The “Avionics Buss” is whatever you want it to be. A wire fits the bill if you want.
The old school single switch that that turned on all the vacuum tubes has no place today - other than creating a single point failure that would take down the whole panel.
Carl
Carl, you have how many switches/contactors in your dual buss/dual battery system?
SNIP
Yep - I have one “vital buss” running from the left battery to half the panel (EFIS #1, COMM #1, trim, etc.) and one “vital buss” running from the right battery to the other half (EFIS #2, COMM #2, Flaps, etc.).
I start up with just the left vital on (brings up the left EFIS and EMS). I leave the right vital off for engine start as it has the power hog GTN-650 and I do not have faith in its ability to protect against voltage droop during engine start. Engine start uses both batteries and I have not experienced any dropout of the SkyView system.
I have two Master solenoids feeding the “non-vital” buss. This is used for engine start, both alternator feeds, and loads not needed for IFR flight “landing lights, strobes and NAV lights, pitot heat, etc.).
The POH emergency procedures cover such things as when to open both masters to isolate a fault.
The system eliminates the need for any avionics backup battery.
Carl
With the usual relay protection diodes in place, there should be no problem starting the engine with the (modern) avionics turned on, and this is in fact recommended at least in the manual for my AFS 5000 series EFIS. I am told that the more significant risk to avionics comes from the surge associated with the collapsing electrical field when the master is shut off while all the avionics are energized, but even this is debatable apparently. I usually start the airplane with avionics on so that I can see oil pressure immediately. I do turn off the avionics master before shutting the engine down and turning off the master switch, Although I am not convinced if that’s necessary.
The small electrical surge from the collapsing field is generated on the activation side of the relay/solenoid, not the pass through side. The contacts are opening as the field collapses so it is only seen on the coil side. Additionally there is no winding on the current side to convert the collapsing magnetic energy into electrical energy as exists on the activation side. The diodes do not protect the current path, but instead protect the activation path. Without these, the surge can create arc'ing across the activation switch contacts, inducing premature wear and potential failure.
Larry
KISS. Master On - all electronics on. If it really makes you feel good, you can turn each one on and off individually via its *own* power switch (which is why it's there, no?).
KISS. Master On - all electronics on. If it really makes you feel good, you can turn each one on and off individually via its *own* power switch (which is why it's there, no?).
..and the ability to "back-feed" any part of the electrical architecture eliminates single point failures (like an avionics bus switch/relay). The ability to isolate buses in the event of an upset condition. Intrinsic shedding of non-essential systems with lose of alternators/generator(s). SPSTs -> SPDTs = pretty manageable. I like the idea of using high quality switches for current iso but dual/redundant system architecture lessens/removes a lot of the related criticality.
Build what you want. If there were a single perfect approach, they'd all look the same.
from MSSTAHLAppreciate your insight! Based on your feedback I think I'll move the GPS to a dedicated switch as all the other electronics are stated by the manufacturer to be OK on the main bus.
WHat's the benefit versus using teh device switch? If understanding you, you've introduced another failure point via dedicated electrical switch
Many items these days don't have on/off switches, pulling breakers, if you have them, is the only option to turn off or reboot an item.
not so sure about any GPS navigators not having on/off capabilities, though).
Garmin GTN650 does not have an on/off switch.
Huh. Didn't know that. So obviously, it's designed for sudden power-on and power-off events.