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GPS Navigator on Main Bus

msstahl

Active Member
The Garmin instillation manual states power to their GPS 175 should be routed through the Avionics Bus.

Has anybody powered their GPS through the main bus and if so any issues?
 
I think the concern is that the main bus in most airplanes sees a significant voltage drop when the starter is engaged, which is generally considered bad for sensitive electronic equipment. Modern electronics may be much less susceptible to damage or rebooting when the voltage gets very low, but it's still a risk I wouldn't want to take every single time I started my airplane, especially on a multi-AMU GPS unit that has no backup battery that could act as a buffer.
 
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
 
I have been told by many knowledgeable people that they can handle the voltage drops/ weird power on startup. I always start with some of my avionics on such as the HDX. However, on my navigator I just can't bring myself to do it. I keep it off with it's own switch and turn it on after running. More usage of the on/off switch, yes. Only downside it is if the switch ever needs fixed...

No avionics master switch for me.
 
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?

My personal opinion is there is no reason not to have an avionics buss/busses, I like having control of loads and when things will get power. Starters/contactors/alternators can all pollute the electrical system, there is no need to have avionics on when starting (starters typically draw hundreds of amps). If I want it on during/before starting it has a backup battery.

However, I prefer to use very high quality switches for buss control rather than a bunch of relays.
 
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
 
Appreciate 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.
 
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

..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
 
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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.
 
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
 
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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

I've had the B&C OV crowbar trip when the starter contactor was de-energized with no flyback diode, the resulting spike was enough to trip it so obviously that spike is on the buss.
 
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No avionics master. Main buss and E-buss per Knuckolls' diagrams, but the takeaway is: everything goes on when the Master switch is thrown, and stays on until the Master is off. GNS-430W, 2nd radio, Dynon EFIS, D6, audio panel, everything out on the networks and such (XPDR, ADS-B, A/P servos, whatever). I need the EFIS up during start to monitor engine parameters, anyway.

Do some things drop off-line and reboot when the engine is started? Yup. Any harm? Nope. These are modern electronics, if they can't handle sudden loss of power, reassertion of power, and "ratty" power inputs, then they have no business on the market these days.

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?).

This is what I do. GPS175 has never rebooted during engine start. I’m using an EXT900.
 
IIRC, the main reason for an avionics master was the old germanium transistors, which could be tender. No longer applicable.

IIRC...
 
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?).

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.
 
..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

I hadn't considered using the power button on the navigator but it does offer a good solution with no extra complexity.
 
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.

I'm not sure what your point is, nor how it changes the discussion of GPS on a main buss, with or without an avionics master. I get that there are plenty of devices that don't have a switch (remote XPDR, e.g., or A/P servos, or other peripherals; not so sure about any GPS navigators not having on/off capabilities, though).
 
My GTN 650 has it’s own separate panel toggle switch as does all my other avionics.

I built my plane partly because I enjoy fiddling around with tools, gadgets, knobs and switches etc. Where is the fun in flicking on just one avionics master? Much more enjoyable flicking on all my 7 avionics switches.

Some will say each switch is a potential point of failure. No failed switch on my panel would cause a disastrous failure. In 14 years and 1250 hours I have never had any of the 28 toggle switches on my panel fail.

Fin. 9A
 
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Huh. Didn't know that. So obviously, it's designed for sudden power-on and power-off events.

Or it’s designed to be on an avionics buss :eek:

FWIW; below is from the Garmin STC install manual (I have this bad habit of trying to comply with manufacturers installation instructions even if not technically required):
This also might make you wonder why modern glass paneled certified aircraft still have avionics busses if they are really not needed?

2.5.7.2 Power Distribution – Single GTN, Aircraft Weight Less than 6000 Pounds
When installing a single GTN in a VFR only aircraft OR an IFR aircraft with a maximum gross takeoff
weight of less than 6000 pounds, the GTN should be connected to the avionics bus. The NAV/GPS and
COM circuit breakers (refer to figure E-4) must be connected to the same avionics bus.
When the GTN is the second NAV/COM unit being installed in the aircraft, the GTN should be connected
to the avionics bus. The NAV/GPS and COM circuit breakers (refer to figure E-4 and figure E-42) must be
connected to the same avionics bus. The GTN and other NAV/COM must be grounded at separate ground
terminal/stud locations on the aircraft. The power and ground wiring for the GTN should be routed
separately from the power and ground wiring for the other NAV/COM. This method, shown in figure 2-7,
will maximize system redundancy if the ground connection for one radio fails.
 
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