Aux Bus

[A64Pilot]

I have a 50 amp CB on the breaker panel marked AUX Bus, I woud have expected a marked line on the panel where everything on the other side of the line would be the Aux Bus.

‘I don’t have that, so what does the Aux Bus power on an 81 J model?

[N231BN]

It runs the line of CB switches on the bottom of the pilot's panel.

[PT20J]

Powers the bus for the circuit breaker switches on the pilot’s panel and the radio bus relay. Note that the radio relay is normally closed and the radio master turns the avionics OFF by energizing it. Thus if you pull the Aux bus breaker with the battery master on and the radio master off, the radios will power up. 

Skip

[A64Pilot]

2 hours ago, N231BN said:

It runs the line of CB switches on the bottom of the pilot's panel.

So I wonder why there isn’t a line that says Aux Bus?

[A64Pilot]

2 hours ago, PT20J said:

Powers the bus for the circuit breaker switches on the pilot’s panel and the radio bus relay. Note that the radio relay is normally closed and the radio master turns the avionics OFF by energizing it. Thus if you pull the Aux bus breaker with the battery master on and the radio master off, the radios will power up. 

Skip

That’s interesting and that is an excellent example of why I’m not a electrician, I guess it’s a normally closed relay as a fail safe.

‘So when you say powers the CB’s on the pilots panel, you mean all of them? Making it I guess a master breaker? So the whole panel is the Aux bus? Makes y9u wonder what the primary bus is?

I guess I just need to pull it and see what happens

[A64Pilot]

OK, so I pulled it. I lost the clock and anything powered off of the rocker switches, fuel pump, strobes, position lights, landing light, electric trim and of course pitot heat.

I did not lose any avionics, but as PT20J said, I lost the ability to turn them off, the avionics master switch became non functional. Flaps worked too. instrument and panel lights worked.

I didn’t try gear of course and the autopilot seemed to be functional but I didn’t try it, if nothing else the electric elevator trim makes me think that axis at least wouldn’t work.

‘The cigar lighter receptacle remains powered too.

At first glance it seems you lose only the rocker switches, I didn’t try the starter.

So I guess the Aux Bus is the rocker switches, and in my airplane the clock? 

Edited May 10, 2021 by A64Pilot

[Yetti]

The Aux bus is over on the pilots side the big switches. "Bottom of Flight panel" Schematics are your friend.

 

http://mooney.free.fr/Manuels M20J/M20J/Mooney Service Manuel M20J Vol. 2 of 2.pdf

 

[201Mooniac]

1 hour ago, A64Pilot said:

So I guess the Aux Bus is the rocker switches, and in my airplane the clock? 

Yes, those rocker switches are circuit breaker switches all powered from a common bus bar fed from that Aux CB

[PT20J]

In the M20J there are three buses:

1 The main bus is powered up when you turn on the master switch.

2. The aux bus taps off the main bus and powers the rocker switches which are also circuit breakers.

3. The radio bus is powered off the main bus through the radio master relay (note that the radio master relay is energized off the aux bus by way of the radio master switch, but the radio bus power comes off the main bus).

Skip

[Ragsf15e]

On older models the gear is powered off the aux bus too.  Not sure about the J.  That one breaker is important if your avionics master isn’t wired like indicated above.  Good to know what happens if it’s pulled.

[carusoam]

That kind of leaves one more question...
 

At what point does this particular Aux Bus switch have value?

 

In modern electrical panels...

During an electrical power emergency...

Shedding load is accomplished by shutting down everything not on the essential bus...

 

In the Middle Ages...

We have two busses... the main and the avionics...

The avionics is kept separate to avoid sending start-up voltage spikes through the avionics...

The avionics is wired in a semi-failure proof method...

European FAA deemed adding another switch to improve on the failure of the relay mode...

 

In the dark ages...

We had only one bus...  it was on or off with the master...

Technically there were two separate bus bars... that could easily be powered independently...

They were split up by left side and right side...

 

Avionics are getting stronger.  The newer avionics are built to handle the spikes that can occur... and come on with the master...

 

PP thoughts... regarding the affects of time on Mooney electrical system designs...

Best regards,

-a-

[N231BN]

It's really not that complicated, the CB is there to protect the two 10-gauge wires running to the CB switches as well as the copper bar that connects them.

[A64Pilot]

It’s nice to know two things, what a CB powers so when one day it trips and won’t reset, you know what you lose.

Secondly if there are any hidden functions, I hate those.

For instance the AH-64A transmission temp transducers were powered off of the engine start CB, so a shorted temp transducer would pop the CB and you couldn’t start the aircraft. Found that out the hard way

[Marauder]

What I found concerning was the number of factor installed inline fuses under the panel. Kind of inconvenient if one of those popped. During my last avionics upgrade, I had them moved to breakers.




Sent from my iPhone using Tapatalk Pro

[A64Pilot]

Yeah, I’m not wild about those in-line fuses either, worse I don’t now what they fuse

[PT20J]

1 hour ago, Marauder said:

What I found concerning was the number of factor installed inline fuses under the panel. Kind of inconvenient if one of those popped. During my last avionics upgrade, I had them moved to breakers.

What did they protect?

[Marauder]

29 minutes ago, PT20J said:

What did they protect?

A bunch of different things. And what was strange was some already had a panel mounted breaker. The clock, the EGT gauge, overhead light all had their own inline fuse but no breaker. Some like the panel lights, radio lights had an inline fuse and a bigger breaker on the panel (I think this may have been done because there were other things connected to that 10 amp panel breaker - like the annunciation panel). And then there were a bunch of inline fuses that were added for other stuff like the intercom, the FP-5, the GEM. I can't remember the exact number but it was somewhere near a dozen of them.

[GJG1MU]

Boost pump went out on my 94 M20J MSE. Had a JPI 730, Garmin 430 and a couple of other small upgrades installed a few years back.
Pulled the Aux CB to remove power to the pump and all my avionics came on with all the rocker switches in the off position. Scratching our heads...

[A64Pilot]

10 minutes ago, GJG1MU said:

Boost pump went out on my 94 M20J MSE. Had a JPI 730, Garmin 430 and a couple of other small upgrades installed a few years back.
Pulled the Aux CB to remove power to the pump and all my avionics came on with all the rocker switches in the off position. Scratching our heads...

PT20J explains that in the third post, I wouldn’t have expected it either, my assumption is it’s fail safe, break a wire etc to lose power and the instruments are fail safe to on.

Edited May 11, 2021 by A64Pilot

[A64Pilot]

Speaking of hidden functions, I went to the (D) Longbow maintenance school years ago, with I guess 10 years or so experience of maintaining A models, I had come to appreciate it was an electronic aircraft, and the more familiar you were with the schematics the better.

‘Well I found in several schematics that there was an MIK switch that had to be closed or a whole lot of stuff wouldn’t work, apparently this MIK was quite important. Now the Apache had many, many acronyms but MIK was new, and I didn’t find it in the list of abbreviations, so I started asking the instructors and no one knew, finally one of the Boeing tech reps smiled when I asked what it was. Each company of course has its own acronyms and names for things, Bell had coolers and fuel controls, Mcdonald Douglas called them heat exchangers and HMU’s for Hydro Mechanical Units.

Anyway for Boeing the MIK stands for “Master Ignition Key” so I learned that if you don’t turn the key on, a whole lot of stuff doesn’t work.

Edited May 11, 2021 by A64Pilot

[PT20J]

A museum I volunteer at has a DC-3. It has a storied history -- flew the hump during WWII. Spent time as a Chinese airliner after the war. Was spirited out of China on the deck of a ship after the Communists took over and converted to a corporate plane. I was mostly helping the mechanic, but I did have DC-3 type rating I'd gotten years ago for fun. They were short of pilots and so I was invited to check out in it. The panel had a cluster of warning lights that lit up with a test button -- much like Mooneys. One was labeled Kamborian. I asked everyone what the heck a Kamborian was. No one knew and everyone just ignored it. I did some research and found that the plane was once the corporate airplane for a company that made shoemaking equipment. The president's name was Kamborian. It was a disused call light for when the boss wanted something.

Skip

[carusoam]

Welcome aboard GJ!

You have stumbled upon the side affects of having an avionics bus...  that is powered with a relay that is failure proof...

It it’s wired in conjunction with the master relay...

 

So... if the avionics relay fails to get power... it operates as a normally closed relay... and closes... thus powering all of your avionics...

There are a few pages written about how this works, why it works, and why Europe uses an additional switch to make the system work a hair better...

Best regards,

-a- 

[GJG1MU]

Thank you guys! I’m going by what my confused mechanic is telling me, so I’m going to go check it out for myself tomorrow.

[Ragsf15e]

22 minutes ago, GJG1MU said:

Thank you guys! I’m going by what my confused mechanic is telling me, so I’m going to go check it out for myself tomorrow.

As @PT20J said, if the aux bus loses power (ie CB is pulled or fails), the “newer” models have the avionics master set up so that the failure will cause the avionics bus to be powered.  It’s like the avionics master switch (in the off position) keeps the avionics bus off when the switch has power.

Now my older F doesn’t work that way and it sort of scares me because that aux bus breaker can take out lots of stuff, including all the avionics.

[Yetti]

Some of the inline fuses in a F of the 1975 version.

There are two on either side of the Ammeter Shunt.   They made a nice place to solder in the Amp meter voltage gauge for the Skyview.

There are three on the transistor plate that has the transistors for the panel lights.   Panel lights also has two CB over there on the CB panel

There is one in the tail by the battery.   I think it is for the External power plug because it is up around there.

With the old avionics I think the Mooney supplied Com relay was undersized and would burn out.   One enterprising AP "fixed the relay.   And yes the electrical tape was falling off exposing bare wires.