Shannon (who is spearheading a very informative discussion -- thanks!) states
the following about his essential bus which I find rather surprising....

<> and then <<(ie flaps, lights, hyd pump which would seem to have this
resistive failure mode, but wouldn’t reside on the Ess bus)>>

I'm curious as to why an EFIS, a Garmin 430, a backup attitude indicator, and
autopilot and a transponder would all be on an essential bus!  Second, why
wouldn't the landing gear be on there?

OK, it's clear that Shannon is using his essential bus as a simple (one
switch?) means of load shedding rather than a true "emergency" bus.  And that's
fine.  The architecture he suggests gives as much power supply reliability in
this backup mode as an average spam can has in it's primary mode.  (In fact,
that's the whole point he's trying to make.)  The only remaining issues are
whether or not anything else is also broken and this is why Jeff and Hamid are
throwing darts  -- with Hamid's being a little less sharp and pointy.  Personally,
I'm looking forward to another round on this topic in hopes of learning even
more than I have already.

However, if this were truly an "emergency" bus I would submit that you don't
need an autopilot, an EFIS or even a transponder.  Hell, I'd rather have the
landing gear than a transponder -- the transponder may keep ATC's blood
pressure down but I'd be more worried about my own.

At work (Aviation Technology Group -- we're working on the Javelin jet which
will be FAR Part 23 certified) we're arguing about these very issues this
month.  We have to live by FAR requirements which basically require a 30 minute
"aviate, navigate, communicate" capability in the event of a generator failure.  
However, we are also interested in an above-and-beyond requirement which
allows us to shut down all of the electrons in the aircraft in the event of an
electrical system fire.  We would then operate on a separate mini-battery
powering a standby attitude indicator ONLY.  This allows up to keep the "canopy side
up" if this happens in IFR.  (The FAR's only require that you be able to
operate VFR after an electrical fire.)  Each of our two PFD's would operate on
separate battery busses and we'd hope to be able to revive one of the busses
without the fire restarting.  With one PFD operating we get "navigate, communicate"
function restored.  We haven't come to any final decisions yet but you can
imagine why I find this entire thread so interesting.  (Actually the Dynon EFIS
would be perfect for this appliaction if it were certified....)

- Rob Wolf
LNC2 51%
Javelin 0%