Mag Check

J

JOhnH

Touchdown! Greaser!
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Twice in the past few months, after a rather long taxi, the mag check for the left mag failed. I will check with my A&P when he gets back in town tomorrow, but for now I thought I would run it by you guys.

Cessna 172n w O360

I have a digital tach and when switching to one mag (at 1800 rpm) the tach normally displays the drop (eg: -45). But these two times the display went to dashes instead. Couldn't really hear any problems and engine kept running smooth.

We ran the engine up to 2000 and leaned for about a minute, then tried again. Failed (dashes) again. Then taxied back to hangar and tried again. It worked fine. Taxied to the runway run up area again and did the run up and mag check and all worked fine so we took off.

There were about 5 flights in between these two instances.

Does this sound dangerous?
Could this be a malfunction of the tach instead of the mag?

I have to wonder if this is a latent mag problem that may have been causing my high CHT problem. (as you can tell, I am not an aircraft mechanic but timing has been mentioned as a source of CHT problems).

I probably wouldn't have posted if I had not read Jay's story about his mag failure and his speculating about winding up in the water.
 
Arbiter419 said:
Do you lean when you taxi?
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My first thought, too. Nothing like a long taxi at minimum power with full rich mixture to foul a plug. However, if a "hot run" didn't clean it off, and it cleared itself mysteriously after shutdown/restart, it might be something mechanical. Having your mechanic check it out thoroughly is a good idea. And yes, a mag timing problem can result in high CHT's.
 
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JOhnH said:
Twice in the past few months, after a rather long taxi, the mag check for the left mag failed. I will check with my A&P when he gets back in town tomorrow, but for now I thought I would run it by you guys.

Cessna 172n w O360

I have a digital tach and when switching to one mag (at 1800 rpm) the tach normally displays the drop (eg: -45). But these two times the display went to dashes instead. Couldn't really hear any problems and engine kept running smooth.
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How would a failed mag (or fouled plug(s)) cause the tach to stop reading completely with the engine still turning?

Where does the tach pick up it's signal?
 
Yep, the EI tach I have connects to the p leads at the ignition switch.
 
Skylane81E said:
Yep, the EI tach I have connects to the p leads at the ignition switch.
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So, when you shut off one mag and are not getting a signal from the other, that would result in an indication of --- (engine off) ? That suggests an open P lead (or open lead from the mag switch to the tach). The sometimes works, sometimes doesn't would be consistent with a broken wire that sometimes makes contact.

Can you say "Hot Prop"?
 
Capt. Geoffrey Thorpe said:
So, when you shut off one mag and are not getting a signal from the other, that would result in an indication of --- (engine off) ? That suggests an open P lead (or open lead from the mag switch to the tach). The sometimes works, sometimes doesn't would be consistent with a broken wire that sometimes makes contact.

Can you say "Hot Prop"?
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When you switch to "mag check", the display shows the drop, not the engine rpm. When you switch back to run, the display again shows the rpm. Two different displays.

And thanks to the rest of you for your comments. I didn't mention this, but on these two times the weather was quite cool and we didn't lean, so that sounds like the answer.

And let me make this disclaimer: (I used to say this all the time but I got tired of repeating myself so now I only say it occassionally). I gave up my ticket years ago for medical reasons. 25 years later, (at 55 yrs old) my wife decided she wanted to learn to fly. One year later she had her ppl and two months after that I bought her this plane. So SHE is the pilot, and she is a good one. But she is less of a mechanic than I am. (Another disclaimer, I used to be a pretty good auto mechanic, but I haven't turned a wrench in 20+ years).
 
Nothing better to teach ya to lean aggressively on the ground than high altitude airport ops.

Come on up for a visit! :)
 
When the engine did not show any indication of ignition problems, it is the indication system.

If it were the mag, "P" lead, or ignition switch the engine would feel it.
 
denverpilot said:
Nothing better to teach ya to lean aggressively on the ground than high altitude airport ops.
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I believe you. We are rarely over 40 ft msl on the ground. The first time we (she) landed at PDK (Atlanta area/elevation ~ 1,000ft) she thought her altimeter was broken.

Come on up for a visit! :)
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Thanks, we'd Love to, but we are planning to head to Phoenix soon and that is about 1,000 nm out of the way. We are gonna find a place this side of the Rockies and try to get some "real" mountain flying training for a few days before proceeding. I say "real" because we did go up to North Carolina and get some "hill top" flying training a few years ago. I hear the Rockies are a little different.
 
Let me guess: its a Horizon digital. yes?
 
Ron Levy said:
For those digital tachs, IIRC, the mags.
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Personally I HATE digital tachs.... The numbers flicker so much it is really hard to pin down an accurate reading.. Give me a regular tach anyday.... YMMV.
 
N801BH said:
Personally I HATE digital tachs.... The numbers flicker so much it is really hard to pin down an accurate reading.. Give me a regular tach anyday.... YMMV.
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+1...
 
N801BH said:
Personally I HATE digital tachs.... The numbers flicker so much it is really hard to pin down an accurate reading.. Give me a regular tach anyday.... YMMV.
Click to expand...

I have to admit it took me a while to get used to it. But it sure is accurate, assuming that hand held thing the mechanic uses is accurate.
 
Did the tach do this on one or both mag checks?

I suspect that there are two leads from the mag switch to the tach. When the switch is set to L, for example, it ground the right mag, and vise versa. Grounding the mag would kill the signal from that line, so there would be two leads that feed back-to-back diodes inside the tach to get the RPM, and one of these diodes might be loose or intermittent. Or one of the lines might be loose at the switch or tach end.

Dan
 
"I have a digital tach and when switching to one mag (at 1800 rpm) the tach normally displays the drop (eg: -45). But these two times the display went to dashes instead. Couldn't really hear any problems and engine kept running smooth."

This is the part that has me confused...the engine keeps "running smooth" but the indicator failed. It seems to be an indicator failure, as was stated before, NOT a mag failure. Forget the indicator for a moment, when you went to each mag on the check
did you hear and/or feel an RPM drop? What do you actually mean by "engine kept running smooth"?
 
Did the tach do this on one or both mag checks?
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Just on one mag. The other one showed a -65 drop.

did you hear and/or feel an RPM drop? What do you actually mean by "engine kept running smooth"?
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I know that is a subjective statement. And to tell the truth, I never actually hear or feel much difference when running a mag check. That is why I rely on the tach. The manual for the tach is in the plane. I am going to dig it out next time I am out there and see if it says what the " - - - - - " display is indicating. We will also make sure we lean more aggressively during taxi. And if no problems, I will add this to the list to be checked during the annual in May.
 
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