Avionics Switch

[mooneyflyer]

Is there any issue starting your engine with the Avionics Switch ON? Currently broken in the ON position.

[N201MKTurbo]

The voltage swings and spikes during starting can be harmful to some avionics. You can just turn them all off individually until you get the switch fixed.

[ArtVandelay]

The modern avionics have voltage regulators, that’s why they can work on 14 or 28 systems. And modern regulators don’t voltage spike on startup, and usually delay charging so not to add resistance to startup. So do you have modern equipment?

[PT20J]

I’ve never measured the drop or looked for spikes, so I only know what others tell me. Garmin installation instructions call for most avionics to be on the switched avionics bus. Some (PFDs mainly) are supposed to be on the main bus. I asked my installer and he said that Garmin explained during training that there are differences in the power supplies (mainly brownout protection) in the units rates for connection to the main bus. Assuming this is accurate, I would be most concerned with a 14V system.

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[EricJ]

35 minutes ago, ArtVandelay said:

The modern avionics have voltage regulators, that’s why they can work on 14 or 28 systems. And modern regulators don’t voltage spike on startup, and usually delay charging so not to add resistance to startup. So do you have modern equipment?

The bigger issue is transients from the starter motor, which is a high-current inductive device that makes a lot of noise.   If the avionics are off when the starter is operated that's the main issue.

 

[ArtVandelay]

The bigger issue is transients from the starter motor, which is a high-current inductive device that makes a lot of noise.   If the avionics are off when the starter is operated that's the main issue.
 

We already have avionics that are on when master is on…glass panels, certified engine monitors, etc. I bet if you take apart the JPI 830 and 900, you’ll not see any special circuitry for voltage conditioning, they will look alike, except 900 has extra AD converter for the extra sensors.

[EricJ]

51 minutes ago, ArtVandelay said:

We already have avionics that are on when master is on…glass panels, certified engine monitors, etc. I bet if you take apart the JPI 830 and 900, you’ll not see any special circuitry for voltage conditioning, they will look alike, except 900 has extra AD converter for the extra sensors.

When things are designed for it, i.e., they're expected to be on when the system is noisy/spikey, it can be different than when things expect to have quieter power input.   Maybe the power input on a device is robust, maybe it isn't, I wouldn't assume.   It'll certainly reduce risk to devices on the avionics bus to leave it off during start or other transients, like connecting external power, etc.

That said, temporarily keeping the avionics bus on to get somewhere to get it repaired is not likely to be a big deal.   Shutting some devices off or pulling breakers for engine start is an easy work-around.

 

[jamesm]

See I can draw fire here..... Like see the error in my ways/ understanding... (you don't have to be gentle with me, I am the youngest of 6 I can take it)

I am not engineer, questionable pilot (wx has been so crappy for way too long in the PNW ) and sometimes A&P , electronic technician from late 80's who occasionally has to works on old equipment for work. 

My '67C never had avionics master switch installed. all the avionics installed except the L3 NGT-9000 ADS-B (which is always on) the other radios and audio panel all have separate on off switches. 

One of the common argument for having an avionics master switch (IMHO) you are adding single point of failure could be setting yourself potential lost com procedure in the future. I am not sure about the voltage spike during start up, I would think the ship's Battery would act as a capacitor dampening any spike during startup of course it's depending wiring configuration. I am also  assuming that most of the  mid 60's and earlier Mooney's have been converted to Alternators. So I wouldn't expect to see a potential  50 amp or 35 amp surge that you might see on start up if you still have the generator installed. It is my understanding, that when the generator comes on line that it could pull 50amps/35amps. In the case of the alternator installation, The Alternator field current circuit draws less than 5 amp for most mid 60's Mooney's that I have seen.

Here is another argument against the avionics master switches installation and operation that I have seen provided that you are leaving all the avionics equipment power switches are on.

people leave all their avionics on and then they throw avionics master switch on  which creates huge an on rush of current to the avionics bus aka SPike on electrical bus from the current draw. that the avionics master switch is supposedly are trying to prevent unless there is some remote circuitry installed to prevent the voltage spike and current draw which my guess there is not. If you switch on  each individual avionics equipment separately  you are not creating this huge on rush of current spike  because it take you a few seconds or so to turn on the next piece of  avionics equipment and the next avionics equipment and so on.  I am going wild ass guess that the biggest current draw for the Garmin GTN's, GNS's, and GNS's, Avidyne IFD's is going GPS/NAV/COM equipment booting up the screen during post check and initialization of the unit.  The other time  that there would be a large current draw is during transmitting.

Another comment most of the newer Avionics are dual voltage range 14vdc  and 28vdc and many have on/off switches.

 

I think that I would more concerned with being able to dissipate voltage  spike from thunder and lightning  or static environments

Just my thoughts,

 

Edited January 6, 2022 by jamesm

[N201MKTurbo]

33 minutes ago, jamesm said:

 

I am not engineer, questionable pilot (wx has been so crappy for way too long in the PNW )

 

Being a weather pussy never killed anyone.
 

I apologize for the politically incorrect statement, but after 6000 hours of flying and 65 years on this planet, I’ve grown into it.

[jamesm]

Ha ha

1 minute ago, N201MKTurbo said:

Being a weather pussy never killed anyone.

I am trying get my instrument rating but the cascade icing machine says no too much ice.

[N201MKTurbo]

4 minutes ago, jamesm said:

Ha ha

I am trying get my instrument rating but the cascade icing machine says no too much ice.

I used to come up there a fair amount. I can’t tell you the times I  scud ran or spun up through holes because of ice. With a Mooney, the instrument rating doesn’t help that much out west. 
 

You FIKI  guys can just shut up....

Edited January 6, 2022 by N201MKTurbo

[EricJ]

1 hour ago, jamesm said:

people leave all their avionics on and then they throw avionics master switch on  which creates huge an on rush of current to the avionics bus aka SPike on electrical bus from the current draw. that the avionics master switch is supposedly are trying to prevent unless there is some remote circuitry installed to prevent the voltage spike and current draw which my guess there is not. If you switch on  each individual avionics equipment separately  you are not creating this huge on rush of current spike  because it take you a few seconds or so to turn on the next piece of  avionics equipment and the next avionics equipment and so on.  I am going wild ass guess that the biggest current draw for the Garmin GTN's, GNS's, and GNS's, Avidyne IFD's is going GPS/NAV/COM equipment booting up the screen during post check and initialization of the unit.  The other time  that there would be a large current draw is during transmitting.

Adding a load doesn't really do the same thing as transients caused by inductive machines (e.g., a starter) or other phenomena.   Clearly it is possible to have a system without an avionics master, and back in the day when radios were made with tubes there was an awful lot of natural immunity to noisy power systems.   As radios got more sensitive to transients and systems got more complex, it got to be a good idea to be able to add another layer of isolation.   One benefit of a separate avionics master is to allow connection of external power in case of a soft or failed battery.   An FBO or lineman hooking up the wrong supply or generation of a transient during connection will do a lot more damage if the avionics aren't isolated than if they are.   Therea are really quite a few reasons to be able to isolate them, but it is true that it isn't totally necessary from a functional standpoint. 

 

1 hour ago, jamesm said:

Another comment most of the newer Avionics are dual voltage range 14vdc  and 28vdc and many have on/off switches.

That doesn't have much to do with susceptibility to transients.   Modern systems easily accomodate either 28 or 14 volt inputs because most of the internal electronics these days run on voltages much less than that, so regardless of the input the power supply needs to regulate things way down, and modern switching supplies adapt fairly easily to varying input voltages.   This does also mean that the internal circuits are actually more sensitive to noise and spikes that manage to get through, so the isolation provided by the device power supply is more critical.   From what I've seen they're pretty decent, because the environment that they're designed to live in is quite a bit more hostile than, say, a desktop.   Personally, I don't see a reason to add risk when it isn't necessary, especially with the prices of avionics boxes these days, but everybody has their own risk tolerance.

[jamesm]

10 minutes ago, EricJ said:

 An FBO or lineman hooking up the wrong supply or generation of a transient during connection will do a lot more damage if the avionics aren't isolated

I don't have an external APU/GPU power plug for the FBO or Lineman to access,  at least that they don't know about.  If I need to isolate the avionics bus I can pull the circuit breaker(s) which I do if I am working on the aircraft. Seems to be  standard practice in the field from what I have heard. 

 

21 minutes ago, EricJ said:

transients caused by inductive machines (e.g., a starter)

My avionics are off (except ads-b no power switch) during  starting It is my understanding the Generator installation is going to create a larger transient than in a Alternator installation. I had forgotten to mention that my installation has pretty good size noise filter  inline with the armature of the Alternator to reduce noise and transients during startup.

[EricJ]

9 minutes ago, jamesm said:

My avionics are off (except ads-b no power switch) during  starting It is my understanding the Generator installation is going to create a larger transient than in a Alternator installation. I had forgotten to mention that my installation has pretty good size noise filter  inline with the armature of the Alternator to reduce noise and transients during startup.

The noise filter helps to attenuate smaller, constant noise that can interfere with audio or communications, like noise from a generator or mechanical regulator.   There can be transients from inductive or other events that are much larger than the noise filter is designed to handle. 

[jamesm]

1 hour ago, N201MKTurbo said:

instrument rating doesn’t help that much out west. 

 I need to work on my finesse and my flying skills besides I am too much of a woose to get ice build up on Mooney or many GA planes out there . I have heard way  to many horror stories flying in icing conditions and been back seat of the scud running flying missions. I am not so sure that my flying skill set matches that of those who have been trained  in the service or have deep background  in flying in adverse weather conditions or flying close to the edge of the flight envelope to know to when to stop.

Edited January 6, 2022 by jamesm

[jamesm]

28 minutes ago, EricJ said:

noise from a generator or mechanical regulator.

I am assuming that most mechanical regulator installations out in the field probably have been replaced if they have converted over to the alternator. I would think that solid state regulators would handle the transits a wee bit better than the mechanical regulators of the past.

[EricJ]

18 minutes ago, jamesm said:

I am assuming that most mechanical regulator installations out in the field probably have been replaced if they have converted over to the alternator. I would think that solid state regulators would handle the transits a wee bit better than the mechanical regulators of the past.

The opposite tends to be true.   It takes a lot to kill or disturb those mechanical regulators.   Just like old vacuum-tube radios, they have a very high immunity to transients compared to silicon (solid state) components, which can be killed by ESD (electrostatic discharge) just by touching them wrong.   

As transistor geometries have gotten smaller and smaller and source voltages gone lower and lower they've gotten even more sensitive to damage by transients, which is why the designs of the power supplies in the boxes and other measures are critical to providing sufficient protection.   It's all doable, but isolating a lot of the critical/sensitive/expensive stuff with a separate power switch goes a long way to reducing risk during known times of potential transients, like startup.

[jamesm]

My point is that in the old generator mechanical regulators There's more arc flashing going on causing pitting of the points causing even more current draw. To my knowledge the old mechanical voltage regulators had no circuitry to help reduce the voltage spikes from getting out of on the bus. I would think another solution would be to treat voltage transit problem at the source rather further down stream.

My guess is that 70% of mid 60's Mooney's probably have converted to alternator of some form or another. It is my understanding that generator is more susceptible creating the voltage transit than the alternator which could be detrimental to the to avionics. I don't believe there were too many (if any) mechanical voltage regulators for alternator at least not that I am aware of. In

Edited January 6, 2022 by jamesm

[N201MKTurbo]

3 hours ago, jamesm said:

My point is that in the old generator mechanical regulators There's more arc flashing going on causing pitting of the points causing even more current draw. To my knowledge the old mechanical voltage regulators had no circuitry to help reduce the voltage spikes from getting out of on the bus. I would think another solution would be to treat voltage transit problem at the source rather further down stream.

My guess is that 70% of mid 60's Mooney's probably have converted to alternator of some form or another. It is my understanding that generator is more susceptible creating the voltage transit than the alternator which could be detrimental to the to avionics. I don't believe there were too many (if any) mechanical voltage regulators for alternator at least not that I am aware of. In

A mechanical generator regulator only has one contact that switches the entire output of the generator, that would be the circuit breaker contact. It disconnects the generator armature from the battery until its output exceeds battery voltage, it then latches on until the engine is shut down. It actually switches at almost zero current, so not much arcing. The other two contacts, the voltage limiter and the current limiter, just switch field current, which is less than the current of your nav lights. They sometimes put a capacitor on the field lead to adsorb the voltage spike caused by the collapsing magnetic field when the field is opened. In other words, there is very little arcing going on in the mechanical regulators.

[N201MKTurbo]

Here is a mechanical alternator regulator. It only has the voltage limiter contact.

[EricJ]

10 hours ago, jamesm said:

My point is that in the old generator mechanical regulators There's more arc flashing going on causing pitting of the points causing even more current draw. To my knowledge the old mechanical voltage regulators had no circuitry to help reduce the voltage spikes from getting out of on the bus. I would think another solution would be to treat voltage transit problem at the source rather further down stream.

Mechanical Regulators don't really arc unless they're faulty.   Starter motors do, though.   

With an inductive machine the voltage spikes are just a consequence of doing business with it.   If you find a way to change inductor voltage behavior, the world will reward you handsomely, but basic physics is hard to overcome.   And when designing a box to go into a panel or a rack or just about anywhere, assuming that things like transients have been sufficiently handled upstream generally doesn't end well.   

 

[jamesm]

I remember people having to tap on the generator's mechanical voltage regulator housing to get the point from sticking. I'm assuming that is because the points were stuck.

[jamesm]

13 hours ago, N201MKTurbo said:

Here is a mechanical alternator regulator. It only has the voltage limiter contact.

I stand corrected

[carusoam]

Summary… sort of.

1) Newer avionics are built to a higher standard… and expect to be on during the start procedure… especially the fancy primary engine monitors…

It is hard to monitor rpm and and OilP if the JPI isn’t on already…

2) Older avionics we’re much more sensitive, so the avionics bus is kept off until after start…

3) The start procedure, was followed by the alternator being connected, then the avionics…

4) Modern avionics busses are protected from the added single point of failure in an interesting way… it’s not just an ordinary switch that could fail big time…

5) There is one situation where a single wire can take out the current to the avionics bus… our European brothers have a solution for that as well…

 

In my case… I would be shutting things off individually to avoid sending power spikes….  Whether they exist or not…

PP thoughts only,

-a-

[jamesm]

I hope I didn't come off as being combative or abrasive and/or arrogant  as not to have avionics master switch  installed. It was more of trying to understand why they are  installed.

Sometimes there tends to be "that is the way we have always done it" attitude/ response on website and other forums.

In aviation as in life there are several "ism"  for a lack a better term  that get carried  over the decades that are no longer true or don't apply to certain circumstances.

Examples

turning the prop backwards  could  cause the vanes in the dry  vacuum pump to break off.  is this true ? Tempest  Rep says no but  perhaps other manufactures of  dry vacuum pumps it could be true.

In the early 90's I was told to turn the  transponder to ON  until just before take off  then switch  the transponder over to "ALT" mode just before take-off. and of course (i) or you forget to put the transponder in the "ALT" mode. I had heard several reasons  from it takes time to warming up magtron<?> tube to it causes ARTCC to see ground returns on there scopes when we taxied on the  ground with the transponder being in the 'ALT" mode. I had asked a Technician who worked on those very ARTCC scopes about the ground returns he said it's wasn't true. Other ARTCC location at the time possibly.

I am sure there are many more examples.

If I was told that during starting sequence the Induction load of the starter or generator could causes spike for instantaneous period of time do to ELI the ICE man a principle which most electronic technician's are going to know this principle. Since  it is common as knowing the resistor color codes (which knowing the resistor color codes is pointless in today age in surface mount components era ) then I could see it.  (ELI = Voltage lead Current in a inductive circuit  and ICE Current leads Voltage in a capacitive  circuit).

I was merely trying  understand the electrical principle as to why we still had avionics master switch and it wasn't a left over or to expensive to make change to the drawings and certification etc etc  etc.

Thanks,