Bypassing the avionics master??!

[TangoWhiskey]

In the new issue of AOPA pilot, the article "Getting to Know You" on page 103 by Bill Kight says something to this effect under point #4 (I don't have the magazine with me right now):

Learn where the avionics master switch is, and know how to bypass it if it fails.

Huh?! I sat there and thought about that for a long time.

I think the only way to "bypass" the avionics master switch (I presume he means so as to get power back to the avionics--what other reason would there be to bypass it?!) would be to use alligator clips behind the panel! Or, just continue VFR and land as soon as practicable.

Maybe he MEANT to say "how to shut down power to the avionics buss" if you get an electrical fire and the avionics master switch doesn't cut power to the avionics... then, the answer would be to pull the correct breaker.

But that's not what he asked... am I missing something? How would you bypass the avionics master switch?

Troy

 

[Len Lanetti]

Troy,

By installing a single avionics master switch a person will have created a single point of failure. In other words, the switch could fail in such a way so there is no power to the avionics. In all of the late 70's era rental aircraft I flew that had an avionics master switch there was only one such switch.

There are a number of ways to solve the issue but newer aircraft all appear to have a second switch to power essential avoinics. When checking out in these aircraft a person would be well advised to understand the systems.

Len

 

[Fast n' Furious]

Try this experiment next time your in the plane. Turn the battery on. Turn on the avionics master power. Make sure all the radios light up and are working. Turn off the avionics master power switch and find the avionics master poer circuit breaker on the aircraft CB panel. Pull the breaker. Since you have now bypassed the avionics master switch, the radios will come back to life. Magic? No, they're designed to work that way.

 

[Let'sgoflying!]

some also have an emergency bus in which one comm (and a few other things) will have power.
then there is sometimes the dispatch/clearance switch which provides power to a radio for ground use when the entire airplane is shut down.

 

[gismo]

Try this experiment next time your in the plane. Turn the battery on. Turn on the avionics master power. Make sure all the radios light up and are working. Turn off the avionics master power switch and find the avionics master poer circuit breaker on the aircraft CB panel. Pull the breaker. Since you have now bypassed the avionics master switch, the radios will come back to life. Magic? No, they're designed to work that way.

That will work on some airplanes, but many aren't wired that way. Besides in the case you mention, there is still a relay that is a single point of failure, if the contacts on that relay fail, you will have no avionics power no matter what CB you pull. Still this is a good thing to try so you can learn your electrical system better.

 

[SkyHog]

Try this experiment next time your in the plane. Turn the battery on. Turn on the avionics master power. Make sure all the radios light up and are working. Turn off the avionics master power switch and find the avionics master poer circuit breaker on the aircraft CB panel. Pull the breaker. Since you have now bypassed the avionics master switch, the radios will come back to life. Magic? No, they're designed to work that way.

You know, I had no idea that you could do that. Why isn't this published anywhere? Could be a really important piece of knowledge to have someday!

 

[wabower]

From a pilots' perspective it's important to know that some circuits fail open (switch required to close them, with most avionics masters) some fail closed. Once you know which is which, the logic is fairly straightforward. RTFM

 

[tonycondon]

RTFM

wayne, i've read a lot of manuals. maybe the manuals of airplanes i fly aren't as comprehensive as the stuff you fly. I don't recall ever seeing anything talking about the situation that John describes, where pulling the avionics breaker, with the avionics switch off, will power the avionics.

 

[wabower]

Several places to find it in the manuals, some more obvious than others. The methods include text describing the system, a check-list item describing how it works, or the wiring diagram in the electrical systems section. Some of the diagrams actually have notations of F/O "fail open" or F/C "fail closed" on the diagram, but I can't recite from memory which ones are so labeled.

This same philosophy seems to be true with other systems as well. For example, another oral question I use for King Air is "did the engineers who designed the King Air make it possible (either on purpose or by accident) for the pilot to pressurize the airplane in flight, even if the squat switch on the main gear has failed? If so, how?" The question should be a give-away to the answer, and the procedure described in the check list is the same as the failed avionics master switch. Pull a circuit breaker. (But be sure that you remember to reset before you land.)

Given the fact that most pilots (including YT) aren't necessarily electrical system wizards (partly due to our typically superficial training methods) it's not surprising they don't know about it until somebody points it out during a part 142 ground school or other such training.



wayne, i've read a lot of manuals. maybe the manuals of airplanes i fly aren't as comprehensive as the stuff you fly. I don't recall ever seeing anything talking about the situation that John describes, where pulling the avionics breaker, with the avionics switch off, will power the avionics.[/quote]

 

[gismo]

wayne, i've read a lot of manuals. maybe the manuals of airplanes i fly aren't as comprehensive as the stuff you fly. I don't recall ever seeing anything talking about the situation that John describes, where pulling the avionics breaker, with the avionics switch off, will power the avionics.

I took a look at all the airplane POHs I had lying around (75 C-172, 71 C-152, 81 C-182, 78 BE36, and 78 M20J) and not one had the avionics master wiring being discussed here. Two of the Cessnas (172&182) did have a NC avionics master relay which was activated (opened) when the starter was engaged or the GPU connection was powering the airplane but in both of those types there was no avionics master toggle switch and the breaker powering the avionics master relay (and starter contactor) was not "pullable". And in any case the only situation where removing power from the avionics master relay would help would be if the starter switch shorted out in which case you've got worse problems than no radio power to deal with (hung starter). In the Mooney the avionics master appeared to be of the normally open type although that's not clearly indicated in the POH so it could conceiveably have a NC relay where pulling the breaker would restore avionics power in the very unlikely event of a failure in the toggle switch controlling the relay (the relay itself is far more likely to fail).

So I'm left wondering exactly what airplanes have the combination of a normally closed avionics master relay controlled by a toggle switch and a circuit breaker powering that which can actually be pulled to restore power if the toggle fails shorted (relay coil powered)? Anyone have any specific examples?

And from my limited sample I must conclude that Bill Knignt's original bit of advice doesn't apply to the vast majority of airplanes flying.

 

[ScottM]

I took a look at all the airplane POHs I had lying around (75 C-172, 71 C-152, 81 C-182, 78 BE36, and 78 M20J) and not one had the avionics master wiring being discussed here. Two of the Cessnas (172&182) did have a NC avionics master relay which was activated (opened) when the starter was engaged or the GPU connection was powering the airplane but in both of those types there was no avionics master toggle switch and the breaker powering the avionics master relay (and starter contactor) was not "pullable". And in any case the only situation where removing power from the avionics master relay would help would be if the starter switch shorted out in which case you've got worse problems than no radio power to deal with (hung starter). In the Mooney the avionics master appeared to be of the normally open type although that's not clearly indicated in the POH so it could conceiveably have a NC relay where pulling the breaker would restore avionics power in the very unlikely event of a failure in the toggle switch controlling the relay (the relay itself is far more likely to fail).

So I'm left wondering exactly what airplanes have the combination of a normally closed avionics master relay controlled by a toggle switch and a circuit breaker powering that which can actually be pulled to restore power if the toggle fails shorted (relay coil powered)? Anyone have any specific examples?

And from my limited sample I must conclude that Bill Knignt's original bit of advice doesn't apply to the vast majority of airplanes flying.

I do not know if I have such a feature as is being discussed here. But I do not recall it in the newer 172's or 182s either. I read those PoH cover to cover.

I know there is nothing in my aircraft's AFM. There is a avionics master but not all the avionics are on the same breaker. Com/nav and GPS on one, ADF on another, the audio panel/intercom on a third. So what breaker would I pull if I could.

The converstation does get me thinking about how to deal with a failed avionics master. But my guess is that it would fail on the ground when turning the radios on and way less likely in the air after contact is made.

 

[gismo]

I do not know if I have such a feature as is being discussed here. But I do not recall it in the newer 172's or 182s either. I read those PoH cover to cover.

I know there is nothing in my aircraft's AFM. There is a avionics master but not all the avionics are on the same breaker. Com/nav and GPS on one, ADF on another, the audio panel/intercom on a third. So what breaker would I pull if I could.

The converstation does get me thinking about how to deal with a failed avionics master. But my guess is that it would fail on the ground when turning the radios on and way less likely in the air after contact is made.

The breaker in question will not be associated with any particular piece of avionics, it would either be labeled "avionics master" or might be for something completely unrelated. In the 172 manual I have the power for the avionics master relay comes from the nav light breaker IIRC.

You'd have to follow the wiring for the avionics master switch in the diagram which should be in section 7 of any modern POH.

As to the time of failure issue, since we're talking about just the toggle switch I would agree that the most likely time would be when the switch was being operated but a switch that was "near death" might fail in turbulence, especially if the bumps were strong enough that something loose in the cabin managed to strike the switch.

 

[ScottM]

As to the time of failure issue, since we're talking about just the toggle switch I would agree that the most likely time would be when the switch was being operated but a switch that was "near death" might fail in turbulence, especially if the bumps were strong enough that something loose in the cabin managed to strike the switch.

As you pointed out simple switches seldom just go bad. They will get more and more intermittent in their operation. If one notices that flipping the switch to on does not always get the power to the avionics and that it needs to be switched a few times then that is a good indication that switch is close to failure and should be replaced ASAP.

 

[kgruber]

I took a look at all the airplane POHs I had lying around (75 C-172, 71 C-152, 81 C-182, 78 BE36, and 78 M20J) and not one had the avionics master wiring being discussed here. Two of the Cessnas (172&182) did have a NC avionics master relay which was activated (opened) when the starter was engaged or the GPU connection was powering the airplane but in both of those types there was no avionics master toggle switch and the breaker powering the avionics master relay (and starter contactor) was not "pullable". And in any case the only situation where removing power from the avionics master relay would help would be if the starter switch shorted out in which case you've got worse problems than no radio power to deal with (hung starter). In the Mooney the avionics master appeared to be of the normally open type although that's not clearly indicated in the POH so it could conceiveably have a NC relay where pulling the breaker would restore avionics power in the very unlikely event of a failure in the toggle switch controlling the relay (the relay itself is far more likely to fail).

So I'm left wondering exactly what airplanes have the combination of a normally closed avionics master relay controlled by a toggle switch and a circuit breaker powering that which can actually be pulled to restore power if the toggle fails shorted (relay coil powered)? Anyone have any specific examples?

And from my limited sample I must conclude that Bill Knignt's original bit of advice doesn't apply to the vast majority of airplanes flying.

The M20J is for sure wired for avionics "ON" with the avionics circuit breaker pulled. Mine was. More than likely also any modern Beech.

 

[poadeleted20]

Not all avionics masters have "normally closed" relays that are powered open when you turn off the switch, and flop closed when you pull the c/b. Some are plain switches, and then the only way to back up the switch is to have a second switch in parallel. That's the situation in my Grumman, with the second switch under the panel and normally left "off" so everything goes off when the main switch is turned off.

 

[SCCutler]

Not all avionics masters have "normally closed" relays that are powered open when you turn off the switch, and flop closed when you pull the c/b. Some are plain switches, and then the only way to back up the switch is to have a second switch in parallel. That's the situation in my Grumman, with the second switch under the panel and normally left "off" so everything goes off when the main switch is turned off.

And, indeed, a great many of the planes in our fleet did not have avionics master switches at all, when they were built.

Mine has a second avionics master switch which is (I am presuming, but guess I should verify it somehow) simply in parallel with the "main" one. Funny, until I read this thread, I almost forgot that I even had the alternate avionics master switch at all... it's WAY over at the right end of the sub-panel.

I'll have to trace it out and see what's up with it one a these days.

 

[wabower]

IIRC, you have to know whether the electrical diagram is depicted with power on or power off. In the King Air manual, the avionics power relays are shown as being closed (completing the circuit) with the aircraft master switch off.

Powering the master (battery) switch opens the avionics bus relays, so most of the the airplane systems receive power, with the exception of the avionics, which can only be powered by moving the avionics master switch to the on position, which again closes the relays and completes the circuit to the avionics componenents. I just looked at the electrical diagram of a King Air B-200 manual that might be helpful for others who are looking at similar diagrams in other manuals.

This concept isn't the easiest to understand, and I'm a business major. After Lord knows how many initial and recurrent classes, however, even those in the back row remember a few things about what was taught in class.

 

[mikea]

...
So I'm left wondering exactly what airplanes have the combination of a normally closed avionics master relay controlled by a toggle switch and a circuit breaker powering that which can actually be pulled to restore power if the toggle fails shorted (relay coil powered)? Anyone have any specific examples?

And from my limited sample I must conclude that Bill Knignt's original bit of advice doesn't apply to the vast majority of airplanes flying.

Being that mine a) doesn't have pullable breakers, and b) the avionics master switch and CB was added probably more than 10 years after the plane left the factory....

 

[MauleSkinner]

The M20J is for sure wired for avionics "ON" with the avionics circuit breaker pulled. Mine was. More than likely also any modern Beech.

Bonanzas, Barons, and King Airs all had this style of avionics master, at least since the mid-1970s. I believe the avionics master switch was standard from the factory. That would probably be hint #1. I do know one avionics guy who says he's taken the NC relay out of the system on a few, because that fails more often than the switch itself.

Another hint might be whether the battery switch is supposed to be on prior to connecting external power...IIRC, Barons, Bonanzas, and King Airs have that written into the AFM, and Cessna singles don't. My understanding is so that it opens the avionics relay, isolating your radios from any voltage spikes that may come from a bad power cart.

Some are plain switches, and then the only way to back up the switch is to have a second switch in parallel. That's the situation in my Grumman, with the second switch under the panel and normally left "off" so everything goes off when the main switch is turned off.

I suspect that this is more common in glass panel airplanes than steam gage airplanes. Did your Grumman come from the factory with that switch, or was it added later?

Being that mine a) doesn't have pullable breakers, and b) the avionics master switch and CB was added probably more than 10 years after the plane left the factory....

This has been the case with most non-Beech airplanes I've flown...the avionics master is "just another switch in the panel".

 

[TangoWhiskey]

You know, I had no idea that you could do that. Why isn't this published anywhere? Could be a really important piece of knowledge to have someday!

Nick, what led you to such an old thread?

This reminds me that I never tried the suggested "bypass"; I wonder if it will work in the newer 172s / 182s I fly...

 

[poadeleted20]

I suspect that this is more common in glass panel airplanes than steam gage airplanes. Did your Grumman come from the factory with that switch, or was it added later?

Both switches were added later. The 1970's Grummans did not come from the factory with an avionics master switch.

 

[wabower]

With all that stuff in yours, it probably needs more than one switch just to get it all turned on.

Both switches were added later. The 1970's Grummans did not come from the factory with an avionics master switch.

 

[poadeleted20]

With all that stuff in yours, it probably needs more than one switch just to get it all turned on.

When you really look at it, there aren't all that many boxes in my panel -- Audio panel, GNS530, SL40, Nav 122, GTX330, and EDM-930. Folks with a typical old technology full IFR panel (dual nav/com, ADF, DME) have just about as many.

 

[brcase]

That will work on some airplanes, but many aren't wired that way. Besides in the case you mention, there is still a relay that is a single point of failure, if the contacts on that relay fail, you will have no avionics power no matter what CB you pull. Still this is a good thing to try so you can learn your electrical system better.

The single point of failure in this system assumes that there is only one relay. It seems to me that having an individual relay for each avionics box on the avionics master would be an excellent way of providing redundancy. If a relay fails you lose only one piece of avionics. If the switch fails then circut breaker will act as a backup to the switch.

 

[gismo]

The single point of failure in this system assumes that there is only one relay. It seems to me that having an individual relay for each avionics box on the avionics master would be an excellent way of providing redundancy. If a relay fails you lose only one piece of avionics. If the switch fails then circut breaker will act as a backup to the switch.

My airplane has no relays for avionics power. I do have two toggle breakers wired in parallel for redundancy as well as a direct feed switch that bypasses the avionics masters as well as the battery master to power the GPSMAPNAVCOM and the MFD so I can have something even if I have to shut the master off to squelch a fire.

 

[wabower]

Yeah, but not in a Tiger. Not many like yours.

When you really look at it, there aren't all that many boxes in my panel -- Audio panel, GNS530, SL40, Nav 122, GTX330, and EDM-930. Folks with a typical old technology full IFR panel (dual nav/com, ADF, DME) have just about as many.

 

[Cpt.B (AMT)]

How the master switched work in aircrafts are backwards then what you would think. When you turn a switch off you are really completing the circuit to keep the item off. It is not the way you would think it would be but it is for safety so that if that master switch fails it wont turn off your avionics and this is true for 99% of aircrafts flying today as long as they have the proper wiring on board. We have to remember that is avition everything have redundancy built in so that we can keep the aircraft flying even under one or two failers. As for pulling the Avionics Master B/S I am not too sure the reasoning about that but if that works too then that is something else you could do if the contacts points on the Avionics Master Relay were to fail.

 

[JHW]

How the master switched work in aircrafts are backwards then what you would think. When you turn a switch off you are really completing the circuit to keep the item off. It is not the way you would think it would be but it is for safety so that if that master switch fails it wont turn off your avionics and this is true for 99% of aircrafts flying today as long as they have the proper wiring on board. We have to remember that is avition everything have redundancy built in so that we can keep the aircraft flying even under one or two failers. As for pulling the Avionics Master B/S I am not too sure the reasoning about that but if that works too then that is something else you could do if the contacts points on the Avionics Master Relay were to fail.

Hardly. It might be true for 99% of the planes built since the 80's. I can't think of the last time I flew anything non-turbine that had an avionics relay. They mostly just have a plain old switch to power the avionics buss, and often a lockable backup switch powered direct from the battery.

 

[denverpilot]

Hardly. It might be true for 99% of the planes built since the 80's. I can't think of the last time I flew anything non-turbine that had an avionics relay. They mostly just have a plain old switch to power the avionics buss, and often a lockable backup switch powered direct from the battery.

Cessna's master closes a solenoid that powers all the busses. Current doesn't flow through the master switch itself for all of the components.

The other side closes to provide power to the voltage regulator to excite the alternator field. (Disregarding the G1000 airplanes for a moment, those are different somewhat.)

And that stupid split master is a cheap POS that only costs $14 in today's dollars. And they do fail. Ask me how I know.

Which light aircraft are you flying that's "modern" that passes the entire buss current through the master switch? I'm very skeptical.

He wasn't speaking of an avionics relay. He was speaking of a solenoid (essentially a big relay) that the master closes. Without that closed, no juice to avionics (or any other) buss in most light singles.

If the other half of the master won't close (or if the voltage regulator is fried), no alternator field current and no charging.

 

[denverpilot]

P.S. You can hear the solenoid close in most light singles during pre-flight when you turn the master on. The click comes from the solenoid mounted on the engine side of the firewall, not the master switch.

 

[Dan Thomas]

How the master switched work in aircrafts are backwards then what you would think. When you turn a switch off you are really completing the circuit to keep the item off. It is not the way you would think it would be but it is for safety so that if that master switch fails it wont turn off your avionics and this is true for 99% of aircrafts flying today as long as they have the proper wiring on board.

I think you are confusing the magneto wiring with the master circuitry. When a master switch is turned off, the contactor it controls opens and the airplane's circuitry is dead, except for the mags, which are entirely separate from the rest of the airplane's electrical system.

FAR 23.1361 requires such an arrangement. http://www.ecfr.gov/cgi-bin/text-id...view=text&node=14:1.0.1.3.10.6.101.24&idno=14

Dan

 

[JHW]

I clearly said avionics switch and avionics relay. What are you talking about?

Cessna's master closes a solenoid that powers all the busses. Current doesn't flow through the master switch itself for all of the components.

The other side closes to provide power to the voltage regulator to excite the alternator field. (Disregarding the G1000 airplanes for a moment, those are different somewhat.)

And that stupid split master is a cheap POS that only costs $14 in today's dollars. And they do fail. Ask me how I know.

Which light aircraft are you flying that's "modern" that passes the entire buss current through the master switch? I'm very skeptical.

He wasn't speaking of an avionics relay. He was speaking of a solenoid (essentially a big relay) that the master closes. Without that closed, no juice to avionics (or any other) buss in most light singles.

If the other half of the master won't close (or if the voltage regulator is fried), no alternator field current and no charging.

 

[denverpilot]

I clearly said avionics switch and avionics relay. What are you talking about?

I was just explaining what the other confused guy was talking about. Heh. He popped in with the discussion about there being an avionics relay, and in most aircraft I've flown, that isn't the case. There's a relay/solenoid powering the buss the avionics master is tied to.

Going back to the original post, I saw one very paranoid person install two avionics masters. Either one could close the circuit between the main buss and the avionics buss and handle the full current.

Our avionics master is a retrofit just like any older Cessna that didn't come with one, and it's a lot higher quality switch than the master, that's for sure. There is an avionics breaker inline with it for the entire avionics buss also. Either one of those fails open, avionics are dead.

 

[flyingron]

My plane had a seperate breaker that bypassed the avionics master that was "pulled" in normal operation. If the avionics master failed, you closed that breaker.

 

[JHW]

Going back to the original post, I saw one very paranoid person install two avionics masters. Either one could close the circuit between the main buss and the avionics buss and handle the full current.

color me paranoid then. I have a backup avionics master which draws it's power direct from the battery. it also provides an alternate source of power to the electric T&B

 

[bbchien]

Hardly. It might be true for 99% of the planes built since the 80's. I can't think of the last time I flew anything non-turbine that had an avionics relay. They mostly just have a plain old switch to power the avionics buss, and often a lockable backup switch powered direct from the battery.

Well mine's got one.

I have the receipt for the replacement....it was NOT cheap and it was a basic Cutler-Hammer....oh never mind.

 

[Dan Thomas]

color me paranoid then. I have a backup avionics master which draws it's power direct from the battery. it also provides an alternate source of power to the electric T&B

Is this a homebuilt? If not, it's got to be an STC'd modification or it's illegal. The regulations don't want any hot lines coming into the cabin when the master is off, with the exception of the hobbs and the clock, both of which are on tiny fuses of maybe 1/4 or 1/2 amp. The reason is to prevent or stop electrical fires.

Even the master switch doesn't have a "hot" wire. It grounds the master contactor's coil, so the line coming to it is already loaded and is on the cold side of the coil. A short of that line to ground simply turns on the contactor and won't start a fire.

A quote from the regs:

-----------------------------------

23.1361 Master switch arrangement.

(a) There must be a master switch arrangement to allow ready disconnection of each electric power source from power distribution systems, except as provided in paragraph (b) of this section. The point of disconnection must be adjacent to the sources controlled by the switch arrangement.

--------------------------------------

Your backup switch is not adjacent to the source, which is the battery.


A master contactor is adjacent to the battery.





Dan

 

[gismo]

First of all, 14 CFR 23 only applies to aircraft whose type certificates were granted under that rule, most of the planes we are flying were certified under CAR 3 and I don't think it contained such a requirement. Second, even 23.1361 allows circuits protected at 5 amps or less to be alive when the master is off provided the overcurrent protection is located near the battery. A vast majority of the piston airplanes flying have such a circuit to power the clock. My airplane has an arrangment with a CB near the battery supplying a "ground avionics power switch" which is also fairly common in airplanes and AFaIK, perfectly "legal" since it's not normally on during flight.

-lance

23.1361 Master switch arrangement.

(a) There must be a master switch arrangement to allow ready disconnection of each electric power source from power distribution systems, except as provided in paragraph (b) of this section. The point of disconnection must be adjacent to the sources controlled by the switch arrangement.

--------------------------------------

Your backup switch is not adjacent to the source, which is the battery.


A master contactor is adjacent to the battery.





Dan

 

[bbchien]

Dan, he's under CAR 3.....

Careful, now, Jeff does REALLY know his stuff.

 

[J_Pilot]

I am still unsure Why there is an alternate avionics master. I understand about the master battery contactor relay (having learned the hard way what happens when it fails) but i don't see what the "alternate avionics master" does. Does it by-pass the switch, the contactor, or what. Does it have any fuse protection in the circuit it creates?