Does anybody perform an inflight, high-power, lean-of-peak mag check

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woxof

Pre-takeoff checklist
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woxof
I am reading an article by John Deakin and he discusses doing this. Never heard of it and I am wondering what the advantages are.

Thanks
 
I do it every flight before i descend. Really good way to find out if you have a mag issue!
 
It's a way to check mixture settings.
 
I have done it a few times


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Can’t get mine to run worth a damn LOP, so I’m a’gonna say, probably not. :)
 
Tom-D said:
It's a way to check mixture settings.
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Is there an advantage to doing this kind of mag check versus a regular one on the ground as per the POH?
 
mscard88 said:
Good article WannFly. But what if you're flying flight school Cessnas for instance without an analyzer?
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without analyzer, there is really no point doing any of these. stick to POH is what i would do if i dont have engine analyzer. if there are any issues performing whats being discussed in the air, you want to see if anything redlines or not and if it does, take corrective actions before things go kaboom.

b/w nice avtar
 
woxof said:
I am reading an article by John Deakin and he discusses doing this. Never heard of it and I am wondering what the advantages are.

Thanks
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A lean mixture is harder to ignite so don't be surprised if you lose power or it runs rough if doing this LOP. However there is no reason you can't do a full power mag check. Maybe just do it rich of peak.

I was about to embark on a long journey in a 152 from NC to Ohio and it had a questionable mag check on the ground. I ran it up and leaned it out to clean up the plugs, then repeated the mag check. Better. So I took off. 10 mins into the flight as I am getting settled in I start thinking about the mountains i'm going to cross and that mag. I decided to try another mag check real fast.... woah nelly it was running rough as hell on that one mag at high power settings. Could barely maintain altitude. I RTB'd and the club mechanic happened to be working. Turns out the mag was totally shot.
 
Yes, I've done them before. It's the best way to test if you have any sort of issues with your magnetos. The normal mag check on the ground is a very basic check.
 
luvflyin said:
How does that work? What are you looking for
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http://content.savvyanalysis.com/static/pdf/SavvyAnalysisFlightTestProfiles.pdf

From the original article though:

"The in-flight mag check is performed at normal cruise power and normal lean mixture (preferably LOP). Run the engine on each individual mag for at least 15 or 20 seconds. Ensure that all EGTs rise, that they are stable, and that the engine runs smoothly on each mag. If you see a falling or unstable EGT, write down which cylinder and which mag, so your mechanic or SAMM account manager will know which plug is the culprit. "

You're looking to do the test and then have someone who knows what they are looking for look at the data...lol
 
BGF_Yankee said:
http://content.savvyanalysis.com/static/pdf/SavvyAnalysisFlightTestProfiles.pdf

From the original article though:

"The in-flight mag check is performed at normal cruise power and normal lean mixture (preferably LOP). Run the engine on each individual mag for at least 15 or 20 seconds. Ensure that all EGTs rise, that they are stable, and that the engine runs smoothly on each mag. If you see a falling or unstable EGT, write down which cylinder and which mag, so your mechanic or SAMM account manager will know which plug is the culprit. "

You're looking to do the test and then have someone who knows what they are looking for look at the data...lol
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OK, so the reason for the Lean of Peak mag check is....."A lean mixture will reveal a weak plug that might be masked with a richer mixture." The weak plug could be due to a problem with magnetos, ignition harnesses, spark plugs, ignition timing, etc. Of course, most piston aircraft don't operate at lean of peak so this check is for a small minority of aircraft.

Thanks.
 
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It is also very useful to do prior to descent, as a heads-up if there is a mag issue so you can repair it before time for your next flight.
 
I would never do a full power mag check in flight unless a emgcy.especially with a key switch set up. I have on more than one occasion turned both mags off only to turn on with a large explosion . lucky no damage . single toggle switch for each mag might be ok/safe
 
woxof said:
Of course, most piston aircraft don't operate at lean of peak so this check is for a small minority of aircraft.
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Even if you don't operate LOP identifying a plug that is incapable of operating with a leaner mixture is a good thing. However, as someone with a JPI 830, I would suggest that there are a lot of people out there that are unknowingly operating at or LOP every flight.

That being said I've never done an in flight mag check.
 
hotprops said:
I would never do a full power mag check in flight unless a emgcy.especially with a key switch set up. I have on more than one occasion turned both mags off only to turn on with a large explosion . lucky no damage . single toggle switch for each mag might be ok/safe
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The explosion thing doesn’t sound very fun.

In the future, if you’d like to avoid that, pull the mixture to cutoff, wait a few seconds, turn the key back to both, and push the mixture back in.
 
jesse with Cessna key mag switches it happens all the time ,your future advice for avoidance in blowing off the exhaust system would most certainly
work well, but in real life not so well.i do not have 15k posts but I have been flying for 50 years.
 
arkvet said:
However, as someone with a JPI 830, I would suggest that there are a lot of people out there that are unknowingly operating at or LOP every flight.
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Interestingly, the 182 I used to fly leaned to best power almost identically with all three methods available...JPI, single EGT gauge, and engine roughness.(or was it best economy...I can't remember.)
 
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YES!

A few years ago while reviewing my JPI logs, I noticed one EGT running about 100 deg hotter than the others, but only in level cruise, and only after I had leaned to peak EGT (best economy setting). If I leaned to 100 deg ROP, all temps were normal. At the time I had no clue what was happening. Even my A&P was stumped. Someone told me the 100 deg rise is indicative of one plug not firing, but a mag check on the ground was fine. My A&P tested the plugs and they tested okay. On my next flight, I did an in-flight mag check at 100 deg ROP, which was fine, and then another at peak EGT - Bingo! Bad plug on the hot cylinder! That one plug would only act up at the higher temps seen at peak EGT. I told my A&P which plug to replace. He didn't believe me, but replaced the plug anyway. Problem solved. Without the engine monitor, I would've never known, and could easily have burned an exhaust valve or subjected my exhaust system to the higher temperatures for a long period. Of course I didn't have a differential alarm programmed at the time, so I'm lucky I looked at the log when I did.

My EGTs run about 1450°F at peak EGT, but that one cylinder was up at 1550 when the plug stopped firing. The 321 SS that our exhaust systems are made of is only good for 1500°F.
 
dell30rb said:
A lean mixture is harder to ignite so don't be surprised if you lose power or it runs rough if doing this LOP.
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Thanks, you bring up a point that was mentioned in the article I am reading. It says...."If you do this mag check while LOP, then you should ignore the magnitude of the mag drop, for it will be huge, two to six times as great as the usual 1700 RPM full-rich check shown in the POH

So the next question is, why is the lean of peak mixture more difficult to ignite, it is igniting just fine with two spark plugs operating.
 
woxof said:
Thanks, you bring up a point that was mentioned in the article I am reading. It says...."If you do this mag check while LOP, then you should ignore the magnitude of the mag drop, for it will be huge, two to six times as great as the usual 1700 RPM full-rich check shown in the POH

So the next question is, why is the lean of peak mixture more difficult to ignite, it is igniting just fine with two spark plugs operating.
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I can't give you a scientific answer off the top of my head (not a chemist), but the fact is that a rich mixture is easier to ignite, as evidenced by carb chokes. Even your fancy computer controlled car starts with a rich mixture and the leans out after the engine warms a minute or so.

Regarding the two spark plugs and lean mixture. On a car or most any engine with a single spark plug, the plug is located on the top of the combustion chamber. The combustion chamber is designed to have a single, hot spark ignite the mixture at a specified point, and the timing of the engine is set so this single spark will occur at the most opportune time for the mixture to cleanly ignite from the top of the cylinder, and spread evenly down the compressed air/fuel charge, driving the piston down in the most efficient way. The compressed air fuel charge does not ignite instantaneously, the ignition happens at a certain point and (very quickly) spreads through the rest of the charge.

Aviation engines are designed with two spark plugs per cylinder. These plugs are offset and not located in the top of the combustion chamber, but in the sides. And the timing of the engine and design of the combustion chamber shape itself, is set for the most efficient burn of the air/fuel charge when it is lit by two very hot sparks at the same time in these locations. When you remove one of the sparks from the equation, the result is a less efficient, incomplete ignition of the compressed air/fuel charge. This is why we get a drop when switching to one mag, the combustion of the air/fuel charge is not as efficient and not producing as much power.

With a rich mixture this loss of efficiency is less pronounced, as the air/fuel charge is easier to ignite and you can achieve more complete combustion. With a lean mixture the loss of efficiency is more pronounced, as the ignition of the mixture may be incomplete.
 
It comes down to slower flame front propagation speeds with a lean mixture. The flame front will propagate more quickly from its ignition point(s) with a rich mixture than it will with a lean mixture. That's why a pre-EGT method of lean adjusting was done years ago with only one magneto operating, then switching them both back on The engine rpm drop with leaning with one mag only is more pronounced and noticeable to the pilot. I don't hear much about this leaning technique anymore with the more sophisticated engine analyzers, but that's what I was taught in my Cessna 150 basic flight training 55 years ago, and my brother (an internal combustion engine major U of MN) filled in the theory.:rolleyes:
 
Thanks guys,

Some good info here and any more good info like this is welcome. At the same time, I have to say that I never had any problem getting a quite noticeable drop in rpm on a C150. But I have seen it on a couple of aircraft types where the drop was very small. One of these aircraft is the Grob 115 with an O-320 engine with the engine not meeting its minimum rpm drop of 50 rpm. I was told that this is due to a Slick mag installation.

Anybody had similar issues of very small rpm drop on mag checks?
 
You only need to be confident that there is a confirmed RPM drop when switching from Both mags, to the Left or Right mag. This lets you know that the engine was running on both mags when the switch was on Both, and that the switch/pleads are working.

I don’t know if there is any real significance to the 50RPM specification beyond that.
 
Just because you get an rpm drop when you do a mag check doesnt mean the mags are working properly. One or both can be mistimed and still give an rpm drop.
 
dell30rb said:
I don’t know if there is any real significance to the 50RPM specification beyond that.
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Thanks,

The 50 rpm minimum drop is listed in the POH. I don't know why. Anybody seen this for any other typical genav aircraft?
 
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