Master On..START Engine

Jaybird180

Jaybird180

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Jaybird180
Every airplane I've flown has as standard procedure to start the engine, the turn on Avionics Buss.

A review of my POH reveals that the Primary Buss is not affected by the starter, yet the start procedure is the same. Why?

Wish I could include a snapshot of the page, but I don't know how from my iPad.:dunno:
 
Of course it will be effected, when engaged the starter becomes the path of least resistance and it it can draw enough off the system to drop the system voltage below a critical point in the surge and then pop back up, we in the old days that would let the magic smoke escape sometimes. Better to be stabilized from large loads before turning on electronics.
 
When inductive loads like starters, solenoids or alternator fields are shut off, their magnetic fields collapse rapidly and generate a large spike in voltage that enters the aircraft's system (before their controlling contacts can open far or fast enough) and can damage sensitive equipment. The avionics bus is therefore left off until after start and is shut off before the master is shut off. Lights and motors don't care much about voltage spikes but electronics sure do.

Sometime when the cowling is off, place the fingers of one hand across the master solenoid's coil contacts and have someone turn the master on, then off. (Don't lean on the airplane with any other part of your body.) You won't feel anything other than the click when it turns on, but when it turns off! Oh boy! I have used an oscilloscope to find a 600-volt spike at those terminals when the 12-volts is shut off. Cessna uses a diode across that coil to shunt the spike, but many others don't.

Dan
 
Dan Thomas said:
When inductive loads like starters, solenoids or alternator fields are shut off, their magnetic fields collapse rapidly and generate a large spike in voltage that enters the aircraft's system (before their controlling contacts can open far or fast enough) and can damage sensitive equipment. The avionics bus is therefore left off until after start and is shut off before the master is shut off. Lights and motors don't care much about voltage spikes but electronics sure do.

Sometime when the cowling is off, place the fingers of one hand across the master solenoid's coil contacts and have someone turn the master on, then off. (Don't lean on the airplane with any other part of your body.) You won't feel anything other than the click when it turns on, but when it turns off! Oh boy! I have used an oscilloscope to find a 600-volt spike at those terminals when the 12-volts is shut off. Cessna uses a diode across that coil to shunt the spike, but many others don't.

Dan
Click to expand...

I scoped the main bus on my Baron when starting and found no spikes greater than 30V. The battery does a good job of absorbing transients and my PM starters don't appear to generate much in the way of a spike either (scoped the starter side of the contactor with similar results). IMO, the primary issue is low voltage while cranking which could cause excessive current in any switching supply (e.g. radios that will run on 12-28v) if they're not designed with brownout protection.
 
Jaybird180 said:
Every airplane I've flown has as standard procedure to start the engine, the turn on Avionics Buss.

A review of my POH reveals that the Primary Buss is not affected by the starter, yet the start procedure is the same. Why?

Wish I could include a snapshot of the page, but I don't know how from my iPad.:dunno:
Click to expand...

push the home button and power button at the same time :thumbsup:
 
gismo said:
I scoped the main bus on my Baron when starting and found no spikes greater than 30V. The battery does a good job of absorbing transients and my PM starters don't appear to generate much in the way of a spike either (scoped the starter side of the contactor with similar results). IMO, the primary issue is low voltage while cranking which could cause excessive current in any switching supply (e.g. radios that will run on 12-28v) if they're not designed with brownout protection.
Click to expand...

I'm really surprised that no one has mentioned Bob Nuckolls over at AeroElectric Connections (http://www.aeroelectric.com). He is a wealth of knowledge and takes a pretty hard stance that a master avionics switch isn't needed. Not only that, but in most traditional applications a single avionics master switch reduces system reliability by introducing a single point of failure.

His position, which granted is targeted mainly to "OBAM" aircraft (owner built and maintained), is that modern avionics built to RTCA DO-160 standards can easily handle the power supply instability in an aircraft electrical system. He provides significant detail on this in an article he published called "Avionics Master Switches: Really Necessary?" ... see http://www.aeroelectric.com/articles/avmaster.pdf. Further, he (and Matt Dralle over at Matronics) maintains a listserv where you can direct questions. You want to learn, listen to Bob. You want a bunch of OWTs, post on the internet and watch the stuff fly.

The article linked above addresses the need, design and necessity of a master avionics switch directly by answering an entire series of questions from his audience. Give it a read - he doesn't argue that you should abuse your avionics, but he does challenge you to think about the system before accepting blindly the advice and direction you may get around the airport or your favorite colored board.

Note - This is not particularly directed to gismo, just thought this was a good jump off point since gismo uses data ... one of Bob's hot points.
 
All that's needed to fix the "single avionics master" problem is a double pole, double pole switch with both legs rated for the full current required.

The chance you're going to kill both sides of a quality DPDT in the number of cycles in a typical aircraft, are just about not worth measuring.

Snap the physical switch handle off, though... you have a problem. Since you don't move it normally in-flight, your chance of doing it when avionics are needed, approaches zero.

Once on, leave it on.

And switches cause spikes, too.

http://en.wikipedia.org/wiki/File:Bouncy_Switch.png
 
It's not like it's a major hot topic with me and i could even be inclined to agree, however the adding a "single point of failure" claim can be simply addressed with a back up tie in switch/circuit. Besides which, especially in a SE airplane it would only be adding one more single point failure in a system chock full of them.
 
Everything is connected to the landing light as well. That's the justification I use to do wiring work under the Appendix A preventive maintenance :)
 
iflyforfun said:
I'm really surprised that no one has mentioned Bob Nuckolls over at AeroElectric Connections (http://www.aeroelectric.com).
Click to expand...
What would the rest of the posters do if all the OWT were debunked? Don't expect anyone to follow your recommendation. If they believed what Bob shows to be accurate, they'd have to question other OWT. Easier to simply repeat what they were told by a previous Old Wife than question their own perceived wisdom.
 
jnmeade said:
What would the rest of the posters do if all the OWT were debunked? Don't expect anyone to follow your recommendation. If they believed what Bob shows to be accurate, they'd have to question other OWT. Easier to simply repeat what they were told by a previous Old Wife than question their own perceived wisdom.
Click to expand...

I don't question what he says, I also know that components will degrade with time to below the standards at which they 'should handle it'.
 
iflyforfun said:
I'm really surprised that no one has mentioned Bob Nuckolls over at AeroElectric Connections (http://www.aeroelectric.com). He is a wealth of knowledge and takes a pretty hard stance that a master avionics switch isn't needed. Not only that, but in most traditional applications a single avionics master switch reduces system reliability by introducing a single point of failure.
Click to expand...

I read all of his articles, and used much of them for my aircraft build. But I still put in an avionics switch, and used it. There is an "essential" switch that gives the avionics power, through a diode, if needed. It's also from Bob's schematics.
 
Henning said:
I don't question what he says, I also know that components will degrade with time to below the standards at which they 'should handle it'.
Click to expand...
And how do you know that, or more specifically what makes you think the transient protection of a properly designed power supply circuit will lose it's ability to cope with transients during the useful life of the product?
 
denverpilot said:
All that's needed to fix the "single avionics master" problem is a double pole, double pole switch with both legs rated for the full current required.

The chance you're going to kill both sides of a quality DPDT in the number of cycles in a typical aircraft, are just about not worth measuring.

Snap the physical switch handle off, though... you have a problem. Since you don't move it normally in-flight, your chance of doing it when avionics are needed, approaches zero.

Once on, leave it on.

And switches cause spikes, too.

http://en.wikipedia.org/wiki/File:Bouncy_Switch.png
Click to expand...

In my airplane I have two separate, paralleled toggle breakers for an avionics master. The only downside is that if you're foolish enough to turn both on at the same time (assuming both are working) the trip current will increase by almost 2x. But as long as you're willing to enable them one at a time this gives complete redundancy on that SPoF. Another SPoF is the main bus itself and to address that I have a 3PDT switch that transfers the power for a few key avionics from the main bus to a direct feed from the battery.
 
gismo said:
And how do you know that, or more specifically what makes you think the transient protection of a properly designed power supply circuit will lose it's ability to cope with transients during the useful life of the product?
Click to expand...

Because eventually the magic smoke always gets out.
 
Henning said:
Because eventually the magic smoke always gets out.
Click to expand...
That's not even true in that lot's of obsolete working avionics end up lying on a shelf never to be used again, but even if it were true I doubt you have any real idea how many failures are truly the result of power transients.
 
Nuckolls says:

Here's where legend kicks in . . . "spikes" simply don't exist
. . . normal or otherwise. Small perturbations in system
voltage are accounted for by design. How about your car?
There's as many transistors, power -AND- microprocessor as
most airplanes . . . how come no "master electronics power
switch" for the electronics in your car?

A car has no master contactor with a coil. A car's ignition switch disconnects the accessory feed during starting. The car's computers are full of suppressors. We drive vehicles that are modern, and fly airplanes full of obsolete technology like felt wheel seals and bias-ply tires. The comparison is completely invalid.

I'm not saying that an avionics switch is necessary, but Nuckolls shouldn't be comparing aircraft systems to auto systems. They're set up quite differently. If one wants to use evidence to make a case, it needs to be pertinent evidence.

Dan
 
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If the avionics master switch fails on most Bonanzas, not a big deal, just pull the avionics breaker and you're back in business. It's funny how many Bonanza owners don't know that and don't believe me until I show them.

I suppose it never dawned on them that their avionics breaker is only 1 AMP...because all it does is power a relay.
 
Skylane81E said:
Every system that is on is affected by the starter
Click to expand...

Good point.......:yesnod:

In everything I operate, whether it be my experimental, race cars, or any toy I play with, I always apply power to the master contactor, light off the motor, let the charging system stabilize and then apply loads. That process takes maybe 5 seconds. I don't care how much people say electric stuff in immune to surges, Voltage spikes give me the willies.. IMHO....:eek:
 
Anyone shut off their alternator as part of the run up procedure?
 
What happens inside the electrical system when you do that...spikes an all that stuff? :stirpot:
 
gismo said:
That's not even true in that lot's of obsolete working avionics end up lying on a shelf never to be used again, but even if it were true I doubt you have any real idea how many failures are truly the result of power transients.
Click to expand...

You are correct, I have no idea, but I can guarantee the number is greater than one so with it being so easy to eliminate, why wouldn't I? SPoF we both agree is easy to eliminate.
 
Dan Thomas said:
Nuckolls says:

Here's where legend kicks in . . . "spikes" simply don't exist
. . . normal or otherwise. Small perturbations in system
voltage are accounted for by design. How about your car?
There's as many transistors, power -AND- microprocessor as
most airplanes . . . how come no "master electronics power
switch" for the electronics in your car?

A car has no master contactor with a coil. A car's ignition switch disconnects the accessory feed during starting. The car's computers are full of suppressors. We drive vehicles that are modern, and fly airplanes full of obsolete technology like felt wheel seals and bias-ply tires. The comparison is completely invalid.

I'm not saying that an avionics switch is necessary, but Nuckolls shouldn't be comparing aircraft systems to auto systems. They're set up quite differently. If one wants to use evidence to make a case, it needs to be pertinent evidence.

Dan
Click to expand...
JOOC, what do you think the master contactor coil has to do with transients on the main bus? It's true that the coil itself can and will generate a transient across the coil when the master is switched off, but that won't appear on the main bus and in many cases there's a diode on the coil to absorb it anyway. And unless your radios are older than something like a KX-155 it has the same kind of transient protection found in automotive electronics.
 
gismo said:
JOOC, what do you think the master contactor coil has to do with transients on the main bus? It's true that the coil itself can and will generate a transient across the coil when the master is switched off, but that won't appear on the main bus and in many cases there's a diode on the coil to absorb it anyway. And unless your radios are older than something like a KX-155 it has the same kind of transient protection found in automotive electronics.
Click to expand...

The coil generates the spike sooner than the contacts open, so the bus will feel something. The diodes aren't present in many makes of aircraft, especially older ones, and I have found those diodes broken, too. Vibration fatigues the wire.

If one looks at the schematic of the coil's wiring, one can see the path to the bus from the coil when the contacts are still closed. That contact is a heavy copper disc, operated by a steel slug, and moves much more slowly than the collapsing field and resultant spike.

The path includes the master switch itself. The spike will arc across the tiny gap in the switch contacts while it begins to open. All of this takes microseconds, and the contactor takes much longer to open.

Dan
 
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Dan Thomas said:
The coil generates the spike sooner than the contacts open, so the bus will feel something. The diodes aren't present in many makes of aircraft, especially older ones, and I have found those diodes broken, too. Vibration fatigues the wire.

If one looks at the schematic of the coil's wiring, one can see the path to the bus from the coil when the contacts are still closed. That contact is a heavy copper disc, operated by a steel slug, and moves much more slowly than the collapsing field and resultant spike.

The path includes the master switch itself. The spike will arc across the tiny gap in the switch contacts while it begins to open. All of this takes microseconds, and the contactor takes much longer to open.

Dan
Click to expand...
While I agree that without a flyback diode the coil will cause an arc across the master switch (a good reason to have the diode) the arc current will be low (can't be more than the initial coil current) and the low impedance of the battery will absorb nearly all the energy that makes it across the switch contacts. Just because there's high voltage at the coil doesn't mean there will be the same on the main bus.
 
gismo said:
While I agree that without a flyback diode the coil will cause an arc across the master switch (a good reason to have the diode) the arc current will be low (can't be more than the initial coil current) and the low impedance of the battery will absorb nearly all the energy that makes it across the switch contacts. Just because there's high voltage at the coil doesn't mean there will be the same on the main bus.
Click to expand...

Agreed. The battery will suppress most of the spike.

Dan
 
My Cherokee does not have an avionics master. I do shut off everything, before starting though.
 
I recently rebuilt my Fly Baby's electrical system, and added an avionics master. I figure there's little downside to a switch failure in my mode of operation, and if it keeps me from having to buy another transponder, it's worth it.

I also added a master solenoid, something the plane lacked, and while I was at it, I included the diode. Also, the digital voltmeter I installed recommended a 30V Zener diode when used in automotive applications, so I put that in as well. Can't turn left anymore, all the diodes are pointed right....

Ron Wanttaja
 
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