Should electric fuel pump be ON at high altitudes?

I cannot imagine there would be a bubble of AIR – air should never be able to enter the fuel lines.

That can easily happen if you happen to run a tank dry, as we’ve discussed on the other thread.

(I hope I have not only stated the obvious, but I can’t see anything more to say about it)

Not everything is obvious to anybody, so it is always worth mentioning! There is another safe alternative to your continuously rising fuel line and that is a loop with a return line to the tank. This is installed with (almost?) every fuel injected engine (piston, turboprop, jet) where more fuel is pumped to the fuel control unit than the engine actually needs. The excess fuel flows back to the tank(s). With this type of installation, every kind of vapor or air lock can quickly be purged out of the system by briefly turning on the electric fuel pumps, even if the engine is not running at all.

That can easily happen if you happen to run a tank dry, as we’ve discussed on the other thread.

Ah, true enough. I didn’t think of it because in my own craft the header tank takes care of this. Even then, grief may come of it, as I related elsewhere**.

As for the return line: Rotax now recommend this for their non-injected 912/914 in all installations, and for the popular C42 the builder (“Musterbetreuer”) insists it should be retrofitted to the whole fleet, to the dismay of the many owners/operators who never suffered from vapour locking, and must now spend a fair deal of money. Concerns were apparently raised by the increasing addition of alcohol to car fuel.

\* \* "Consequences of low levels of fuel ", #02

Vapour lock is just a simple description of this effect: Really it is cavitation in the fuel pump when high feed rates are needed at takeoff. Fact is a pump is usually not very good in sucking fuel from a tank lower than the carb and with mogas and high fuel temperature in summer at high revs you get cavitation in the pump so no more fuel can be delivered. This is critical for mogas but even avgas when temperature is warmer, which has an effect on Dampfdruck/ vapour pressure. So in these conditions you better turn on the extra electric fuel pump that should be placed best way down past the collector tank in the fuselage so the mechanical pump on the engine does not have to suck. No good practice to put the electric pump high near the engine so you´d get cavitation there instead. A return line from pump back to the tank is not required for some pumps if check valves are in this pump and it is a low pressure system as on carb engines. There you would see max. 0.5 bar for supplying the carb and no problem to have the electric pump running long time. The float in the carb has no problem with this. As was said, this problem should not exist with high wing types because the fuel pump does not have to suck – if there is a pump at all. And no, there is no such thing as a bubble somewhere in the line that might block fuel flow. It is cavitation really. Vic

Here is an AVGAS spec sheet. ASTM D910

If you look down to vapor pressure: 38kPa to 49kPa. 49kPa is FL200, 38kPa is about FL260.
All these values are for 38C.
Vapor pressure is the pressure where fuel starts to change its phase from liquid to gaseous. The boiling point.

Might be a bit different with a turbo charger. 22mins to FL200, a rubber bladder tank (more insulation) and you might be able to test how good your AVGAS is.

Mogas is different: DIN 228
DVPE: 45–60 (Summer) and 60–90 kPa (Winter)