Another PMag Switch Question

[AirplaneDetective]

Planned dual e mag/p mag on io390.

I would like to have a traditional OFF, RIGHT, LEFT, BOTH, START rotary switch/key

I see that it requires a circuit breaker, so I’ll plan for that. Duh.

Now my off, right, left, both, start rotary switch seems that it can satisfy the “ignition kill switch P lead” requirement.

And I would still need a separate set (for each p mag) for ignition power switch for testing the internal alternator?

I’ve seen some add an indicator light, can these be wired to show P mag fault?

 

[Bavafa]

I’ve seen some add an indicator light, can these be wired to show P mag fault?

I have that indicator light and its main purpose for me is to remind me to flip the toggle switch back to ON position after testing or if accidentally my hand bumped them to OFF position. One other function that it serves, it proves that I actually have juice on that line and the fuse is not blown. The indicator light comes ON when the toggle switch is turned to OFF position.

 

[penguin]

A P-Mag requires a power supply from the battery/bus-bar to start up.
That supply needs circuit protection, I think a 3 amp breaker is now recommended.
Once you have some confidence in your system you will only test the internal P-mag generator occasionally, so nothing complex required.
I use a switched breaker - several types available.

 

[Bavafa]

Once you have some confidence in your system you will only test the internal P-mag generator occasionally, so nothing complex required.
I use a switched breaker - several types available.

Common practice is to check the internal generator at the run up, each time, at least this is what I do.But I understand each has their own procedure*and tolerances for risks.

 

[AirplaneDetective]

So would the rotary switch alone suffice?

 

[MED]

No. You need to keep the particular p-mag (l or r) operational (i.e., firing its set of spark plugs), but turn off its access to ship’s power to confirm the internal generator is working. I use switches to temporarily interrupt ship’s power on each p-mag during the ‘mag’ check. Takes just a moment for each.

 

[Carl Froehlich]

So would the rotary switch alone suffice?

A traditional ignition switch to control the pMags (just like standard mags) and a single pull breaker for each - done.

Test the pMag internal generator after engine start by pulling the breaker.

Carl

 

[sibriggs]

Push button off

I spoke with one of the employees that works with Brad at p-mag.

He told me about his approach to checking the internal power of the p-mags when operating above the 600-700 RPM requirement that generates pmag power in the event of a aircraft power loss (i.e. battery and/or alternator).

He used the rotary on-off-right-left key switch described above. I copied his idea and installed two push button mom switches, one each directly above the right and the other directly above the left positron of the key.

When you do the mag checks and the key is in the left or right positron checking that pmag.... you reach up with your finger and depress the momentary off button which kills aircraft power to the pmag. This lets you know the internal alternator of the pmag is working properly.

When testing the for a wire break to a pmag you select the left or right key position, hold in the corresponding off button and let the RPM drop below the 600-700 RPM range. The engine should die. Or just hold in both off buttons at the same time and the engine should die when the RPM drops below the range.

I had to search the internet a bit to find the push button momentary off switch, most of the are momentary on, like a doorbell button.

 

[MJarreau]

Carl's Approach

Just an opinion, but I like the disconnect feature offered by Carl's idea.

Best regards,
Mike

 

[rleffler]

So would the rotary switch alone suffice?

No, the rotary switch just connects/disconnects ground as appropriate. You need to interrupt the power source.

I did something similar to Carl, except I used a switched breaker. I find them a little easier to use, but it’s personal preference.

 

[penguin]

Common practice is to check the internal generator at the run up, each time, at least this is what I do.But I understand each has their own procedure*and tolerances for risks.

I would suggest it is not common practice but a procedure used by some.
After 15 years of running P-mags I have found the generator to be reliable.
I check the generators every month or two. It would only ever be an issue if the aircraft generator failed and the battery is weak.
Unlikely for both to fail at the same time.
Its your airplane, run it as you see fit!