Al Gietzen wrote:

As I puzzled over the issues of vacuum system, backup vacuum source, backup
up electric power; it occured to me that, while a combination of vacuum and
electric gyros is standard aircraft fare; it may not apply to us.  The
vacuum system is for engines with magnetos and standard electrical systems;
which, in the era of the old aircraft engine, made sense.  We don't have
mags, and we must have electric power to fly.  So we design our electrical
systems to be fail-safe (as possible); with backup.  Of course, if we lose
electric, we lose our motive power, and, we lose vacuum.

According to sources I've checked, the reliability of electric gyros is much
higher than vacuum driven.  Mean times to failure can be more than 5 times
higher than vacuum driven.  So other than saving some money up front (which
you will pay later in service and replacement) why put in a vacuum gyro, and
vacuum system with all its pump, regulators, expensive tubing, etc.?

So you might consider a backup alternator instead of a pump; but I don't
think that is necessary either.  Design your dual battery system properly,
with isolation and essentials buss, and you should be OK.  Alternator
fails - you have 2 batteries to run on which will give you time to choose
from a dozen airports (just illustratively speaking) at which to land.
Massive failure of main power which takes out alternator and main battery,
you have 1/2 dozen airports from which to choose.

 If you wonder about drawdown time of a good battery; go turn on the
headlights and interior lights of your car and see how long it takes for the
voltage to drop to about 8 1/2 volts.  If you shed non-essential loads when
you have a failure, and assuming EFI/IG, I'm guessing that you would be able
to fly about twice that long.

Anything wrong with my logic?

Al Gietzen

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