Mag Check in Flight
 

My mechanic is chasing a mag drop in my front engine and chastized me for checking my mags in flight. I told him that's the only way I found the problem. I have checked mags in flight for almost 50 years. It's not a routine thing but I've never hestitated to make a check just randomly or when I thought I detected something.

He says the mag might arc inside the cap and damage the mag if checked at high power (75% cruise power) in flight. Anybody have any comments about not checking a mag in flight?

Mag Check
 

I had a problem with a rear mag last year, and my mechanic specifically told me to do an in flight mag check.

Keven
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mag check
 

This does not make a lot of sense.

down side : possible "backfire" or detonation.

upside: you can find a problem with a mag or cylinder and help isolate a problem......... especially if you have an engine monitor.

I do not see any harm. You will see an increase in CHT and EGT which is normal so make sure you are running well ROP if possible to avoid possible damage. I believe it delays the fuel burn causing an apparent "leaness".

bob

Mag check in flight
 

I do check my mags in flight now and then and never experienced adverse reaction. My IA does not seem to be bothered by that practice.
Jerry

That makes zero sense. It can't arc or crossfire if it is turned off. There is no secondary voltage. Does the mag know it is airborne? Is it any different than a high power mag check on the ramp?

Bawb

I too occasionally check the mags in flight.

The only way I can see that you're going to have any arcing in the mag (in normal operations) is if the capacitor is u/s...then it's going to arc whether you're in flight or not, and not just when you're checking the mags! I have experienced a mag failure in this way - capacitor u/s, leading to arcing, leading to a dead mag. It is hard to find - since the mag appears to run fine...until it doesn't!

Mind you, for the same reasons Bob mentioned to ensure you're well ROP, I would recommend lower power settings when checking the mags in flight. Running on one mag will reduce the detonation margin.

Cheers,
Damon

mag check
 

BTW

Altitude is a consideration as there is much greater chance of crossfire while in "thin air". You can get carbon tracks on the inside of the cap, however, only way of proving this is by inspection. Once started it will only get worse.

Condensation in the mags can cause xfiring until sufficiently dried out. All the more reason to pre heat engine in the winter. Just need one carbon track to create a permanent problem requiring a cap replacement.

If not mistaken there is a greater chance of crossfiring due to load on the engine. A worn rotor can have an affect as well.

A bad capacitor will cause the points to pit and wear prematurely. I don't think a failed cap (open) would cause much change in the potential )voltage). A shorted cap......... dead mag.

bob

I have heard what he said before. I don't know enough to suggest pros or cons, but I've taken a practice of not checking them inflight (at least at a higher power setting than that recommended during runup) unless there's an emergency and I needed to switch from one to the other to see if it solved a problem.

I did have an interesting experience in a Piper Turbo Arrow a few years back. They are notorious for developing arc-ing in the mags over 12,000 feet unless a $250 (at least that's what it cost then) "pressurized mag kit" was installed. All it consisted of was a hose running from the turbo to the mags to keep the ambient pressure higher - which in turn made less resistance across the cap than across the points and hence discourage arcing.

I had asked the FBO about this before taking the plane high, and he didn't know if it had the kit or not. So I rounded up an A&P to check and it did not. However the FBO owner assured me that it was "no problem" and that if I didn't get up to 18,000 or more it wouldn't be a factor. (In those days I was more willing to take someone's word on such a thing than I am now...) So off we went. Early in the AM, still dark and at 14,500 over central Nevada, on O2, cruising along with a cloud deck at about 12,000 below, bingo, the mags start to arc. Really gets your attention. It turned out OK in that the airplane flew just fine on into Elko (1/2 hour flight), even holding the altitude just fine. But that was the exeperience that made me decide I was going to fly twins. For IMC, night, and with family aboard, I don't like the idea of one mill quitting without having another to depend on.

I quit using that FBO. They never did find the problem, even though I told them what it surely was. I talked to the A/C owner a year later and it was still running rough. I told him to have them replace the mags (or caps or whatever) and have them stop fooling around with the fuel flow and etc. Don't know if he ever got them to admit and solve the real problem.

I believe there is some risk in doing a mag check at high power settings. I was cautioned not to do that by someone I trust. I will check into this, see what I can learn, and report back. It had something to do with rapid changes that could occur if the mag you switched to was bad. Can't remember...

Kevin

Further Explanation
 

Earlier in this thread I stated that my IA specifically told me to perform an in-flight mag check. The rest of the story is that he told me to do this because I was starting to get RPM fluxuation at 5000'. So, he asked me to go up to 5000' and perform the mag check and see what resulted.

Don't know if or how this additional info helps, but I wanted to clarify my circumstances.

Cheers,

Keven
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mag check
 

KEVIN

Agree with you....Would less likely do it with a turbo because of possible damage to downstream components such as waste gate.
Personally ... only for diagnostics.... at <70% power... ROP... and for short duration.

bob

Here's the scoop from my previous mechanic, now retired. This gentleman REALLY knows turbocharged twins and singles, his was the premier shop for these type aircraft in our area until he retired. Here is what he had to say about in-flight mag checks:

"As to the in-flight mag check. It is a great deal of risk. If you have a Mag that is "dead" the engine goes from making 65% power to being driven by the prop, Negative 10%? The real problem is when it does that you take a chance of detuning the engine, i.e. damaging the counter weight bushings and pins. The procedure that I like to use to check mags in-flight is to reduce the power to about 25% power and check the mags, if both mags work at that power then increase the power to about 50% and do the same thing then increase to your cruise power and check it again. IF IN ANY OF THESE CHECKS IF THERE IS A "DEAD" MAGNETO, CUT THE MIXTURE TO THAT ENGINE AND WAIT ABOUT 2 TO 3 SECONDS AND TURN THE MAGS BACK TO BOTH AND THEN SLOWLY RETURN THE MIXTURE BACK WHERE IT WAS BEFORE THE TEST. Don't forget that at cruise speeds you may not see a drop on the in the rpm as the prop governor makes blade angle adjustments to compensate. Even is a Mag is "dead" the rpm may not droop very much. If you haven't got the message yet, this procedure is dangerous to the pocket book. You only use this procedure when you suspect that you have a single cylinder shutting down under high power, and in conjunction with your multi-probe EGT gauge you can determine what mag, what cylinder."

FWIW.

Kevin

mag check
 

Thank all of you for the input. I failed to mention in my original request that I have a 337G normally aspirated engine. Perhaps the turbo is more sensitive but the premises mentioned should apply to both engines. I think my mechanic will agree with everything Kevin says in his response right above this one - do it with great caution.