Fred, I accept all that, but can't quite visualise what sort of lesion could
wipe out the alternator and battery simultanoeusly, particularly if the
alternator is wired into the system through a fuse or contact breaker. This
seems effectively to give two independent systems with one pump running off
battery even if alternator switch off and the other running off alternator
even if master and alternator switches both off. I don't incidentally have a
battery relay.
It is entirely reasonable to allow for one unlikely event , but the
odds of two unlikely independent events happening simultaneously are such
that I am happy to use gliding for the back up mode. But perhaps I am
missing something and displaying my ignorance! David Joyce----- Original
Message -----
From: Fred Fillinger <fillinger@ameritech.net>
Subject: Re: Europa-List: Batteries
<fillinger@ameritech.net>
>
> > 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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