Low Fuel Pressure w/ Backup Pump On

Are you using the fuel distribution block, part number 851328?  That's what I was referring to.  It has a sort of pressure regulator built in.  It's item 7 on the schematic I posted previously (which IS for carburetted engines)

From the Installation Manual: The installation of a fuel return line is mandatory. If the fuel distributor piece with regulator from ROTAX® is not available, the fuel pressure must be regulated by a restriction in the fuel return line, which ensures that the fuel pressure is under all operating conditions within the operating limits specified by ROTAX®.

Well, while contemplating the cause, please plumb a bypass with one-way check valves around those facet pumps per the Rotax installation manual. There is a thread somewhere on this forum where a mechanic performing a mishap evaluation at the request of the NTSB relays the story that he found a facet cube pump had failed in the closed position- which facet advertises they should not- and blocked fuel flow through the pump. 

Hi Jim,

I am all for redundancy and your/Rotax advised plumbing of a bypass of boost pump sounds perfectly reasonable BUT we are mainly talking light aircraft here and almost by definition, simplicity of systems.

By my count, a bypass will require a check valve x 1,  a  T or Y piece connections x 2.

So we have now moved from a connection on either side of the pump (2 points of failure or 3, if you count the pump itself) to an additional 8 connections (10 in total).

If you put the pump & the valve into the sum , we now have 12 points of potential failure - a 4 fold increase in risk.

I guess it comes down to the owner/builders comfort zone -

   What is the likelihood of a, usually reliable, pump failing?

   If it does what is the likelihood of it failing closed?

   Is there other fuel (tank) available in flight?

and

   Weighing that against the additional joints/connections failing (fuel leak), potential for a one way valve (not in frequent use) sticking           open/closed/ failing anyhow.

.  

I've attached some pix of the fuel system.  Reading the part number on the wing tank facet fuel pumps, I see now that I have part 40105 which has a max psi of 3 to 4.5 psi with no check valve so that fuel can be pulled in either direction once the pump is turned off.  Here's a link to the specs for it...

https://www.pegasusautoracing.com/productdetails.asp?RecID=8440

It looks like I have Rotax part 851328 fuel distribution block as you can see in the engine compartment pix and while I couldn't actually find the part number on it anywhere, it looks like what is shown in the Rotax schematic of it.

Also, looking at the engine compartment pix, you can see 2 fuel lines going back to the firewall.  The smaller line is for the fuel pressure sender on the firewall and the other larger line is for the fuel return line which goes back to the right wing tank.

In the wing well pix you can see the single inlet and outlet lines going into and out of the Facet pump with the outlet line above the inlet line.  Any other lines you see in the wells are either the return line or pitot tube.  There is no bypass for the Facet pumps.

In the cockpit, an Andair fuel selector is used to select either left or right wing tank or off with the wing tanks joining into a single line just aft of the fuel filter which sets on the floor under the fuel selector.

All fuel lines are metal aft of the firewall and all fuel and oil lines are teflon forward of the firewall. 

What is the black cylinder, just forward of the Andair fuel selector valve? Pump/Filter??

Have you checked the operation of the Andair fuel selector valve ? Fully open left/right no partial opening/restriction.

Ideally you should have a filter on each tank - this allows for one to be blocked/restricted and the other to get you home. 

Its very common to have an earth return through the airframe however I feel that 12 V systems should always have a dedicated earth/ground return circuit for optimum performance/reliability..