If the starter contactor sticks, you have no way to shut
the starter down until the battery gives up (had one melt
down severely in a Glasair) or the starter burns out.
/NOT SURE HOW TO INTERPRET THIS. THE EXP USES THE STARTER
/INPUT TO AUTOMATICALLY SHUT DOWN THE AVIONICS BUSS DURING
/STARTS. A BIG PLUS IN MY BOOK IN THAT IT'LL MAKE MY VISITS
/TO THE RADIO SHOP FAR MORE INFREQUENT.
Actually, starter "glitches and spikes" were never really
a radio killer issue . . . low bus voltage was. I was at Cessna
in the 60's when the avionics master switch was born. Airplanes
sitting in the delivery patch would be 30-60 days before delivery.
Batteries got low and when a pilot jumped in to take delivery
of the airplane, quite often the battery would just barely
pull the engine up on compression and quit. The fragile,
germanium transistors of the time would come out of saturation
in the power supplies, go into second breakdown and voila, "my
starter just killed my radio."
It took us years later to understand the phenomenon and in the
mean time, DO-160 says you'll design a radio to take anything
an airplane will throw at it. Germanium has long since been
replaced by more rugged silicon. Radio manufacturers have become
a lot smarter. Alternator runaway overvoltage is the only
"gremlin" left to guard against.
The issue I was speaking to was the sticking of the main terminal
pathway in a starter contactor . . one of the most common occurances
in the Stancore 70 series contactors (old RBM controls devices
used in most Cessnas and offered by Aircraft Spruce). If your starter is
powered directly from the battery hot post via a single contactor,
sticking of that contactor gets you an uncontrollable starter
runaway. If you look at how certified ships have always been
wired, shutting of the battery master always shuts off the starter
too . . . irrespective of the starter contactor condition.
---------------
The self-reseting nature of poly-fuses can hide a latent failure;
you can be suffering intermittant short that you don't catch
because the poly-switch resets when the short clears. I'd prefer
to have the fuse/breaker open immediately letting me know that
something is going on that needs attention before the next flight.
/FORGET ABOUT THE ANNUNCIATOR PANEL? ALSO BREAKERS RARELY OPERATE
/AT THE SAME VALUE ( ERROR CAN EQUAL X 100s%) EACH TIME AND I REFUSE
/TO CONSIDER OR DISCUSS FUSES.
Don't understand . . most airplanes have no annunciator panel. The
latent failure is a condition that is hidden to the pilot and if
intermittant in nature, can lurk around for a long time before
it's presence is noted. If a self reseting poly-switch masks it's
existence, then many opportunities to note it and fix it are lost.
-----------
The last time I looked at an EXP-Bus and it's competitors at
OSH last year, they were both dropping wires directly to pads on
a p.c. board with no insulation support. After going to the trouble
to use PIDG terminals on the rest of your wiring, it seems prudent
that insulation support in the rest of the system would be nice.
/A BUCK OR 2 AT THE LOCAL ELECTRONICS SUPPLY WILL SOLVE THIS ONE
How? One could apply RTV around the wires where they go into the
board for additional support. . . .
--------------
One version of the EXP-Bus installation uses the terminals of the
switches to suport the etched circuit board. This adds mechanical
stresses to the switch terminals for which they were not designed.
Slight mis-alignment of the row of switches would aggravate the problem.
/AN HONEST CONCERN OF MINE. REMOTELY MOUNTED SWITCHES ELIMINATE THIS
/POTENTIAL PROBLEM.
Sure . . . so when you've dismounted the switches the number of
wire segments required to assemble the airplane is the same as if
you'd used either fuse-blocks or circuit breakers . . . therefore,
no time or effort saved as compared with fuse-blocks, it does
still eliminate the time to lay out and fabricate a breaker panel
and it saves the panel space too.
It just occured to me that EXP-Bus wiring as I perceive it currently
offers no opportunity to incorporate the a battery-maximizing
essential bus concept for dealing with alternator or battery master
relay failures.
Regards,
Bob . . .
AeroElectric Connection
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