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Pmag Switch Operation Confusion

rockitdoc

Well Known Member
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I am wiring my P Mag switches. I intend to use a three position Honeywell 2TL1-50 from Steinair with down (power on ignition off), center (power on ignition on) and momentary up (+12v power interrupted for testing internal alternator).

I think I understand why the center and up positions of the switch do what they do. What I do not understand is: why would we want +12v going to the #5 pin on the P mag at the same time we are killing it by grounding the #4 P lead? Don't we want to just ground the P lead to kill the P mag?
 
Page 14 of the install manual:

Setup Mode is entered by turning 12volt bus power ON, WHILE the p- lead switch is OFF (grounded). If the LED is not lit up, you are not in Setup Mode.

Carl
 
I think a drawing or a schematic would help us answer the question.

Yes, of course. But, I thought you should be able to read my mind...:confused:

Here is how Steinair suggests wiring the Honeywell 2TL1-50 switch, which is a DPDT on-on-mom type with a locking center position. I like the concept because it allows operating and testing the Pmags in one switch per mag instead of having a separate switch for the test operation. Also, with the center locking feature, moving the switch out of the run position requires pulling out the lever, so accidental movement is reduced.

Anyway, here is their diagram which makes sense to me except for the down position, which sends +12v to the mag at the same time it is killing the mag. I understand this is something that needs to happen when setting up the pmag (p14 of the Pmag manual, as Carl noted) but is this something we would want as a permanent feature of the panel switches? Seems like when you want to shut down the pmags, having just the kill function would be desirable. You know, seems like the down position of the switch is doing two conflicting functions, ie, providing +12v and killing at the same time. It's probably cuz I don't understand how the Pmags work.

Pmag Switch Wiring Guide.jpg



S
 
Applying or removing power to the pmag doesn’t really do anything besides help with putting in setup mode and ensuring that the self generating power is working correctly. It also needs the power to start. Above 900 rpm even if you remove power the ignition will continue to provide spark. That is what the momentary up on your switch is testing. I don’t test this every flight but when I do, I remove power via the efis vpx control. I am assuming your 12v power control on the switch is wired to a vpx input and the pmags wired to independent vpx outputs. This will allow you to choose on/off via the vpx control in your efis regardless of what the switch is doing. It’s ends up being kind of a confusing runup procedure compared to just removing power via the efis to both mags and testing ignition like you would in a normal runup to ensure it keeps running.
 
. It’s ends up being kind of a confusing runup procedure compared to just removing power via the efis to both mags and testing ignition like you would in a normal runup to ensure it keeps running.

Agree with the above, to test the system you have to hold one switch up while turning the other switch off, not intuitive at all.
That’s why I install 2 breakers, pull the breakers during run up, test the mags like a normal airplane, push breakers in.
 
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Agree with the above, to test the system you have to hold one switch up while turning the other switch off, not intuitive at all.
That’s why I install 2 breakers, pull the breakers during run up, test the mags like a normal airplane, push breakers in.

Listen to Walt, please. Regardless of the specific component selection, try to design things so they operate as much like the theoretical "normal airplane" as possible.

Yes, I know the arguments. It's yours and you can wash it any way you want...but none of us last forever. Eventually, someone else will own it and fly it.
 
Actually fairly easy

My pmags will be wired with an on-on-mom switch which makes run up procedure pretty easy. Down the mag is killed, Center the mag is powered (‘contact’), Up the ship power is removed from mag and if engine dies, mag is not generating its own power. If engine keeps running, mag is generating power.

Pretty simple. One switch for each mag. And with the 2TL1-50 the Center is locked unless pulled out. Less chance mag will be accidentally shut off.
 
I just ran power through some CBs. When I check the self-powered feature (not every run up), I pull the CBs. Otherwise I control the p-leads like any other magneto. My particular install uses a key-switch but regular switches could work as well.

Not sure if inject a bunch of funky wiring to put ship-power to the pmags on the same switch as the p-lead.
 
Agree with the above, to test the system you have to hold one switch up while turning the other switch off, not intuitive at all.
That’s why I install 2 breakers, pull the breakers during run up, test the mags like a normal airplane, push breakers in.
Maybe I'm stuck on an old way of thinking but I've never been a fan of treating a breaker as a switch.

I don't have any empirical data and maybe my concerns are not valid but I've always felt that a pullable breaker can experience premature wear when repeatedly pulled and reset.
 
I know little about pmags. However, why would you ever want a normal switch position that puts the unit into setup mode? Sounds a bit scary. Isn't this the mode they use to reset the timing? I must be missing something, as I would expect that applying power to the pmag with the P lead grounded would be somewhat normal at startup.

Larry
 
I have 2 momentary switches that are used for testing the PMAGS that normally apply power and remove power when held in the test position. Some use breakers for this function, but breakers have a limited number of operating cycles so I went with switches. I also have a keyswitch which I know some people don't like, but it operates in a standard configuration to perform the PMAG grounding to disable the PMAGs. If I was to use separate switches for this function instead of a keyswitch I would still want to keep these functions separate. I know this creates lots of switches related to the PMAGs but personally still think this is the best approach.
 
Figure somewhere between 5k-10k cycles on klixon units, the below is at rated load so closer to 10k for pmags.

Klixon stats (7277/7274):
Endurance 2,500 cycles: 120 VAC, 400 Hz inductive
5,000 cycles: 120 VAC, 400 Hz resistive
2,500 cycles: 30 VDC inductive
5,000 cycles: 30 VDC resistive
10,000 cycles: Mechanical, no load

So, if you fly every day of the year, and use the breakers each time, you may only get 27 years out of them.
 
My pmags will be wired with an on-on-mom switch which makes run up procedure pretty easy. Down the mag is killed, Center the mag is powered (‘contact’), Up the ship power is removed from mag and if engine dies, mag is not generating its own power. If engine keeps running, mag is generating power.

That's assuming you turned off the other mag first, otherwise the engine will keep running on the mag not being tested.
 
That is true…

That's assuming you turned off the other mag first, otherwise the engine will keep running on the mag not being tested.

However, if the internal alternator is not working in the one pMag that is having ships power removed, you will see an RPM drop the same as you would if you turned it off. If there is no change in RPM, all is good.

We have the three position Honeywell switches being discussed in this post, as well as a CB for each pMag. 300 hours and all works well.

The CB is used occasionally to remove power from the PMag when the master is on and a lithium battery charger is connected to the airplane. The PMags reportedly do not like the pulsing output of the battery charger.
 
The CB is used occasionally to remove power from the PMag when the master is on and a lithium battery charger is connected to the airplane. The PMags reportedly do not like the pulsing output of the battery charger.

Not a good idea for expensive avionics either.

Carl
 
That's assuming you turned off the other mag first, otherwise the engine will keep running on the mag not being tested.

True. But, only on 4 plugs if the tested emag isnt working. So, any change in rpm or sound indicates a problem.
Ah, what Katana said…
 
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Agree with the above, to test the system you have to hold one switch up while turning the other switch off, not intuitive at all.
.

Offering a contrary point of view, I also use the 2TL1-50 from Steinair with my dual P-Mag set up and I find it intuitive, ergonomically simple and it flows well on my checklist. When you stop and think about a mag check using the old rotary switch, when you switch to “L” you are actually controlling (killing) the right Mag and vice versa. Same deal with the P-Mags using the 2TL1-50 except it is more transparent to the pilot what is actually happening. The only real difference is that the test is a two-step process. The first step occurs automatically when you kill the other P-Mag to make sure the engine runs on the P-Mag under test. The second step is accomplished by switching the P-Mag under test to “TEST” using the momentary (up) switch position which ensures the P-Mag under test continues to run without ship power. My checklist reads as follows
Run Up 1800 RPM
L Mag – OFF, R Mag - TEST, L Mag - RUN
R Mag – OFF, L Mag - TEST, R Mag - RUN
Note: You do not have to hold one switch up while turning the other switch off.
 
Offering a contrary point of view, I also use the 2TL1-50 from Steinair with my dual P-Mag set up and I find it intuitive, ergonomically simple and it flows well on my checklist. When you stop and think about a mag check using the old rotary switch, when you switch to “L” you are actually controlling (killing) the right Mag and vice versa. Same deal with the P-Mags using the 2TL1-50 except it is more transparent to the pilot what is actually happening. The only real difference is that the test is a two-step process. The first step occurs automatically when you kill the other P-Mag to make sure the engine runs on the P-Mag under test. The second step is accomplished by switching the P-Mag under test to “TEST” using the momentary (up) switch position which ensures the P-Mag under test continues to run without ship power. My checklist reads as follows
Run Up 1800 RPM
L Mag – OFF, R Mag - TEST, L Mag - RUN
R Mag – OFF, L Mag - TEST, R Mag - RUN
Note: You do not have to hold one switch up while turning the other switch off.

Warning: Honeywell has discontinued some of their product lines, including this one. "This product is no longer manufactured", to be exact.

VV
 
Not that there is anything wrong with this but here is my take.

Why spend$140 on a set of switch that actually complicates things vs. $7 on a set of toggle switch that does the job simple and rather reliable.

Here is how I have done mine. I have two mini toggle switch that is ON-OFF which supplies the power to the PMAG, one for each PMAG. When the switch is to the OFF position, there is a LED light right above which will turn on, reminding me of
1- The +12v is alive from my fues/CB
2- Don't forget to flip it back to ON position after your PMAG test.

During the run up, I flip both to OFF and check the mag with my R-L-Both key switch.
If I ever want to crank the engine and make sure it does not fire up, I just flip both to OFF.

However, build what fits your way of thinking and liking.
 
Listen to Walt, please. Regardless of the specific component selection, try to design things so they operate as much like the theoretical "normal airplane" as possible.

Yes, I know the arguments. It's yours and you can wash it any way you want...but none of us last forever. Eventually, someone else will own it and fly it.

Perhaps my perspective is a little different, practically 100% of my customers purchased their RV and bring it in for a CI or panel upgrade.

For the panel jobs my goal is to make everything as "standard" as possible, so any pilot can get in and immediately understand the operation without having to go to training.

And you'd be surprised at how many folks come in and don't understand the systems installed in their new RV, they don't know what switches do or what indications mean, someone just told them turn this on, but they don't know what it does or what happens if they turn it off.

Often times I even have trouble understanding what folks did and end up spending lots of time in "discovery" mode trying to figure sh** out.
(multiple busses and pmags come to mind especially)

KISS is a really good principle, stick to it. Make is standard and obvious for the next owner.
 
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Walt,

I offer that no RV should follow a standard setup. There are far superior ways to do this besides duplicating a 1960 era Cessna.

If the plane has poor documentation, shame on the builder. If the new owner does not take the time to gain the system knowledge to get the most out of his new plane, shame on him.

We should not limit matching capability to mission just to satisfy the pilot that is too lazy to learn his own plane. If a pilot wants a simple, day VFR ship that is what he should buy (or build).

Carl
Two independent avionics busses
One “non-vital” buss
Dual pMags
And would not have it any other way….
 
Walt,

I offer that no RV should follow a standard setup. There are far superior ways to do this besides duplicating a 1960 era Cessna.

If the plane has poor documentation, shame on the builder. If the new owner does not take the time to gain the system knowledge to get the most out of his new plane, shame on him.

We should not limit matching capability to mission just to satisfy the pilot that is too lazy to learn his own plane. If a pilot wants a simple, day VFR ship that is what he should buy (or build).

Carl
Two independent avionics busses
One “non-vital” buss
Dual pMags
And would not have it any other way….

And I would suggest that increased complexity doesn't always equate to increased capability/reliability.

You never did post an answer to my question in another thread about your procedure for a 'smoke in the cockpit event', because turning off your dual masters doesn't actually kill power to the multiple busses?
 
And I would suggest that increased complexity doesn't always equate to increased capability/reliability.

You never did post an answer to my question in another thread about your procedure for a 'smoke in the cockpit event', because turning off your dual masters doesn't actually kill power to the multiple busses?

Procedures mean little without an understanding of how the system works - and I’m weary of trying to explain it as most don’t put in the time to do the study.

Carl
 
Sorry Walt, my last post was flippant.

The system has two PC-625 batteries, with backup modes to fully split the system so one battery powers half the panel, and each side of the panel can be powered from either battery.

The short version out of my POH for any electrical casualty (loss of alternator, smoke, over voltage or other weird stuff):
1. Open both masters. Risk analysis tells me this eliminates the worst outcome problems (the fat wires). Both sides of the panel are still fully powered for continued IFR flight.
2. If problem persists, open one or both avionics masters. Here odds are there will be some other indication of which side the problem is on (an EFIS blanking out, etc.)
3. Now the the casualty has ended, the pilot can opt to bring back the avionics master that was not effected - this restores one EFIS, one Comm and one GPS.

Carl
 
No issue here Carl, and I respect the design of your system from a redundancy standpoint, but from a safety standpoint not so sure.

A quick google search on 'smoke in cockpit', whether it be cirrus or any other model aircraft, will bring up multiple examples of it occurring, so the possibility is certainly there, no matter how careful your installation.

So, my only point is killing ships power in the event that you have smoke needs quick and decisive action, there is no time to troubleshoot at this point.

Killing all ships power (master contactor off) does that for you in a single action. If you have a G5 or backup battery system, you will retain important flight instruments even with the master off. The G5 has its own battery and something like a TCW will keep the PFD and AHRS alive, all you need to continue safely hopefully and buy you some time. Basically, these small back up batteries take the place of back up busses.

If you haven't tried it, I suggest you take a length of Tefzel wire say 10' long 20 ga coiled up and short it across a battery, try it in a bathroom or other small room to get the full effect. Let me know what you think after that.

Or maybe we could get fireman Dan to do a test for us, I know he likes playing with fire :D

I did the above "test" in an RV7 with a single non-protected wire off the battery (turning off the master had no effect), I can tell you it wasn't pretty, had I been flying when it happened panic would be just around the corner.

Hoping others can learn from my experience is all.
 
This is basically my setup as well (minus the LED's- but I like that idea). P-Mag internal alternator test for me is:

Both P-mag toggles off after achieving run up RPM. Then during run-up normal mag check: Both-Right-Both-Left-Both (If one side dies, the alternrator is not working on that mag). Toggles back "on", then complete other run up checks.

As I recall from the install/set up, you should also do a minimum operating speed (at least once for each side of the igniton) to know at what RPM your particular P-mags will not be able to produce sufficient power to operate should you lose the external power source.


Not that there is anything wrong with this but here is my take.

Why spend$140 on a set of switch that actually complicates things vs. $7 on a set of toggle switch that does the job simple and rather reliable.

Here is how I have done mine. I have two mini toggle switch that is ON-OFF which supplies the power to the PMAG, one for each PMAG. When the switch is to the OFF position, there is a LED light right above which will turn on, reminding me of
1- The +12v is alive from my fues/CB
2- Don't forget to flip it back to ON position after your PMAG test.

During the run up, I flip both to OFF and check the mag with my R-L-Both key switch.
If I ever want to crank the engine and make sure it does not fire up, I just flip both to OFF.

However, build what fits your way of thinking and liking.
 
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