Do you have a number that represents a minimum head pressure to prevent vapour lock, some suggestions for a filter and the pressure altitude I should limit my flying to

 

Ian;

 

I know you asked this of the “other” Al, but here’s my $.02 worth.

 

We know that there are fractions in auto fuel that have (partial) vapor pressures at ambient conditions that are ½ - 2/3 of atmospheric (absolute).  This suggests that we don’t have to draw the pressure down very far below atmospheric (negative gauge pressure, psig) for incipient bubble formation. And as temp goes up, or our altitude goes up, the margin decreases.  So reiterating what Leon pointed out, we should have a system which maintains some positive gauge pressure at the pump inlet; enough so even at altitude we have a something around zero guage or better.

 

Remember, we are talking ‘at the pump inlet’.  If you have some line, and/or some filtering before the pump, that loss has to be accounted for.  I have about 18” of level 3/8 line from the bottom of my sump tank to the pump, so I want a minimum of about 1 ft head of fuel to the pump inlet (that’s only about 0.5 psig) when I’m down to that last gallon of gas (which should never happen except when I’m testing on the ground).  I’ll have over 2 ft of head with full tanks.  Generally something less than could be OK, but we want margin for those hot days, time on the taxiway; whatever.

 

BTW; speaking of the other Al, Al Wicks, I would only comment that statistical failure analysis requires statistically significant numbers of cases, which for much of what we are doing doesn’t exist.  But there were good points buried in there about determining root causes; and measuring and/or calculating whatever unproven aspects we can to verify function PRIOR to taking to the air.  It was disappointing to see rude and even vulgar remarks here.  We can look for what can be of value and either ignore, our simple say we disagree and why, for stuff we don’t agree with.

 

Al G.