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Restarting dual Pmags

cojaken

Active Member
I have been running one Pmag and one Slick on my RV-9A O-320 for about 150 hours and 18 months now. I am very happy with the Pmag.

It is about time to inspect the Slick and I am considering replacing it with a second Pmag.

I have been searching for discussions on "robustness" of dual Pmags, that is, dependability in all scenarios.

If I have an engine out due to a fuel problem, which I manage to solve, and an electrical problem that I cannot solve, will I be able to restart the engine?

I would assume that without power to the Pmags, as long as I can get the prop to turn at 800 rpm, they will self generate and begin to produce a spark. But will I be able to get the prop turning at 800 rpm?

I may be overthinking this and I would appreciate the thoughts of the Airforce.
 
If I have an engine out due to a fuel problem, which I manage to solve, and an electrical problem that I cannot solve, will I be able to restart the engine?

I may be overthinking this and I would appreciate the thoughts of the Airforce.

Well, since you asked-------my $0.02 is;

To even dream of this kind of low probability dual failure mode, it sounds like you do not really trust the Pmag even after flying with it for a year and a half.
 
Thanks Mike. I have full confidence in the Pmag.

None of the failures I imagined were of the Pmag itself, but imagining these dual failure modes in the airplane may be paranoid?

Anyway my question is, would I be able to get the prop to turn at 800 rpm?
 
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Do you have a constant speed prop, or a FP? With my constant speed on my RV8, I never saw less than 1000 RPM in flight with the throttle at idle (not so with my current FP O-360/RV4 though). If you loose your engine, that might not be the case, but typical windmilling RPM is probably north of the minimum required to sustain the Pmag alternator. Easy enough to check if you are comfortable with your dead stick landing capability. Get high above your home airport (5,000+ feet), turn off your mag, then turn off your master switch. It’ll get quiet on the radio (depending on back-ups), but your engine should continue to run. Slow to your minimum glide speed, check your indicated RPM, then see if your engine is still operating (try increasing power). If it is still operating, slow down to see what low RPM is needed to power that ignition. If the engine quits, note the RPM, set mixture to ICO, throttle slightly open, then if the prop is still spinning, turn on the mag. If the prop is not spinning anymore, leave the non-impulse mag off, Pmag on, and engage the starter. Slowly richen the mixture (may need your boost pump), and restore power. If you can’t get the fan spinning, exercise your honed dead stick skills to a successful dead stick landing.

However, if you have a battery, or back-up battery connected to one of your Pmags, it should still operate no matter the RPM, as long as your battery is still good.
I had one Pmag and one Slick on my last RV8 originally, but then went to two Pmags. One of the Pmags was connected to my back-up battery to provide redundant power supply. If I lost my alternator, I had both Pmags powered by my primary battery. When that battery dropped enough, I had one Pmag powered by my back-up battery, and the other one still powered by my main battery, until it got down to a voltage that wouldn’t support it anymore. The Pmag on the back-up had much more endurance because there wasn’t much load on that back-up bus. The Pmag doesn’t draw very much. At max RPM, it’s only .25 amps and should operate on as little as 9 volts. Main battery power should keep the engine running on both Pmags for quite a while.

When I did my min RPM check, my engine (IO-360) continued to run without ships power down to 750-800 RPM. Pmag mentions 900 RPM based on engine/RPM indication variances. You should respect that minimum 900 RPM recommendation.
 
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Bob Nuckolls says.
Things fail.
Design a system so that when things fail, it presents no immediate hazard.
Things needed for comfortable termination of flight require a backup.


When evaluating a system, dual failures are generally not considered. Rather Look for a single failure that can cascade into multiple failures.

For example: an alternator is installed that is internally regulated with no over voltage protection. When it fails to an over voltage situation, it will put out 30V. If this is paired with an Earth X Battery, then the Earth X is programmed to shut itself down and cascades into multiple failures.
 
If I have an engine out due to a fuel problem, which I manage to solve, and an electrical problem that I cannot solve, will I be able to restart the engine?

I would assume that without power to the Pmags, as long as I can get the prop to turn at 800 rpm, they will self generate and begin to produce a spark. But will I be able to get the prop turning at 800 RPM.


I think you are overdoing/overthinking this. Dual/multiple critical system failure (fuel and electric) is not even covered in the certification process for certified aircraft (if i remember correct, if not, please step in).

Concerning the complete electrical failure, your engine will continue running on PMAGs as long as the RPM is more than 900-ish.
Once the RPM is below the value which is required for the internal Alternator to produce the necessary electric power, and due to the shortage of electrical power due to complete electrical system failure, your engine will remain silent.

The PMAG manual says:
Both the E-models and P-models need outside electrical power to start. You cannot prop-start the engine with either type ignition if the battery is missing, or totally dead. However, a low battery that barely “bumps” the starter motor, or can only “click” the solenoid will likely have enough energy to power


Cheers.
 
I think it's good to question stuff like this, but that scenario seems really unlikely.
The only way I can imagine it was if the causal chain looked something like this:

1. charging system failure
2. either over voltage frags the battery or you fly long enough for it to go dead
3. continue flying until any sort of backup battery is also dead
4. run a tank dry
5. switch to not quite empty tank
6. prop is windmilling less than min rpm for p-mags to stay self excited while you were switching tanks.


It seems like the best way to deal with it is to just say "if I ever have a total electrical failure, I'm going to land before theres any chance of running a tank dry."

I'm going with dual P-mags. I think the technology is there at this point.
 
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My opinion is you’re overthinking it. Two unrelated critical system failures on the same flight? Extremely unlikely unless you’re fighting in a war zone. LOL

I have 2 Pmags, and Precision fuel injection. During flight testing, I killed the engine once with the mixture control when at idle and best glide speed. The engine continued turning at same rpm. As soon as fuel was re-introduced, the engine started to run normally without using the starter.

Proven by testing on the ground, the Pmags do not require power from the main battery to run when at 1000rpm (or a bit less). So they’re not using ships power anyway normally.

I love my dual Pmags.

Bevan
 
Bob Nuckolls says.
Things fail.
For example: an alternator is installed that is internally regulated with no over voltage protection. When it fails to an over voltage situation, it will put out 30V. If this is paired with an Earth X Battery, then the Earth X is programmed to shut itself down and cascades into multiple failures.

Had similar situation several years ago. Alternator failure with 100+ Volt spike. Earth X shut down (temporarily, but I found that out on the ground), both EFIS fried. Multiple other modules fried including LEMO powered headsets. The dual P Mags continued to work flawlessly until clearing the runway after landing. When the RPM dropped on rollout, the engine quit.

The PMAGS required service. Was told there is an internal diode that protects them in an over voltage situation but has to be replaced at the factory.

Several changes have been made on our plane to prevent this from happening again, but the PMAGS performed well in my opinion.
 
Battery on a switch?

I think you could easily add a 9v battery to the p-mag, put a switch on it. In the event it does not spark, flip the switch and it will spark... from what I have been told, back country pilots have used 9v batteries have been used to hand prop when starter battery was dead...
 
I have been searching for discussions on "robustness" of dual Pmags, that is, dependability in all.

Bill Harrelson completed his around the world flight over the poles in his Lancair with dual eMags. That’s likely one of the most brutal tests for an ignition system, let alone the complete engine package, with extreme hot and cold temperatures along the way.

They work, they weigh less than a mag, much easier to time, cheaper/better spark plugs, easy to install, self powered once started.
 
I think you could easily add a 9v battery to the p-mag, put a switch on it. In the event it does not spark, flip the switch and it will spark... from what I have been told, back country pilots have used 9v batteries have been used to hand prop when starter battery was dead...


I've heard the same thing and have thought that might be a cool feature. But I was talking to an avionics guy who used to work for brand G and now runs a shop that does extensive work with with experimental aircraft. I asked him his opinion on including analog engine monitoring in the event you needed to jump start, hand prop, whatever.

His take was that it was cheaper and lighter to go all glass and add a $200 backup essential battery to run that stuff, in which case you have a backup source for energizing your p-mags without messing around with 9v batteries etc.

If I was planning on an analog solution I'd re-visit the 9v feature just because it's kind of a neat idea, but he convinced me that starting from scratch, theres a cheaper/lighter/better way that doesn't give up functionality or redundancy.
 
No one has answered the OP's question: Will the engine windmill at 800 RPM ?
I do not know the answer, but my guess is yess, in a slight dive. This sounds like agood thing to test hight above the pattern.
My duel P-mags externl power supplyand my digital tach are both tied ito my master, so I would not be able to check my engine RPM, but I would find out if I can get a re-start.
 
Thanks everyone for your observations, which greatly helped my thinking process, resulting in a decision to go with the second Pmag.

I ordered it on Monday and it arrived Tuesday! They are no longer gold colored, but black, The removal of the gold has not reduced the price :)

I am intrigued by the idea of using the EFIS backup battery as backup power for the Pmag. Anybody have an electrical schematic addressing that?
 
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