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Vapor Lock at FL82

C

constantin_w

Member
Hi guys,

today I went to FL82 for a while and had to drop to somewhere about FL65. As soon as the descent started, the fuel pressure sensor started screaming and showing high numbers and the engine started running rough. Originally I felt the oil may be the root cause as the oil temp didn't rise above 70°C but after descending a bit the problem disappeared immediately. In the end, I think I experienced vapor lock (as I know the symptoms from the summer time near the ground). I own a 2013 RV12 with the 912 ULS engine and I use Mogas .
I am a bit surprised that FL82 does already trigger vapor lock. Did anyone of you experience similar issues at that altitude?

BR
Constantin
 
constantin_w said:
Hi guys,

today I went to FL82 for a while and had to drop to somewhere about FL65. As soon as the descent started, the fuel pressure sensor started screaming and showing high numbers and the engine started running rough. Originally I felt the oil may be the root cause as the oil temp didn't rise above 70°C but after descending a bit the problem disappeared immediately. In the end, I think I experienced vapor lock (as I know the symptoms from the summer time near the ground). I own a 2013 RV12 with the 912 ULS engine and I use Mogas .
I am a bit surprised that FL82 does already trigger vapor lock. Did anyone of you experience similar issues at that altitude?

BR
Constantin
Click to expand...
It normal to see reduced fuel pressure during a climb at that altitude if fuel is varm at take of. This can trigger vapour lock.
Rising fuel pressure during a decent is not related to vapor lock.
Rather it is more likley the carb had problem to adjust the fuel level due to
quick power reduction.

Good luck
 
I would say the Mogas is the culprit. The vapour pressure of the Mogas is much lower than Avgas meaning that as the aircraft climbs the pressure differential keeping the fuel liquid drops, usually not a problem with Avgas but with Mogas who knows?
 
Some places in Europe Flight Levels start as low as 4000’ MSL. A big gotcha for US based pilots not doing their homework.
 
constantin_w said:
Hi guys,

today I went to FL82 for a while and had to drop to somewhere about FL65. As soon as the descent started, the fuel pressure sensor started screaming and showing high numbers and the engine started running rough. Originally I felt the oil may be the root cause as the oil temp didn't rise above 70°C but after descending a bit the problem disappeared immediately. In the end, I think I experienced vapor lock (as I know the symptoms from the summer time near the ground). I own a 2013 RV12 with the 912 ULS engine and I use Mogas .
I am a bit surprised that FL82 does already trigger vapor lock. Did anyone of you experience similar issues at that altitude?

BR
Constantin
Click to expand...

Read this:

SL-00078 REV 1 - Van's Aircraft Total Performance RV Kit Planes

Discusses risks of vapor lock in high operating temperature environments and when using non-aviation fuels, as well as some information related to prevention and recognition of potential signs of vapor lock. Rev 1 updates scope of applicability to all RV aircraft running MOGAS.
www.vansaircraft.com www.vansaircraft.com

Also make sure the following items have been addressed:

SD-00017 - Van's Aircraft Total Performance RV Kit Planes

Fuel return line test – verify fuel return line is not blocked
www.vansaircraft.com www.vansaircraft.com

N 18-07-12 - Van's Aircraft Total Performance RV Kit Planes

Change to higher pressure rating aux. fuel pump
www.vansaircraft.com www.vansaircraft.com


Also the MM ch 13 specifies all the fuel system annual mx requirements. I usually replace the gascolator screen annually and also the o ring. Flushing the fuel pressure hose sometimes is surprising the amount of dirt that you see in the glass bottle even if everything else is clean. (13-9)


Couple of other things:

1. Engine driven fuel pump has a 5 year life span replacement like other rubber parts
2. Carb rebuilds after 200 hrs/ 4 years should be part of the ELSA routine as well IMHO
 
Last edited:
Does your country change to a more volatile gasoline blend in the winter? Any chance "they" went to winter blend early in the year?

Once you are heat soaked, reducing power to descend lowers fuel flow, which can give a boil until the engine cools.

FL65 is perfect for vapor lock, I long ago found.
 
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