Electrical Buses

[Mike S]

Randy, almost exactly my thoughts---------one difference, my GPS has built in batteries, so it is not on my essential.

I agree with your turning on the main for landing idea------same thought. The idea is to shut off the main batt while there is still power left in it, so you have the landing reserve. This mandates the use, and monitoring, of a charge warning system.

Mike

[grjtucson]

Gentlemen,

This is a terrific thread: very thought-provoking and useful. Based on this, my revised plan is:

Battery Buss
Battery Contactor
Cigar Lighter Socket
Ignition - E-Mag / P-Mag incl. Maint Ann Light
Any avionics "keep alive" circuits
Panel flood light

Essential Buss
AHRS / Magnetometer
Audio Panel / Intercom
Autopilot
EFIS
EIS
Elevator Trim
GPS(s)
Nav/Comm
Panel power for portable electronics
Transponder
Avionics fan / defroster
Low Voltage Warning Light (also serves as a master/e-bus switch reminder)

Main Buss
Cockpit Light(s)
Add'l Panel Light(s)
Fuel Boost Pump
Flaps
Seat Heaters
Landing Light
Taxi Light
Position Lights
Strobes
Starter & Contactor

My rationale is that I will fire up the Main Buss to land, or keep it fired up and load shed individually if needed. The E-Buss, though, is everything I need for continued comfortable flight if I'm running on battery only, and the switch makes it easy to quickly load shed.

Paul & Kevin, good catch on the starter. It does belong on the main buss, an oversight in my first go, and for the purpose of killing a runaway. The fuel pump too as it is like flaps and should only be needed at the end of the flight.

I suspect we all want a system that is as simple and robust as possible, with very logical and easy to perform tasks for as many forseeable events as possible. How we each express that is quite different and good to pay attention to.

George

[airguy]

Personally, I would put the trim, and possibly the AP on the main buss - especially if you intend to fly mainly as VFR or IFR "lite" as your primary mission. If you intend to fly regularly in IMC or night conditions, then yes I can see the desire for those on the essential buss. Portable electronics should have (fresh) internal battery backup in any case - so panel power for them comes from the main buss.

My way of thinking is for the essential buss to be only what is absolutely critical for flight - not what is comfortable or nice to have. In the event things start smoking in the cockpit, you want to be able to kill all current flow and selectively bring items back online to isolate the trouble. A very lean essential buss gives you flight-critical items without a large probability of powering up the smoke-emitting item. For me, my idea of "essential" means about 30 minutes of emergency battery backup for the "Gotta have that" instruments and I'm looking for the nearest safe landing area - you don't need a lot of instrumentation for that kind of "essential" conditions - just enough to complete a night instrument approach. If you're in VFR conditions you can kill EVERYTHING and still get down - if IMC without vacuum backup, you simply have to take the chance that your essential buss will not smoke. If it smokes, you MAY still make it down - but without it your odds are very poor indeed, as the NTSB accounts will testify to.

[f1rocket]

If it were me, I would also take the alternator warning light circuit and the avionics fan off as well. My logic for the light is that I'm probably on the E-bus because my alternator failed. I don't need to see the light continously on to remind me and I want to save the current draw.

I don't have an avionics fan even though I have a stack of Garmin radios. I'm not sure one is needed but if I had one, I'm sure my radios could survive an hour without it.

[avpro56]

Originally Posted by Kevin Horton

If the starter current does not go through the main contactor, what is the procedure to use in the unlikely event that the starter relay ever sticks closed?

I have three warning lights installed:

1) Low Voltage -Yellow
2) Fuel Pump On -Blue
3) Starter Engaged -Red

So if the start contactor sticks you'll know it.

[gmcjetpilot]

-Anything you MUST have to fly the airplane, to physically stay in the air.
-The idea is to extend the battery power by cutting out all unnecessary items, especially big drain items.

This is not an argument but a thought

-Flaps? you can land flaps up.
-Boost Pump? The engine runs fine without it.
-Transponder? Don't need to stay in the air (live near class B, C nice 2 have)
-Com? Probably a must in most builders books unless you have a handheld
-GPS? Probably a good thing and handhelds have low drain, but also can have their own battery
-Electronic Ignition? A must to stay in the air if you don't have backup (magneto)
-Glass?A big one now, since all pitot/static & mechanical gauges are all-in-one EFIS & Engine Monitors. (Can physically fly without it, but its a must for most).
-Lights? You can fly without lights in an emergency, must land, dark runway at night, Landing lights a must?

The idea is extend the battery.
What do you need to stay in the air?
Don't confuse legal & min necessary to stay in the air.
The idea is extend the battery, for that item or items you MUST HAVE!
Have you ever tested you "essential items on the ground to see how long they last on battery power?

Autoengines with electronic fuel injection and ignition are the most demanding. Since the engine is so electrically dependant you may NOT want to have the EFIS as essential if you have backup mechanical/pitot-static instruments.

I see essential busses with everything on it, why? (especially if you can individually turn off individual items).

The question is DO you need an individual essential buss at all?

Why an essential buss, just turn stuff off you don't need when you want to conserve power. If you don't want to use, lights, flaps and boost, leave them off. (Not recommending this, but I know a guy in a Cessna that flew freaking 1/2 way across the US but turning everything OFF, wet compass, map, airspeed, altitude and watch. Piper J3 has no electrical system. I think the first goal with the electrical system is make it NON-essential if at all possible.

Food for thought to simplify wiring. Our planes are mostly day VFR (most fly day 90% of the time) and we are wiring them like a B737. Nothing wrong with it, but....
Keep it light
Keep it simple
Keep it per plans (has advantages)

[jcoloccia]

I must say, I'm with George on this one. If I were setting up for VFR, I'd just as soon just hit the Master and be done with it. A big reason for the e-bus architecture is for redundancy 'cause loosing everything in the clouds is a bad place to be. For VFR, a handheld with VOR reception (like Sporty's), two eyeballs and a map (or even better, a cheapy handheld GPS) would give me all the navigation and communication I need to make any VFR flight with no pucker factor whatsoever.

[McFly]

Why an essential buss, just turn stuff off you don't need when you want to conserve power

An advantage of using an essential buss is to take the master solenoid out of the loop. If you turn everything off individually but leave the master on, you are still depleting your battery. Just how much juice does the master solenoid take, I?m not sure but 1 amp sticks in my mind.

[Mike S]

Originally Posted by f1rocket

Here's mine and the reasons:

Essential:

• GPS - Need to know where I'm going

• NAV/COM - Need to talk to people to ask for help

• TPDR - I want Center to be able to find me

• INTERCOMM - Can't hear the radios without it

• EI - Need to spin the big prop out front

• EFIS - Need to keep greasy side down

• AH - Backup instrument, same as above

• Engine Monitor - Critical fuel information contained here

EVERYTHING else goes on the primary. I don't need it for continued flight ops. When I get ready to land, I can switch the Master back on to use the fuel pump, flaps, or trim motors.

After re-reading this, and other posts here, I just came to the realization that Randy and some others (myself included) probably have dual battery setups-----or dual alternators, or both---------and others dont.

It really makes a difference in how you look at the idea of an "essential" buss if you have a second, parallel power source.

Anyway, the idea of shutting off the main battery and running on the "essential" source is something that is much more adaptable to a dual system.

Food for thought.

Mike

Big batt, little batt, 60 amp alt, 8 amp alt.

[grjtucson]

Thanks Randy, the fan is off the e-buss now and back to the main, if I even put one in (desert living means I'd like as much air movement for radios as possible).

The low-voltage warning light has been a maybe from the get go since I'll have an EIS with low-voltage warning. I like the light, though, to remind me that the master and/or the e-buss feed is on, which is why I'd put it on the e-buss.

Great stuff all! Here's some more rationale...

1. If I were going VFR only, I would likely end up with the same electrical system but less avionics.
VFR
Transponder
Garmin SL40 or similar
Panel mounted Garmin 496 or similar
EIS (GRT likely)
Steam Airspeed & Altitude
Homebuilt LRI

"Light" IFR
Transponder
Garmin SL30 or similar
Handheld VHF as backup
Panel mounted Garmin 496 or similar
EIS (GRT likely)
EFIS (dual screen GRT is frontrunner)
Marker Beacon
Homebuilt LRI

At this point, I'm not likely to go with an IFR capable gps unless GRT is shipping theirs. If they're not, I'll probably put a Garmin 81 as a second backup. I also plan on the 496 as backup to the EFIS - for the level of IFR I'm planning, I believe that to be sufficient and will train accordingly.

2. I'm likely going E-Mag/P-Mag so will have something of an out in the P-Mag for total battery and alt. failure. Not quite magneto level independence, but close.

3. Alternator (likely Plane Power int. reg. w/int. OVP) failure leaves battery only and the need, if in IMC, to shed non-essential loads as fast as practical. Radios/glass/even auto-pilot are all low enough draw that putting them on an E-Buss is fine to me, more importantly I can shed them later if I choose. I don't, however, want to lose any of them if I want to shut the main down in a hurry. I can refire the main when I need flaps, pump, and lights to land. Even battery only with no alternator should leave 1-hour plus times with the loads and battery I am looking at.

4. Taking the main contactor offline saves 1 amp, quite useful if on battery and no alternator. This alone is reason enough to me for the simple addition of the E-Buss.

5. I am either going to have a backup vacuum pad alternator from the start, or be wired for it to be added later. It simply makes the main alternator failure a non-event assuming everything on the E-Buss can be powered entirely by the 8 amp backup (reasonable based on my load analysis thus far).

6. I'll likely put a backup battery only on the EFIS. Main battery failure, while unlikely, would lead to an instant desire on my part to land.

7. Smoke in VMC: power it all down right now. Smoke in IMC: power down main and see if it helps. Add or subtract items as needed/able. Hopefully not have to power down e-buss.

8. As complicated as this all sounds, the actual system will end up quite simple, the additional weight of the e-buss is negligible, same with the complexity. Dropping the e-buss would mean one less panel switch, a larger main fuse block and no e-buss fuse-block, and one less diode.

9. Unfortunately, simple, light, inexpensive, and IFR capable, don't all seem to be destined to live and play well together. I think I'm going to have to settle for relatively simple, relatively light, stupidly expensive, and IFR capable.

George