To bypass or not to bypass, that's the question!

Hi Mark, 

I have a low wing, with Facet boost pump. I have installed a bypass system for the pump but no additional filter. My only fuel filter is a gascolator between Facet & mechanical pump (Note: all my refueling is done through a filter funnel).

Thinking out loud:

Your high wing tanks may deliver sufficient fuel flow to the carburettors, without  the need of a boost pump ie gravity is giving you built in fuel supply redundancy. This would need to be checked before making a decision one way or the other.

Installing a boost pump has potentially added a point of failure, than can only be mitigated by having a bypass system.

Not sure, however Facet pumps are supposed to fail open (maintain flow).

Fuel flow is what the carburettors need, not fuel pressure.

I guess I am suggesting that , with reserve fuel quantity in your tank(s), you measure your gravity fuel flow at the carburettors - if sufficient to maintain engine operation, ditch the boost pump & its added complexity/potential failure point.😈

d

I  have an inline Facet pump that has worked fine for 6 years now without issue.

The facet "cube" pump has different models, these vary by flow.  The operational characteristics of this type of pump allow for flow even when off.  There is no bypass needed however some have a check valve for anti-flowback of a fuel system.  

One issue with this type of pump, commonly called a "clicker" pump by the sound it makes when running dry, is that they will pulse fuel by design.  This can in some cases lead to fuel pressure fluctuations on your fuel pressure.  Some aircraft have been known to be worse than others and this depends on the type of fuel line, size and material, as well as the length of line.  In most designs this is run constantly.  

Be aware that just because you have a high wing you can still have low pressure at your fuel pump.  This is due to the location of the pump in relation to the fuel tanks.  In a climb attitude you may have little to no gain in pressure by just gravity feed.  Again line size, restrutions and materials, can affect your pressure. 

Cheers

Thanks guys for your responses.

- relying only on gravity feed is a little outside my comfort zone, so that's why I prefer to have the elec pump.

- my fuel installation doesn't have a return line and it's not straightforward installing one. My hope with the elec pump is to have a way to push trapped air bubbles through to the carb and thus avoid vapor lock (the pump is installed in the cockpit, where temperatures are lower than under the cowling).

- I'm aware the facet pump normally fails in a way that fuel can flow through with little resistance, if any. But I'm not sure if there are other failure modes which could warrant having a bypass line. But from the feedback here, I'd err on the side of caution and install it.