cavitations pumps. I think ill be able to develop some healthy mitigations from this experience.

It would be really cool to gather/coalesce a small list of RV-12iS flyers that have experienced the same situation with a more detailed list of events and serial numbers of aircraft affected. At the very LEAST a warning or caution added to the POH would make me feel better. Something about hot weather weather, heat soaking, and high altitude climbing ops.

Thanks again for your response Jeff!

Hi Jeff,

Debate and sharing ideas is a way to learn from each other. I have never been bothered or upset just because others have different perspectives and approaches.  Many times you get to look at an issue from a different angle you either didn't know or just forgot about.

I love to learn from others experiences and like to debate different ideas and approaches to problems. It's a good way to expand a person's knowledge base and helps understand people. Plus learning from others helps me help others later on.

Life's good if you don't fight it. :) 

Jeff Blakeslee wrote:

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.  

Jeff, can you please post how you test the RVP of fuel? (I don't see how to PM on this forum.) Thanks!

Hi Mike, 

Send me an email at Jeffb@BlakesleeElectric.com and I’ll send you complete instructions. I’d rather just send it to interested individuals than post it.  It’s safe and simple, and yields results more than accurate enough to determine if you have summer or winter fuel.