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Hi Shannon,
This has been an interesting thread so far. I wanted to confirm a few
numbers and see if the data holds up under scrutiny. I believe your
essential equipment list consists of the following (your brand choices may
differ):
Equipment Current Draw @ 28V
--------------- ------------------------------
Chelton EFIS 3 - 5 amps (we'll use 4.0)
Tru Trac Autopilot 2.5 amps (2 servos)
Garmin Audio Panel 2.2 amps
Garmin 430 GPS/Com 1.1 amps
Garmin XPNDR 1.0 amps
Electric AI 0.6 amps
---------------- ---------------
Total 11.4 amps
If we look at the selection of Valve Regulated Lead Acid batteries offered
by Concorde as an indication of what is available in the market, their
largest 28v battery weighs 42 lbs and has a 19 amp-hour capacity at 23c. If
we assume an end of life condition of 80% capacity compared to new, we are
down to 15.2 AH capacity. Now if we consider a cold soaked battery at
altitude, say -18c per the Concorde spec sheet, capacity has fallen to 68%
of our room temperature value or 10.3 AH.
Given the above figures, you would not have enough battery capacity to fly
even 1 hour, let alone to fuel exhaustion. Pitot heat could easily double
the current requirements in IMC, and could leave you with little time to get
back into VMC and land.
The other aspect of your system that struck me as a potential failure mode
that could leave you in dire straits is the ALT FEED system that is fed
directly from the battery via a 30 amp fuse. What happens if one of your
essential items, let's say an autopilot servo, should develop a short while
you are completing your trip after the rubber coupler for your gear driven
alternator has sheared and you are in the limp-home mode with your ALT FEED
activated? What would happen if the 30 amp fuse blew first, rather than the
circuit breaker for your auto pilot (it may only take a millisecond
difference between the two reaction times to cause this to happen)? What if
your limp-home trip took more than an hour and the main contactor opened
because there wasn't enough voltage across the coil to keep the main
contactor closed?
While I agree that your electrical system is likely better than 99% of the
GA aircraft in the sky, there is still a need for sound decision making and
judgment as Jeff suggests. Most accidents are the result of a sequence of
small events that, individually, are seemingly innocuous, but when stacked
together, lead to an emergency situation. For example, to continue in IMC
after an isolated charging system failure would probably be imprudent,
whereas the same failure in VMC may be manageable. It's not the first
failure that usually causes the accident, but how the pilot manages the
situation and decreases his risk and or workload such that he can handle a
second failure and avoid an emergency situation (OK, I'm off my sop box
now).
Food for thought. I'm sure there are plenty of issues and details that may
mitigate the impact of the scenarios above, and I would honestly like you to
hear counterpoints.
Sometimes I wish someone could just design a failure proof electrical system
and publish the schematics so we could simply implement the same design in
our own aircraft and not have to go through all of this brain damage. I
guess there is still plenty of room for innovation in the real world. Keep
up the good work.
Best Regards,
Mike Hutchins
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