Thought provoking post..looking forward to the responses from the brains
trust
On Sun, 21 Apr 2024, 01:20 wsimpso1, <wsimpso1@comcast.net> wrote:
>
>
> Recently I spotted a document from a well known brand of EFII recommendin
g
> an =9CEssential Buss=9D backed up with a Aux Battery and simp
le switch to
> connect it. While this is supposed to make powering the engine possible
> even if the Main Battery Bus goes cold, it also appears to make possible
> powering the cold bus with many things connected through a single or few
> fuses on the hot bus. Sounds like a recipe for blown fuses followed by
> forced landings and maybe even dark avionics to me. What do you guys thin
k
> about how to power engine buses?
>
> Then I came across some videos (IIRC, Bob Nuckolls, but I can not find
> those videos now) where two LightSpeed ignition modules were powered
> through a common connection and caused them both to go cold followed by a
> deadstick landing.
>
> With my consciousness suitably raised and supported by Bob Nuckolls
=99, I
> pored through switch and wiring schemes in an attempt at avoiding back
> powering cold buses, and even thought about mechanically gating these
> switch pairs to prevent switch setting combos we should avoid.
>
> The danger discussed by Bob (and reinforcing my recently raised
> consciousness) is that you can have one element connected to both buses.
> Let=99s just think of two switches wired to power two pumps from tw
o buses by
> having one buss on one switch with both pumps, the other bus to the other
> switch also for both pumps. With two 700-2-10=99s we have several s
witch
> states, two of which are powering the same pump from both buses. (Main
> A/Aux A or Main B/Aux B). Yeah, the pilot should remember not to double
> connect elements, but pilots ARE human.
>
> Look it over and it looks like =9Cno big deal=9D to have both
buses connected
> to one pump while both buses are hot. But we want to be failure tolerant.
..
>
> Imagine one bus goes cold =93 all the elements on the cold bus are
connected
> through one fuse on the hot bus =93 fuse goes pop! Among all the ot
her fuss
> with a cold bus, the engine is windmilling. Once the pilot gets the light
> back on, the attitude and navigation gadgets rebooted, our pilot sees the
> fuel pressure alarm and wants to restore pressure to the injectors. Yes,
> the switches for the EFI pump will get exercised, looking for a setting
> that restores fuel pressure. Trouble is one state is off, another state
> will blow the other fuse, more states are now unintentional =98off
=9Ds, and
> only one state will run the airplane. Remember to fly the airplane while
> getting all of this sorted. This difficult human factors situation has hi
gh
> likelhood for becoming a =9CGreek Tragedy=9D. Nope, let
=99s avoid all that.
>
> First thought was =9CCan I build some sort of mechanical gate that
prevents
> the back power combinations?=9D Best scheme I came up with is kind
of
> inelegant with a little plate with notches for switch handles on a pivot
> attached between two switch handles. Try to move one handle to a prohibit
ed
> combo and it drives the other switch to an allowed position. Not sure I
> want to do that five places in my airplane, but it could work.
>
> Another scheme is to run only one element (like one pump) per switch and
> select from buses on each switch. No back powering, but we do have some
> reduced redundancy at lower probability orders. So, how do we make our
> decision on just how to build?
>
> And are there other schemes that could work for this scenario, access bot
h
> buses and both pumps or other elements, and keep us from having a
> windmilling prop?
>
> More in the next post.
>
> Billski
>
>
> Read this topic online here:
>
> http://forums.matronics.com/viewtopic.php?p=513480#513480
>
>
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