nuckolls.bob(at)aeroelect
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 Posted: Fri Feb 19, 2010 10:40 am Post subject: Failure modes with Z12 (ignore earlier) |
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I'm eagerly awaiting system restore disks for my computer . . . seems
it's been invested with little gremlins that are not virii/malware
but instead associated with every software designer's desire to link
his/her application with everyone else. After 5-7 years of tug-a-war
between all those applications . . . their little squabbles are now
spilling out of the play-pen. Makes the computer do silly things like
"send now" before I've even edited the message!
. . . ain't this 'lectronix stuff wunderful . . .
At 10:44 AM 2/19/2010, you wrote:
> "I am still vacillating between Z13/8 and Z12."
If you are undecided, go with Z-13/8. It is simpler, costs less, and
weighs less
than Z-14 and is more reliable than most factory-built systems.
Agreed . . . keeping in mind that when airplanes come
down or even get into trouble with root causes in electrical
systems, somebody had to do something really . . . uh . . .
what's the PC term . . . inappropriate? Stacking
backup on top of backup without a carefully considered
FMEA (failure mode effects analysis) backed up with
equally well considered Plan-B is fraught with
risk.
An incident I'm working on right now involves seemingly
cool approaches to redundancy that set the stage for
single-points of failure for power to both ignition
systems in spite of TWO properly working alternators
and TWO charged batteries.
You are concerned about a battery contactor failure. How will you
recognize that a battery contactor failure has occurred?
Excellent question that drove the features of our 9011
multi-channel OV/LV monitoring device. A channel dedicated
to monitoring battery voltage will flag a battery contactor
that has opened after pre-flight . . .
There might not be any symptoms because the alternator would probably
keep working. It would be necessary to monitor the battery voltage
to recognize a problem. Even with a failed battery contactor,
voltage on the main power distribution bus will be normal as long as
the alternator keeps working. The low voltage warning light on the
regulator does not warn of low battery voltage due to contactor failure.
Correct.
If using Z-13/8, turning on the E-Bus Alternate Feed Switch will
provide a parallel path around the contactor from the main alternator
to the battery. Although voltage will be dropped across the diode
and current is limited by the 15 amp fuse, it should be adequate for
a fuel pump and ignition. In addition, the aux alternator is
connected to the battery side of the main contactor. Multiple
failures would have to occur before power is lost to critical engine
ignition and fuel systems that are connected to the battery. You
might consider connecting one ignition and fuel pump to the battery
bus and the other set to the E-Bus.
Yes!
Plans-B should also consider shutting down one of two ignition systems
during the endurance phase of flight at altitude. Keep in mind that
there are many, Many, MANY OBAM aircraft flying with one electronic
ignition and one mag. For all practical purposes the airplane flies
on a single ignition. 95% of all performance gains come when you
add the first electronic ignition . . . the mag is coasting. The second
electronic system is untaxed redundancy that consumes power from your
limited resources in the endurance mode of flight.
Doing so insures that one set will keep working in the event of a bad
electrical connection.
What do you think Bob N.?
Excellent example of the FMEA thought processes.
In the accident case I cited, simple direct feeds
to EACH ignition from SEPARATE batteries would have
offered a near bullet-proof hedge against risks.
In a Z-13 or Z-12 system, feeding one ignition from
the battery bus and the other from the e-bus is
a good move. Primary and secondary fuel delivery
systems should be spread around too.
There's no single-golden-righteous recipe for
success here. There are no two OBAM aircraft wired
the same way. This FMEA thingy is something as
necessary as learning how to buck a good rivet.
Bob . . .
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