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You are flying level at 5500 feet and 65% power set in an aircraft with a constant speed propeller. The air is a little turbulent and the airspeed is fluctuating somewhat. Your wife asks a question at the same time as one magneto fails completely.  What instruments will give an indication that the mag has failed.

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EGT gauge. Because only one mag is working, the mixture is only being ignited on one side of the combustion chamber.

 

This results in the flame front having to be propagated across the entire combustion chamber, from just the one plug.

 

This, in effect, is similar to retarding the spark, and will result in lower power output, and a higher EGT registering on the gauge.

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Been there, done that. C172 O300C. Indicated by very rough running.  First mag failed and second very weak. Luckily at 5000ft after long climb and still close to home base. Was planned to fly over divide to Benalla!  Mags (slick) were supposedly overhauled about 10hrs previous and after inspection found to have damaged parts installed. Now fly rotax

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If you have 2 good mags you probably will not notice a slight decrease in engine performance. My taco is only connected to 1 mag so I may loose that if the right one fails but this is an unusual setup.

It's why we have to do mag checks because if we dident most people would fly around unaware one had stopped working a long time ago.

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The ASI will show a reduction in airspeed. The engine has lost a bit of power, and the constant speed prop has gone a little finer to keep the rpm's up.

It wasn't a Jabiru engine in which you would not notice anything, not that Jabiru's have those fancy props.

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Tried this yesterday.

 

190 kts IAS, 2000', 2400 rpm (constant speed prop), 36lph (50 deg Lean of peak on richest cylinder). IO360.

When I turned one of the mags off the engine when very rough the EGT started to rapidly rise. I did not leave it running on one mag for more than a few seconds because of how rough it got, Both Mags behaved the same.

So for me if I loose a mag I will notice straight away.

Not what I expected.

Edited by SplitS
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interesting splitS

gather it had passed mag run-up tests with ease ?

when u got it back on the ground, any chance you did another mag test to see if it still passed mag run-up test ?

 

 

 

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It's to be expected when you are running lean of peak. Richen it up and do a retest but Mag checks at cruise power are not recommended. A backfire at those power settings can ruin a motor. Any roughness is not good. It's not a happy motor then even doing it for a short time. Nev

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SplitS. I read what you are saying as that turning off one mag resulted in roughness and high EGT, but I don't understand that you say both mags behaved the same. Are you saying that if you cut the right mag off, you get rough running and high EGT and exactly the same if you cut only the left mag off?

That means that the smooth running needs both mags to be fully operational. Is that what you were saying?

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I should be clearer sorry.

The Mag test great on the ground run, you have to pay really close attention to even notice a change between both, 1 or the other.

When I did the air test both mags behaved the same way.

I wont do it again.

to quote facthunter "never stop learning"

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  • 3 weeks later...

I aborted once a takeoff as the engine developed less than 2000rpm in the t/o roll.

 

Then the 2nd mags check on the ground confirmed fouling which was cleared with high rpm's and leaned rich of peak for a short period of time.

 

Since then, I agressively lean the engine from the moment the engine is started until takeoff, then lean again in the cruise, and before landing.

 

New sparkplugs helped a lot!

 

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PA28 no CSP,i was flying gave a slight shudder and i noticed EGT begin to rise.Mag gear had sheared apparently,nylon?i didn't get to see that.

cheers

colin

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They can do that.  Reduce throttle and try each mag separately. It will often run quite acceptably on the good one but leaving the damaged one still working mucks it up. I think all Mags have to be serviced every 5 years regardless of TIS. Nev

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  • 5 months later...

I heard Huey pilots in Vietnam would rotate all the engine instruments in the panel, so the needles all pointed straight up....you could see a problem with one quick glance.

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3 hours ago, F10 said:

I heard Huey pilots in Vietnam would rotate all the engine instruments in the panel, so the needles all pointed straight up....you could see a problem with one quick glance.

That has been discussed before in reference to rally cars- where the driver has even more reason to keep his eyes outside...

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Yeah, in my experience, fortunately not often...but once you have a few hours on an aircraft, when something changes, it’s the changed needle position that first gets your attention, rather than reading the gauges. Modern systems have attention getters and bells and whistles now.....

Edited by F10
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