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P-Mag test

Ron B.

Well Known Member
Having never flown behind a P-Mag I'm wandering if my thought are correct for testing the internal power supply on a P-Mag. I would assume that while running the engine on the right mag (P-Mag) only you would toggle the P-Mag test switch to see if the engine stops or runs rougher proving the reliability of the internal power supply?
 
Manual is located here: https://emagair.com/downloads/

Test procedure is on page 22:
"P- Model (self-powering) Alternator Check: - You can check the internal alternator operation on the P model during a ground run-up (900+ rpm) by switching to the P model ignition and cutting bus power (not the p-lead switch) at the breaker. If the built-in alternator is working, the engine will continue to run. If it is not working, the engine will quit."

I'm not certain what your "test switch" does but I know some people have wired a 2-10 switch for the p-mag switch to perform this function (cutting bus power).
 
Steinair made the panel and they have a rocker momentary switch labeled P-Mag test. It disconnects the ship supplied power for the test. So yes my assumption was correct. Read the manual a while back, it's out in the hangar and I thought I would ask while the question is fresh in my mine.
Thanks
 
My two P-mags are on separate pullable CBs which I use to test.

As an aside, most of my electronics are on spade fuses except for a few things which are on pullable CBs along the switch row - things like P-Mags, auto pilot, and alternator field, i.e. stuff that I might want to pull for emergency, test, load shedding, etc.
 
I have two normally on momentary switches for my PMAG test functions. Its really not more complicated. They are normally on if I want to test then I turn them off. It does take some panel space, but not a problem. Circuit breakers are really not intended to be switches but some folks use them that way.
 
Having never flown behind a P-Mag I'm wandering if my thought are correct for testing the internal power supply on a P-Mag. I would assume that while running the engine on the right mag (P-Mag) only you would toggle the P-Mag test switch to see if the engine stops or runs rougher proving the reliability of the internal power supply?

To answer your question directly: No

If your engine was running only on the right Pmag and you tested it (cut external power) and the mag was not providing spark, thats setting yourself up for a big backfire when you switched the Pmag back to external power.

My understanding of testing Pmags.
Both Pmags running normally (or one mag, one Pmag), remove external power from the right Pmag, look for the telltale RPM drop. If there is no RPM drop, that confirms the right Pmags internal power is working. Repeat on the left Pmag if you have dual Pmags.

Now...I'll have to admit my knowledge is from my understanding of the system, not from operational experience. If I'm all wet I'm sure someone will set this straight :p
 
I have two normally on momentary switches for my PMAG test functions. Its really not more complicated. They are normally on if I want to test then I turn them off. It does take some panel space, but not a problem. Circuit breakers are really not intended to be switches but some folks use them that way.
That is exactly what I have and exactly why I have them. 800hrs of flying with this setup and still going strong. Here is a photo of the switches in my panel.

eMag Switches and Display.JPG

"P- Model (self-powering) Alternator Check: - You can check the internal alternator operation on the P model during a ground run-up (900+ rpm) by switching to the P model ignition and cutting bus power (not the p-lead switch) at the breaker. If the built-in alternator is working, the engine will continue to run. If it is not working, the engine will quit."
This is the correct way to check the internal alternator as per eMag. I follow this procedure as part of the engine run up BEFORE each flight.

This is the circuit diagram eMagAir provides in it's installation manual. I added a wire from each Pmag pin 5 to a voltage input on my SkyView EMS. This way I can monitor the battery voltage to each magneto and insure the normal closed switch is actually working. You can see the two green dots (top middle of the SkyView EMS display) indicating the mags are receiving 12v in the photo above.

PModel 114 Wiring.jpg

:cool:
 
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To build on Michael's very valid point, I've experienced that backfire and it isn't pleasant. As the engine is spinning down from lack of spark it's still sucking fuel and air in on every intake stroke. Now we've got an engine full of unburned fuel, and we re-introduce spark. BANG!

I've opted to do my P-Mag test at engine shutdown in order to avoid this nasty little surprise. At a 1000RPM ground idle I switch off my right (P-Mag) ignition aircraft power by pulling its breaker, then switch off my left (Slick) magneto's P-lead and witness a slight decrease in RPM. I then slowly back off the throttle and note the point at which the engine dies from lack of ignition. Once that point is reached I pull the mixture to ensure the engine dies quietly and the cylinders aren't filled with fuel/air mixture in case somebody happens to touch the prop when ground-handling.
 
I have a fuse under the panel.

So I simply pull up to the hangar, leaned for ground ops, 900 RPM.

ACS Switch to P-Mag only, then shut down ALT, BATT, and E BATT.

Engine runs (slightly rougher)! Slowly retard the throttle, the engine usually cuts off around 700 RPM. No backfires on next start.
 
Circuit breakers are really not intended to be switches but some folks use them that way.
Yeah, I don't necessarily disagree since the Potter-Brumfeld / Tyco etc. CBs which, though they have been standard / approved / blessed on certified A/C forever, are nevertheless stone age technology and frankly not as robust or reliable as all that history might imply (and yes, I have read Nuckolls' take on traditional CBs!) Still I used them for these specific things that would be normally ON with the master but might still want to be pulled for one reason or another, and the CB protection is there with only the one device. Or, that's my philosophy anyhow, YMMV and lots of ways to do it.
 

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