Test procedure/schematic for single Pmag

Hello all,

I'm working on my electrical system, specifically control of the ignition sys. I'm going to run a single bendix and a Pmag, and would eventually migrate to two Pmags. I've come up with the following schematic for how I intend on controlling the mags, and enabling a push-button start switch. I searched the archives, but could not find anything similar. I'd appreciate feedback from anyone who has done something similar.

http://imageshack.us/photo/my-images/705/ignitionq.png/

Based on this schematic, here is my procedure for starting, mag checks and shutdown;

START;
PMag: On
LMag :On, momentarily hold up to start position to enable starter
Start Button: Push

MAG CHECK:
Lt Mag: Off (slight drop in RPM)
PMag: Test (No drop)

Shut Down:
Pmag: Off (slight drop in RPM)
Mixture:Cut off
Lt Mag: Off

Again, feedback appreciated. Feel free to use if you find it useful.

David

I understand that there are a few valid reasons for going away from a standard keyed switch (cost and reliability among them), but they work just fine controling two pmags or a split system like you propose. Also, why do you intend to shut off the Pmag before you pull the mixture?

Perhaps also check the Pmag power by shutting off its power from the ship and force it to demonstrate its ability to self-generate its own power.

Caveat - that's if I understand how it works.... I'm not a Pmag expert.

Dave

PDF

Dave,
You really don't need the push button start switch as you have it. If the conventional mag switch has a momentary up position that will activate the starter. If you have the desire to put the second P-mag in the starter button will be really useful and you will still have the ability to check the self powering of the P-mags. I'm in the process of putting in another P-mag and will add the push button to start.

FYI - The 114 series P-mags work different than the old 113's. If you are above around 800 RPM the 114's are self powering. Thus, if you cut the power to them you aren't really testing anything because they have already dropped ship's power.

Below that number, say for starting, they are running off of ship's power, so you just tested that by starting the engine.

For this reason, I would still the mag check just to make sure everything is good but I'm not sure you need to pull the power on them.

FWIW, even with the EICommander telling me all good with my dual P-mags, I still run a power test because I did it with the 113's and continue to do so with the 114's.

N941WR said:

Thus, if you cut the power to them you aren't really testing anything because they have already dropped ship's power.

Click to expand...

But you are testing the built in alternator. If it is bad and you remove ship's power, the engine will die as long as the other ignition is turned off.

If ships power is never removed, there is no way to tell if the internal alternator is working or not.

I have to agree with Brian on this one. I just think it's a good idea whether it's a 113 or 114 to check the internal alternator to confirm that it's still working. Never had an ounce of trouble with the 113 that I have and don't expect any when I add the 114.

Clarification

I guess I should have clarified a few items in my original post...

I don't want the weight of a keyed switch, and feel it adds no add'l functionality or security. Honestly, I like the 'coolness' factor of a bush button switch that I designed myself, as long as I can do that reliably (hence this post).

I know I don't need the push button switch, as I have it designed, but this way it takes two deliberate actions to enable the starter, so I should not inadvertent activations. This push button has a built in LED, which i hope to use as a starter engaged annuciator to indicate whether it's engaged or not. PaulR brings up a good point, that my present methodology would not work with dual PMags, as my present start position for the second switch would be for the test position of the second mag. I suppose I could just start on a single mag, or make it so either PMags would enable the starter, which would still require starting on a single PMag, which should not be a problem.

The TESt function of the PMag simply disables the internal alternator. Doing this during the run-up should not cause any drop in power, as the internal alternator should be powering the PMag, and the Bendix should still be operating. If I get a drop in power when running this test, it means the PMag jphas dropped off line, and I'm running solely on the Bendix.

I plan to turn it off before shut down, to test running on the Bendix alone. Actually, I'll probably ground the Bendix briefly first to ensure that it's P-lead is functional, reapply power to it, and then turn off the PMag to ensure that it can be powered down and the Bendix can carry the load.

I think this covers all the questions I received thus far, thanks for your feedback to date, anything else?

David

Beer30? said:

...I don't want the weight of a keyed switch, and feel it adds no add'l functionality or security. Honestly, I like the 'coolness' factor of a bush button switch that I designed myself, as long as I can do that reliably (hence this post)...

Click to expand...

OK, that's valid.... There seems to be a number of people who feel they really NEED something other than the standard keyed switch for PMags - just wanted to be clear that that is not the case.

And FWIW, after hundreds of hours flying behind PMags, I rarely check the "alternator function"... This test is reserved for condition inspections and oil change intervals in my world. One thing that HAS been added to the daily check is the LOP, cruise power "mag check". This has shown me a lot more than any end of runway, 1800 RPM checks ever has.

What is the LOP mag check

Mike,

I'm not familiar with the cruise power LOP mag check. Can you elaborate?

Thanks,

David

http://blog.savvymx.com/2010/03/mag-check.html

"The usual pre-flight mag check is a relatively non-demanding test, and will only detect gross defects in the ignition system. To make sure your engine's ignition is in tip-top shape, we recommend performing an in-flight mag check at cruise power and a lean mixture (preferably a lean-of-peak mixture). Because a lean mixture is much harder to ignite than a rich one, an in-flight LOP mag check is the most demanding and discriminating way to test your ignition system. It's a good idea to perform one every flight or two.

The in-flight mag check is performed at normal cruise power and normal lean mixture (preferably LOP). Run the engine on each individual mag for at least 15 or 20 seconds. Ensure that all EGTs rise, that they are stable, and that the engine runs smoothly on each mag. If you see a falling or unstable EGT, write down which cylinder and which mag, so your mechanic or SAMM account manager will know which plug is the culprit."

Beer30? said:

...I'm not familiar with the cruise power LOP mag check. Can you elaborate...

Click to expand...

Sure,

The end of runway mag check is not much of a test - anything but a gross ignition failure is going to show up as "good". OTOH, high altitude, LOP operations are challenging for ignitions. Just getting the fire lit at all is tough for some magnetos. The fact that EI's do this job better than magnetos does not mean they aren't getting a workout. So to see if they are both actually healthy, simply do a conventional "mag check" when established in your normal cruise power setting. For me, that's ~ 8,000 feet, 50 LOP. On the ground, my "mag check" produces zero RPM change between ignitions, so I have to look for the EGT rise to verify that one ignition is in fact shut down. If my ignitions are healthy, the cruise LOP method delivers the same result. I started doing this after a plug change once and discovered a weak coil. Worked fine on the ground, worked fine on ROP climb, but once I went LOP, the one side just couldn't hack it. Now, this is my normal cruise checklist item.

EDIT: someone beat me to the punch....

On those same lines...

If you're cruising along LOP (let's say 50 degrees) and notice that suddenly you're some significant value warmer - say 10 LOP or even slightly ROP, don't adjust the mixture right away.... There's a good chance that you have lost an ignition. Do a mag check first, but expect the engine to fall on its face or quit when you shut down the "good" side. If the ignition checks out, then you might think about chasing the mixture back to where it needs to be.

PMags (like most EI's, I suspect), work so well that the engine runs quite happily, even at altitude, on just one. That's why having one drop out on you often produces only the slightest clues. The first time it happened to me I just assumed I bumped the mixture a little rich. There was no obvious indication the ignition checked out.