Mag check question

[par]

I have a question for you guys regarding the mag check. It used to be that my left mag would show about a 60-80rpm drop while the right mag usually showed around 90-100. However, I have noticed that they are both showing about a 90-100 rpm drop over the last couple of flights. Is this something I need to be concerned about?

Edited April 3, 2017 by par

[N201MKTurbo]

As the point followers in the mags wear the internal timing changes. This changes the amount of power the mag produces and it's timing to the engine. 

You might want to check the mags to see if there are loose. The timing could have slipped. Loose mags are bad.

[Yetti]

How many hours on the mags?

[N201MKTurbo]

It could just be a flaky plug, but you can usually feel that.

[gsxrpilot]

An inflight mag check would tell you a lot more.

[Hank]

The runup for flight that my left mag failed on gave ~30-40 rpm drop. But the failure was poor Kelly rebuild, leaving the condenser wire touching the gear, and it was cut in half, maybe an hour into the flight. There's more to magneto health than runup and inflight checks.

[Guest]

Internal wear, poor maintenance or assembly, incorrect timing to the engine, incorrect gaskets and mounting clamps are all possible cause.

Clarence

[DXB]

Are such small changes in RPM on mag check as described by the OP really a cause for concern in absence of any other problem?  I can't say I pay close enough attention to my precise rpm drop that I would ever notice such a small change.  On a mag check before takeoff, I mainly look to make sure that all 4 EGTs rise when on one mag and come back down when returning both, and that there's no obvious roughness on either side alone.  

I do my run ups full rich per the old school POH instructions and see drops in the 150-200 range.  I imagine it would be less if i kept it leaned for the runup. So subtle variations based on mixture and other factors from day to day may contribute to variability? 

[Hank]

30 minutes ago, DXB said:

Are such small changes in RPM on mag check as described by the OP really a cause for concern in absence of any other problem?  I can't say I pay close enough attention to my precise rpm drop that I would ever notice such a small change.  On a mag check before takeoff, I mainly look to make sure that all 4 EGTs rise when on one mag and come back down when returning both, and that there's no obvious roughness on either side alone.  

I do my run ups full rich per the old school POH instructions and see drops in the 150-200 range.  I imagine it would be less if i kept it leaned for the runup. So subtle variations based on mixture and other factors from day to day may contribute to variability? 

The Manual for my C says 150 RPM drop max, and no more than 75 difference between the two.

Edited April 4, 2017 by Hank

[N201MKTurbo]

I wouldn't rush out and buy new mags, but the fact that it changed is the worrying part. I would check them for security and throw the timer on them. If everything was within spec I wouldn't worry about it, just keep an eye on it.

[par]

56 minutes ago, DXB said:

Are such small changes in RPM on mag check as described by the OP really a cause for concern in absence of any other problem?  I can't say I pay close enough attention to my precise rpm drop that I would ever notice such a small change.  On a mag check before takeoff, I mainly look to make sure that all 4 EGTs rise when on one mag and come back down when returning both, and that there's no obvious roughness on either side alone.  

I do my run ups full rich per the old school POH instructions and see drops in the 150-200 range.  I imagine it would be less if i kept it leaned for the runup. So subtle variations based on mixture and other factors from day to day may contribute to variability? 

This was my thought. The engine runs absolutely perfect and there is nothing obvious that stands out. I'll going to have my IA check if everything is secure and possibly check the timing. If all looks good, I'm not touching anything else.

[DXB]

56 minutes ago, Hank said:

The Manual for my C says 150 RPM drop max, and no more than 75 difference between the two.

This Mike Busch article has been the basis for my not making too much out of the precise numerical RPM drop: 

http://blog.savvymx.com/2010/03/mag-check.html

Back when I had no graphic monitor and just an analog rpm gauge, I'd eyeball a drop in the 100-200 range,  but I could really only discern a problem if it was way off from this, which would likely also make the engine run rough.  Now I suspect with a digital RPM gauge, it's far more precision than was ever intended per the POH criteria. According to Mike Busch, the most pertinent info is the direct readout of ignition system integrity based on the EGTs.  It makes sense to me that this is now the best available readout one has when on the ground. An excessive drop may also reflect overly retarded timing on that side, but the OP's drop doesn't sound excessive.

Of course I'm no expert, and I'm not saying what Mike Busch says should be taken as gospel - I'd certainly be curious to know  if the logic is faulty here. 

[carusoam]

Each part of the run-up tells us a lot of info...

the engine monitor can show us

- a mag failure.

- a plug failure. Or a dirty plug

- a switch failure.  Both mags always on.

- mag timing issues.  Excessive drop in rpm.

- the POH usually gives two math calculations.  The drop of rpm between both and L,R and the difference between these drops.

- showing an EGT rise on one mag is a good hint the one mag is working well while the other is shut down.

- expect wear of a mag will or may start showing itself after 500hrs.  See how long your mags have been in  can be OHed... ask for recommendations...

- exercising the prop is pretty good. it gives a few hints about the health of the governor, it's oil pump and it's valve and if it has any internal leaks. If you exercise the blue knob and nothing changes... know that your day just got longer...

Did I miss anything or get anything wrong?

Best regards,

-a-

 

Edited April 4, 2017 by carusoam

[Skates97]

1 hour ago, Hank said:

The Manual for my C says 150 RPM drop max, and no more than 75 difference between the two.

Lycoming issued a Service Instruction that changed that to 175 max and 50 difference between the two on June 18, 2010. They also give steps to take to lean and then recheck if necessary.

Lycoming Service Instruction No 1132B

[Shadrach]

Two well performing mags that are precisely timed will show almost no drop between them. A full rich run up mag check is not a very robust test of ignition health. It's good in that it verifyies function and continuity. Testing in flight at high power (especially LOP) will often reveal when a mag is underperforming.

[Wakeup]

17 hours ago, gsxrpilot said:

An inflight mag check would tell you a lot more.

I have never done an inflight mag check.  How do you do it, besides the obvious?? 

Thanks,

Troy

 

[Shadrach]

3 hours ago, Wakeup said:

I have never done an inflight mag check.  How do you do it, besides the obvious?? 

Thanks,

Troy

 

Just like a mag check on the ground except at cruise power (WOT and 2500RPM in my case). Set your mixture as lean as you can while still maintaining smooth operation. 

https://www.savvyanalysis.com/articles/in-flight-diagnostics

 

[DonMuncy]

9 minutes ago, Shadrach said:

Just like a mag check on the ground except at cruise power (WOT and 2500RPM in my case). Set your mixture as lean as you can while still maintaining smooth operation. 

https://www.savvyanalysis.com/articles/in-flight-diagnostics

 

Just remember that if one mag setting completely shuts down the engine, pull the mixture back before switching back to both. Then advance it after restart.

[Shadrach]

Just now, DonMuncy said:

Just remember that if one mag setting completely shuts down the engine, pull the mixture back before switching back to both. Then advance it after restart.

This is indeed sound advice.  One would have to disregard a lot of symptoms to not realize they were running on one mag before doing the check (decreased performance, Significant increase in EGTs, decrease in CHTs etc.)...

[par]

37 minutes ago, DonMuncy said:

Just remember that if one mag setting completely shuts down the engine, pull the mixture back before switching back to both. Then advance it after restart.

Just to clarify, you are saying that we should shut down the engine completely as we would on the ground before selecting both and attempting a restart with the mixture full in? How would the engine start with the mixture pulled all the way back? This may just be a stupid question but it's a good idea to clarify these things in a 1-g environment on the ground.

[carusoam]

Nooooo.....

he is saying...  in the event the engine gets shut down, don't let it run fuel into the exhaust system while getting it restarted.

finding a dead mag at altitude will kill the engine when the dead mag is selected.

the prop will be spinning, but no fuel will be burning.  Raw fuel will be delivered out through the exhaust.

you know, Booom, what I mean?

PP thoughts only, not a mechanic or CFI...

Best regards,

-a-

Edited April 4, 2017 by carusoam

[Andy95W]

The engine won't start with the mixture pulled out, which is good- with the magnetos off but the fuel system still putting gobs of fuel into the engine, that fuel winds up in the muffler which will give a helluva backfire and can damage the muffler.

In flight, the propeller will keep windmilling. With the mixture pulled out, the fuel will clear from the engine and muffler.  You turn the magnetos back to both and slowly push the mixture back in.  The engine will resume combustion on its own. 

That whole process will only take about 5-10 seconds.

[par]

Just now, Andy95W said:

The engine won't start with the mixture pulled out, which is good- with the magnetos off but the fuel system still putting gobs of fuel into the engine, that fuel winds up in the muffler which will give a helluva backfire and can damage the muffler.

In flight, the propeller will keep windmilling. With the mixture pulled out, the fuel will clear from the engine and muffler.  You turn the magnetos back to both and slowly push the mixture back in.  The engine will resume combustion on its own. 

That whole process will only take about 5-10 seconds.

Got it. That is the explanation I needed.

[DonMuncy]

Andy said it better than I did.

[gsxrpilot]

One thing to note, an inflight mag check should take some time. Assuming the mag isn't dead and the engine keeps running on a single mag, leave it on each of the single mags for 30 to 60 sec. If you have a good engine monitor, put it in normalized mode first. This levels all the bars. As you switch to a single mag, notice all the EGT's rise together. Let them stabilize. Then back to both, again until EGT's stabilize, then the other mag. 

I typically do this at altitude in cruise LOP just before I start my decent.