Back up fuel pump

[gizmos]

What is the thoughts on a second electric back up fuel pump for the Rotax 912uls? If so, what fuel pressure? If installed, would you use a bypass back to the tank with restriction installed? I know there were issues with some stock lumps producing to much pressure and causing leaks in the carbs.

[Dorsal]

Being a high wing craft I do not have a back-up pump.

[jiott]

No back-up pump in mine.

[Dave S]

Gizmos,


When our S7 was built, I installed a Facet electric fuel pump low under the console as a backup to the 912ULS mechanical pump. There are a confusing number of pumps available from facet via aircraft spruce, however, the one I selected had a maximum pressure of 4.5 PSI and no check valves (Facet 40105). In doing the original post build/preflight testing, this pump would raise the pressure at the carbs, with the engine running, a very small amount but well under the specification Rotax stipulates.



I understand that Rotax once had a batch of mechanical pumps on their 912XXX engines that exceeded the upper fuel pressure limit; although, that was a defective mechanical pump and has nothing to do with a properly selected backup pump.



I think there are various reasons for installing a backup electric pump, or not doing so. Based on what I know about the plane now with the 912 ULS, I think it is a toss-up, but certainly not necessary.


At Least on our plane, the gravity flow would keep the engine running without the help of the mechanical pump or a backup pump if there were no other problems as determined by flow testing.


A electric backup pump may solve some problems but it would not solve everything. In the event of a somewhat restricted fuel flow (plugged filter or similar) it would likely help but it would depend.



It might help with startup to assure the fuel system and carbs are full at that point.


It would help if the mechanical pump failed due to its internal valves messing up.


It would not help with a catastrophic failure of the mechanical diaphragm except to spill fuel past the rupture (the 5 year replacement of the mechanical fuel pump I sincerely endorse)



Considering the fact that the carbs are on top of the engine, It may be possible to get the nose up high enough to prevent gravity feeding from the main fuel tanks with a dead mechanical pump - probably some help with this if the mechanical pump didn't pump.


I use the electric backup primarily for assuring the fuel system is full to the carbs for starting on a cold winter day; however, I don't think it is needed for any normal operation of a Kitfox.

[gizmos]

Thanks for the info, who needs google when you have kf forum 🙊