That's the old pump setup for the O-540 for early Sixes. I'll bet the test aircraft was
an older model with the twin electric fuel pumps in series with the mechanical pump, and
since that was what it was tested with, that is what all planes aspiring to the STC have
to be 'upgraded' to.

wrote:

Another more subtle change for the Cherokee setup was a change from electric
boost pump being in parallel with the engine pump. Now the two electric pumps are in
parallel, but in series before the engine pump. Not the engine pump diaphram is a
single-point of failure. Also, the gascolator and cabin primer is not on the
*pressure* side of the electric pumps.... subtle changes that have significant safety
ramifications IMO.


-Cory
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