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-   -   P-Mag - Requirement for power line switch? (https://vansairforce.net/community/showthread.php?t=114650)

Kevin Horton 06-29-2014 03:10 PM

Quote:

Originally Posted by jclark (Post 892909)
My thinking is that the P-Mag (114 Series), once fired up is "on it's own" so to speak. Whether I have power to it or not. If I want to take it out of firing for the engine then my action is the SAME as if it was a mag that I wanted to remove.

I have not yet envisioned a power problem that would apply to the P-Mag but not to the "other" EI. So if I have smoking EI wire then I need to turn the power off to BOTH and LAND ASAP.

Probably not within your risk profile but that is how I think about it.

James

By "goes crazy", I mean fire the spark plugs at the wrong timing, so the engine runs rough or quits.

If there is smoke in the cockpit, my thinking is that it will be pretty much impossible to quickly identify the source. My Smoke in the Cockpit procedure is to open the battery master/alternator switch, which will kill the whole electrical system, except the Lightspeed EI and my turn and bank, both of which are fed from an always-hot battery bus.

Larco 06-29-2014 03:10 PM

Because the P-Mag loses internal power below about 750 RPM I installed a DPDT switch with 2 slow blow fuses. Reason-----One day I turned off the P-Mag switch to check the internal power during the run up, got distracted and forgot to reenergize the circuit. Got to destination and on roll out with idle power engine quit! So now I have a light to indicate when the switch is OFF! Just another way of doing it. :-) Larry

Ironflight 06-29-2014 03:35 PM

No power switches for any of our P-Mags Kevin - they are powered through pull-able breakers in order to test when needed. SInce I added them well after the -8 was flying, I also didn't have a good spot for the breakers on the norma panel - I put them on the inboard face of the left gear tower where I can reach them. It made adding the wires easy as well - buried them in the gear tower.

krw5927 06-29-2014 03:59 PM

One possibility would be to ditch the lightspeed and get 2 pmags, each with its own three-position switch. Your sense of (dis)order would not be offended.

Andy Hill 06-29-2014 04:02 PM

We have 2 P-Mags with a CB for each for Power.

Only use the CBs for the "once every so often" self-generating test. I would say not having this ability might not be ideal in a Dual P-Mag installation? But cannot see an issue in a single installation if you are willing to kill all ship's power.

When you kill all power, do you still have RPM indications? The full test is to remove power from the P-Mag, having removed the other ignition system - then gently slow engine and note RPM at which self-generating stops.

N941WR 06-29-2014 04:47 PM

The 114's switch to internal power around 800 RPM, which works seamlessly. However, I had one of my 114's internal generators go bad and the only way I found it was to pull the power on them, one at a time, because it was running on ship's power.

So, yes, you want a way to kill the power on them.

Kevin Horton 06-29-2014 04:49 PM

Quote:

Originally Posted by Andy Hill (Post 892936)
When you kill all power, do you still have RPM indications? The full test is to remove power from the P-Mag, having removed the other ignition system - then gently slow engine and note RPM at which self-generating stops.

That's a great point. No, if I killed all power, I would no longer have an RPM indication. That is definitely a point against my draft plan.

Kevin Horton 06-29-2014 04:54 PM

Quote:

Originally Posted by N941WR (Post 892942)
The 114's switch to internal power around 800 RPM, which works seamlessly. However, I had one of my 114's internal generators go bad and the only way I found it was to pull the power on them, one at a time, because it was running on ship's power.

So, yes, you want a way to kill the power on them.

Did the internal alternator fail completely, so the engine would not run at any rpm if there was no ship's power? Or, did it have a partial failure so that it would generate power at some rpm higher than the normal 800 rpm cutout?

Larco 06-29-2014 05:27 PM

Quote:

Originally Posted by N941WR (Post 892942)
The 114's switch to internal power around 800 RPM, which works seamlessly. However, I had one of my 114's internal generators go bad and the only way I found it was to pull the power on them, one at a time, because it was running on ship's power.

So, yes, you want a way to kill the power on them.

And this is why I installed a switch as per P-mag recommendation. To ensure that the internal power mechanism is operating properly before flight. Not a big deal until it becomes one. :-)

GalinHdz 06-29-2014 05:48 PM

Quote:

Originally Posted by N941WR (Post 892942)
The 114's switch to internal power around 800 RPM, which works seamlessly. However, I had one of my 114's internal generators go bad and the only way I found it was to pull the power on them, one at a time, because it was running on ship's power.

So, yes, you want a way to kill the power on them.

I echo that you really want a way to kill power to the 144 mag for testing. If panel space is tight then a combination CB Switch is the way to go.

:cool:


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