Air/Vapor Lock


Dear mark
I am Nico van Dijk, living in the south of     Spain and owner of a stol avid flyer EC-YFP. Engine Rotax 582
After flying approx 25 hours total in this     plane, I had a engine failure at take-off. I immedeately did cut the engine     power and could brake in time. The engine remained running. After checking     about everything, I could not find any problem. In order to be more sure,     took the carburetters apart and cleaned them. Nothing found, no water,     everything very nice and clean. Checked for water in the fuel again,     nothing. Checked the free flow at the position of the fuelpump, approx 0.7     Litres/minute, ok.
So I mounted a fuel pressure meter and a     fuel level alam at the header tank and tested the engine at ground, all     OK. I did take-off again, EG temp ok, power ok worked very fine. A week     later I flew at 5400 RPM and had en other engine failure in air. Did cut the     engine wich remaied running at 2000 RPM and looked for a place to land.     While doing that, I saw the fuel pressur rising to normal, opened the engine     slowly and could return to the airfield.
There I tested everything again and found the     problem, air or vapor lock.
From the wing tank I had a small fuel     valve from Aircraft Spruce number 05-0104x mounted in the line going to the     header tank. There is an other of such fuel valves for panel mounting in the     panel in order to prevent that the carb’s flooding when parked for a long     time.
During a test run, I saw a vapor or air bubble     underneath the fuelvalve between the wing tank and the header tank. This     bubble did not pass the fuelvalve and did completely shut-off the fuel flow.     After a few minutes, the fuel level in the header tank lowered and my alarm     went off. I knocked the airbubble wich bit by bit started passing the fuel     valve. As soon as it was gone, the fuel started flowing and the level in the     header tank was restored.
I checked the main tank end the fuelvalve for     debris, were perfectly clean. So I took the fuelvalve apart and… surprise.     The connections are 5 mm interior, but the valve has only a throughput hole     of 3.15 mm (half the expected flow!), leading to the vapor lock problem. The     bubble and the fuel can’t pass each other in this small hole. I mounted a     fuelvalve with a througput hole of 6.5 mm and the problem was gone.    
But there is an other such valve in my panel.     So also vulnerable to Air / vapor lock. In order to prevent that I am going     to mount a Facet pump (low pressure!!) near the output of the header tank so     the rest of the fuel system is a little pressurized.
I contacted Aircraft spruce about this. I     explained, but no results yet.
I think that the fuelvalve should have     throughput hole the same size as you can see at the connections. This     situation is dangeroeus and a vapor or air lock can happen any time. When     selling fuelvalves, the seller should advertise the throughput     hole!!!
Kind regards
Nico

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