> I am not one bit convinced of the need for a spare
> battery. The plane carries on flying without a blink if you switch off
> master and alternator switches!
That's because it's wired as it's supposed to be! It's picking off
regulator current before fed to the bus, but it's not redundant. If one
has a battery relay, and the alternator/regulator fails, and the overall
problem is such the battery must be disconnected, the engine will quit.
There's no way to wire the 914 pumps to any single electrical system
which would meet current FAA rules, were this a production A/C. It's
also interesting to note that this is a relaxation of the rule prior to
1965, since way back then one pump must have been engine-driven. Now
redundant electrical pumps are permitted if there's two independent
sources of power (spelled batteries).
One partial solution is to wire one pump "off-master" - direct to
battery. Satisfactory if one will never leave the A/C with it still
switched on. Not an independent source of power, but redundant if the
rest of the electrical system has failed and battery still good.
For full compliance the FAA has approved, as a separate independent
power source, a simple pack of dated alkaline "D" cells. I believe this
will provide enough alternate power to a pump to get to some nearby
airport if the main electrical system has smoked.
Not trying to convince; just suggestions for any builder considering
options.
Fred F.
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