Low low fuel pressure and flow at high altitude

Walt,

I always turn off my Boost pump once cruise altitude has been reached - so my observations above are for  mechanical fuel pump only.

As I said - there is more to your engine fuel starvation than altitude.

Despite RV's advice to run the Boost pump at all times, this should not be required - unless there is a feature of the RV 12 fuel plumbing that somehow restricts fuel flow. If this is the case, I would see this as a fault, try diagnose/fix that problem, rather than relying on overcome the issue,  by running the Boost all the time.

Dont get me wrong - I dont think running the Boost pump continually is a problem per se, although there is the potential for this to mask a developing fuel supply problem..

Sean, what airplane do you fly? Do you fly it on 105f days? It appears your highest temperature in Australia is in the low 80’s compared to mine in SoCal USA of 110f. That will make big difference. 

Walt,

Last aircraft ATEC Zephyr/912 ULS 900+ hrs

Current new aircraft Sonex Legacy/912 ULS 92 hrs

Where I am, just SW of Sydney, NSW, our annual day temperature ranges from about 05 C (41 F) - 47 C (116.6 F). 

I try and avoid taking off in summer temperatures much above 35 C, by starting at first light, confining operations to mornings or late afternoon.

The first time I encountered fuel vaporisation, was on a trip from home to Queensland Central Coast - One landing/TO, for bladder relief,  was at about 45 C (113 F). I was only a little over half way on the journey, so in a hurry to get airborne and back up into cool air.

On the ground the hot ambient + hot engine, vaporised the fuel above the engine. I recognised the problem immediately - Boost pump rattled like crazy and then settled down. Engine started & stopped within a second or so and then hard to start. Application of choke + Boost while cranking,  got her going. Did extended run up, in the hope of "flushing" all vapour & cooling engine bay from prop blast. Seemed good to go BUT engine died about  1/2 way on ground run. Taxied back - longer run up -  second go all good & we climbed out, backup to cool air.

The Zephyr had a very simple fuel supply system and single in fuselage fuel tank. The restrictor jet fuel return concept came after the Zephyr had been commissioned however she did have a return line with a big/heavy controllable fuel flow return system.

The Sonex has wing and single fuselage tank, a much more complicated plumbing system. Being new she has the, now standard, fuel return system .

Have not done much summer flying in the Sonex due to early engine  temperature issued - now solved (I hope)