>
>Now, I have a general question regarding the Z-14 schemes. For the
>crossover switch, up is to engage the starter with both batteries,
>center is non-crossover, normal mode with batteries working to power
>their bus independently and down is both batteries working together
>on both busses......If I understand correctly. So, in the Z-14
>Fadec scheme, the start button engages the starter, then one
>position on the switch is independent alt to each bus and the other
>switch to have the systems work together? Or, is on position Off and
>the other switch position engaging both alternators to work
>together? If it is the second scenario, does that mean the two
>systems ALWAYS work together? If so, is there any reason not to
>have both systems always working together in the standard Z-14
>scheme. I assume the FADEC label is since it is always engaged
>there is no pilot intervention. Just looking for a better
>understanding. Thanks.
Two ideas drove the crafting the first iteration of of Z-14
many moons ago.
(1) For operators that plan a highly
redundant electrical and instrumentation system
for flight in marginal to adverse conditions,
two independent systems is desirable/
practical by teaming a conventional main alternator
with a vacuum pump pad driven alternator.
(2) Both systems need their own battery. With fully
redundant systems having cross-feed capability, one
may strive for smallest practical batteries with the
notion that battery-only flight is exceedingly
improbable. However, small batteries don't crank
engines well . . . so auto-crossfeed or paralleling
small batteries during engine cranking is a useful
thing to consider.
The two systems are never cross-connected for normal
operations. Each system receives the benefit of
a load analysis. Pairs of redundant electronics
are distributed between the two systems.
Some years later we were offered electro-whizzies
with desirable performance characteristics
under normal operating conditions but not designed
to live in the real-world of vehicular DC power systems.
During starter motor inrush time, system voltage can
sag sufficiently low, sufficiently long . . .
http://tinyurl.com/6tbry6
that some appliances wander off into the weeds or
take an untimely interval to reboot.
These appliances include but are not limited to
electronic ignitions, electronic fuel injection,
EFIS systems, FADECs, etc.
This prompted the addition of two Band-Aids to
the Z-figures. The FADEC version of Z-14 suggested
elimination of the auto-crossfeed feature during
cranking.
Another version of Z-13/8 was crafted to add
a brownout protection battery and auto-disconnect
relay to support e-bus loads during engine-cranking
http://tinyurl.com/7q2usj
The supposition you offered at the end of your post
is not correct. The cross-feed switch in the FADEC
version of Z-14 is never closed except to allow
ONE working alternator to support both sides of the
system (within that alternator's limitations). The
switch is left open all other times. Recalcitrant
systems are operated from the Aux Battery which is
NOT loaded during engine cranking.
Bob . . .
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( . . . a long habit of not thinking )
( a thing wrong, gives it a superficial )
( appearance of being right . . . )
( )
( -Thomas Paine 1776- )
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