Recently I spotted a document from a well known brand of EFII recommending an Essential
Buss backed up with a Aux Battery and simple 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 think 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, 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. Lets just think of two
switches wired to power two pumps from two buses by having one buss on one
switch with both pumps, the other bus to the other switch also for both pumps.
With two 700-2-10s we have several switch 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 no big deal 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 all the elements on the cold bus are connected through
one fuse on the hot bus fuse goes pop! Among all the other 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 offs, 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 high likelhood for becoming a Greek Tragedy. Nope, lets
avoid all that.
First thought was Can I build some sort of mechanical gate that prevents the back
power combinations? 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 prohibited 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 both 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
