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Franklin Fuel Pumps.
I have a Pat Goodman installed fuel pump. Between the recent article by Simon Aegerter and the recent tragedy that sounds like fuel starvation, I decided to dismantle my pump to see how it really works. FYI, bypass valves are not used in diaphragm pumps because they are not needed. They are an integral part of rotary vane pumps. I have included a crude drawing of what's inside the automotive type pump.
The problem that Simon had sounds like one of the valves was defective and would only partly open. This would explain why the electric pump couldn't deliver full volume through the engine driven pump. As far as connecting the pumps in parallel, I own a Piper Cherokee that came from the factory that way and hasn't missed a beat in 30 years and 2400 hours. The electric pump has been replaced 3 times but the engine driven (AC) pump was still working fine when it was replaced with a rebuilt when the engine was rebuilt at 2135 hours.
The little information we have on the accident would lead me to believe that this was not a fuel pump failure but a fuel system failure, perhaps a clogged filter or a kinked line.