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

Discussion in 'Maintenance Bay' started by woxof, Mar 27, 2018.

  1. woxof

    woxof Pre-Flight

    Joined:
    May 6, 2015
    Messages:
    68

    Display name:
    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
     
  2. EminiTrader

    EminiTrader Cleared for Takeoff

    Joined:
    Jun 15, 2013
    Messages:
    1,059
    Location:
    JAX

    Display name:
    Emini Trader
    I do it every flight before i descend. Really good way to find out if you have a mag issue!
     
  3. timwinters

    timwinters Ejection Handle Pulled

    Joined:
    Feb 23, 2008
    Messages:
    13,737
    Location:
    Conway, MO

    Display name:
    LTD
    Occasionally.
     
  4. Tom-D

    Tom-D Taxi to Parking

    Joined:
    Feb 23, 2005
    Messages:
    34,736

    Display name:
    Tom-D
    It's a way to check mixture settings.
     
  5. WannFly

    WannFly Final Approach

    Joined:
    Nov 28, 2016
    Messages:
    6,409
    Location:
    KFAR

    Display name:
    Priyo
    I have done it a few times


    Sent from my iPhone using Tapatalk
     
  6. denverpilot

    denverpilot Tied Down PoA Supporter

    Joined:
    Nov 8, 2009
    Messages:
    55,446
    Location:
    Denver, CO

    Display name:
    DenverPilot
    Can’t get mine to run worth a damn LOP, so I’m a’gonna say, probably not. :)
     
  7. woxof

    woxof Pre-Flight

    Joined:
    May 6, 2015
    Messages:
    68

    Display name:
    woxof
    Is there an advantage to doing this kind of mag check versus a regular one on the ground as per the POH?
     
  8. Tom-D

    Tom-D Taxi to Parking

    Joined:
    Feb 23, 2005
    Messages:
    34,736

    Display name:
    Tom-D
    I don't think so.
     
  9. mscard88

    mscard88 Touchdown! Greaser!

    Joined:
    Oct 1, 2015
    Messages:
    23,226
    Location:
    Alabama

    Display name:
    Mark
    Never have.
     
  10. WannFly

    WannFly Final Approach

    Joined:
    Nov 28, 2016
    Messages:
    6,409
    Location:
    KFAR

    Display name:
    Priyo
  11. mscard88

    mscard88 Touchdown! Greaser!

    Joined:
    Oct 1, 2015
    Messages:
    23,226
    Location:
    Alabama

    Display name:
    Mark
  12. WannFly

    WannFly Final Approach

    Joined:
    Nov 28, 2016
    Messages:
    6,409
    Location:
    KFAR

    Display name:
    Priyo
    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
     
  13. OkieFlyer

    OkieFlyer En-Route

    Joined:
    May 16, 2011
    Messages:
    3,181
    Location:
    Lindsay, OK

    Display name:
    Andrew L.
    What? A carbureted 182 that won't run LOP? Haha!
     
    denverpilot likes this.
  14. dell30rb

    dell30rb Final Approach

    Joined:
    May 18, 2011
    Messages:
    7,126
    Location:
    Raleigh NC

    Display name:
    Ren
    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.
     
  15. Cpt_Kirk

    Cpt_Kirk En-Route

    Joined:
    Jan 20, 2014
    Messages:
    3,297
    Location:
    Georgia

    Display name:
    Ted Striker
    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.
     
    EminiTrader and nrpetersen like this.
  16. luvflyin

    luvflyin Touchdown! Greaser! PoA Supporter

    Joined:
    May 8, 2015
    Messages:
    11,221
    Location:
    Vancouver, WA

    Display name:
    Luvflyin
    How does that work? What are you looking for
     
  17. BGF_Yankee

    BGF_Yankee Line Up and Wait

    Joined:
    Jan 12, 2015
    Messages:
    659
    Location:
    Winchester, TN

    Display name:
    BGF_Yankee
    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
     
  18. woxof

    woxof Pre-Flight

    Joined:
    May 6, 2015
    Messages:
    68

    Display name:
    woxof
    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.
     
    Last edited: Mar 30, 2018
  19. SCCutler

    SCCutler Administrator Management Council Member PoA Supporter

    Joined:
    Feb 27, 2005
    Messages:
    17,118
    Location:
    Dallas

    Display name:
    Spike Cutler
    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.
     
  20. hotprops

    hotprops Line Up and Wait

    Joined:
    Nov 12, 2015
    Messages:
    730

    Display name:
    hotprops
    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
     
  21. arkvet

    arkvet Line Up and Wait

    Joined:
    Feb 8, 2016
    Messages:
    776
    Location:
    Arkansas

    Display name:
    Brent
    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.
     
  22. jesse

    jesse Touchdown! Greaser!

    Joined:
    Oct 2, 2005
    Messages:
    15,775
    Location:
    Lincoln, NE

    Display name:
    Jesse
    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.
     
  23. hotprops

    hotprops Line Up and Wait

    Joined:
    Nov 12, 2015
    Messages:
    730

    Display name:
    hotprops
    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.
     
  24. MauleSkinner

    MauleSkinner Touchdown! Greaser!

    Joined:
    Oct 25, 2005
    Messages:
    10,050
    Location:
    Wichita, KS

    Display name:
    MauleSkinner
    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.)
     
    Last edited: Mar 30, 2018
  25. pburger

    pburger Pre-takeoff checklist

    Joined:
    May 26, 2007
    Messages:
    304
    Location:
    Houston, TX

    Display name:
    Paul B.
    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.
     
    flyingcheesehead, WannFly and arkvet like this.
  26. woxof

    woxof Pre-Flight

    Joined:
    May 6, 2015
    Messages:
    68

    Display name:
    woxof
    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.
     
  27. dell30rb

    dell30rb Final Approach

    Joined:
    May 18, 2011
    Messages:
    7,126
    Location:
    Raleigh NC

    Display name:
    Ren
    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.
     
  28. nrpetersen

    nrpetersen Line Up and Wait

    Joined:
    Jan 4, 2013
    Messages:
    588
    Location:
    Minnetonka MN

    Display name:
    nrpetersen
    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:
     
  29. woxof

    woxof Pre-Flight

    Joined:
    May 6, 2015
    Messages:
    68

    Display name:
    woxof
    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?
     
  30. dell30rb

    dell30rb Final Approach

    Joined:
    May 18, 2011
    Messages:
    7,126
    Location:
    Raleigh NC

    Display name:
    Ren
    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.
     
  31. coloradobluesky

    coloradobluesky En-Route Gone West

    Joined:
    Mar 19, 2014
    Messages:
    3,621
    Location:
    Colorado

    Display name:
    coloradobluesky
    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.
     
  32. woxof

    woxof Pre-Flight

    Joined:
    May 6, 2015
    Messages:
    68

    Display name:
    woxof
    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?