Strange mag check

[AndreiC]

Hello all,

yesterday, while flying out of the Truckee-Tahoe airport (KTRK) I had a strange experience and would like to ask the maintenance gurus if this is something to be concerned about. 

Flying into TRK in the morning, from sea level, everything was normal, no abnormalities in how the IO-360 performed. However, when departing in the afternoon, my mags check was strange. The engine was perfectly smooth on either of the two mags, full rich, but the RPM drop was much higher than usual when switching to either one of the mags. Close to 200 RPM (max allowed is 175). The differential between the mags was about 50-60 RPM. I usually do my mag check at 1800 RPM, and normally see about a 70 RPM drop on each mag, with a very small differential. I tried it several times and saw no change. Then to double-check I did a mag check at 2200 RPM, and there the drop was lower and I decided to take off. Things went normal and the flight was uneventful on the way back (to KCCR).

The density altitude was a bit on the high side, 7200 feet -- 55 F, 5900 pressure altitude. Could this have been a factor? Was I supposed to lean somewhat before doing the mag check (and/or before takeoff)?

Thanks.

[hammdo]

I lean vs full rich. I pretty much do that all the time to test the ignition system health. I lean so I can get to the RPM I run without it running rough. Just my habit…

-Don

[PeteMc]

30 minutes ago, AndreiC said:

5900 pressure altitude

I always ground lean even at sea level and 5900ft seems pretty high to be running full rich.  I'm not sure if you're POH has anything about departing higher elevation fields, but you might check.  I don't remember the procedure for the Mag check, but do remember you needed to lean for max RPM at altitude before Takeoff, keeping an eye on the temps in the climb.

Hopefully someone that flies a non-turbo plane in the mountains all the time chimes in with some recommendations. 

 

 

[EricJ]

Yes, if your mag check is funky like that the first thing to try is to lean it a bit and try again.    It usually fixes it.

 

[PT20J]

At that density altitude, full rich is too rich. The overly rich mixture burns slower than a best power mixture which is why the CHTs are lower and the power is lower. When you go to one mag, the mixture burns even slower because there is only one point of ignition. That causes a larger than normal power drop which shows up as lower rpm. It's a good practice to lean to max rpm for the mag check regardless of altitude in a NA engine because then you are always testing under the same conditions. At density altitudes above 5000 feet, I always do a full power run up and lean to max power for best takeoff and climb performance.

[AndreiC]

49 minutes ago, PT20J said:

 It's a good practice to lean to max rpm for the mag check regardless of altitude in a NA engine because then you are always testing under the same conditions. At density altitudes above 5000 feet, I always do a full power run up and lean to max power for best takeoff and climb performance.

This relates to a question I wanted to ask -- how do you lean to max power (or max rpm)? I thought running the engine full power on the ground was a really bad idea, since it does not cool off properly, even with cowl flaps fully open. If I understand correctly from what you write above, above 5000 ft DA the engine cannot produce enough power, even at full throttle, to damage itself? Or do you lean for max RPM at a lower power setting?

[Ragsf15e]

14 minutes ago, AndreiC said:

This relates to a question I wanted to ask -- how do you lean to max power (or max rpm)? I thought running the engine full power on the ground was a really bad idea, since it does not cool off properly, even with cowl flaps fully open. If I understand correctly from what you write above, above 5000 ft DA the engine cannot produce enough power, even at full throttle, to damage itself? Or do you lean for max RPM at a lower power setting?

You can do a full power run up without overheating. You probably don’t want to run for an extended time like that, but a runup is fine.  Also be careful that the ramp is clean (to save paint dings and your prop).  One thing to be cautious of is setting “max rpm”.  You’re looking for max power yes, but with enough ROP to provide some cooling from the extra fuel.  If you do your max power runup, set around 50rop and then takeoff with the thin air and slow climb, you’ll quickly have very high chts.  You need to be well rich of peak egt.

Personally I like the target egt method and it works pretty well at higher altitudes, but a full power runup is certainly another option.  I set my target egt (egt from a full rich sea level takeoff) during the takeoff roll, but it requires knowing the egt, having an engine monitor, and doing it during the takeoff.

[Utah20Gflyer]

I always lean to max rpm on every mag check as well. I’d be worried about fouling my plugs doing it at full rich.  You also might take an excessively large power penalty if you take off full rich.  I normally pull the mixture out about a quarter inch so I can make full power.  This makes heat management a little more difficult but the loss of power is too much at full rich for my liking.  

[Hank]

At high altitude (5900 msl counts, much less 7200 DA), go full rich, full prop, WOT, then slowly pull the mixture back while watching RPM. Lean for max RPM and smooth running, do the magneto checks, then pull throttle to idle. Leave the mixture where it was.

When you pull onto the runway for takeoff, push the throttle in and go. Make sure Prop stays forward, and Mixture stays wherever it was.

I don't go out that way often, but this worked for my C in the Dakotas and Wyoming.

When you get home, or near to sea level, take off on as close to standard temp and pressure as you can find (59°F, 29.92", 0 msl) and record your full-power EGT as you climb out. Look up "Target EGT" before your next trip to high altitude airports, and follow those more modern procedures instead.

[PeteMc]

1 hour ago, AndreiC said:

how do you lean to max power (or max rpm)?

It shouldn't take too much time at all and you have a couple of options.  If you've flown that plane a lot you should have an approximation of how far to lean, so maybe go 1/3 of the way there.   Then if it is a busy Apt, when you're #1 go full throttle and lean as you watch the RPMs and then reduce to idle.  Then taxi onto the Rwy and go.  If it's a slow airport, similar to a short field TO.  Taxi into position, go full power and lean and then just release the breaks.  

Remember to Watch The Temps on climb out and enrichen a little if the temps are going up too high. 

 

[EricJ]

2 hours ago, AndreiC said:

This relates to a question I wanted to ask -- how do you lean to max power (or max rpm)? I thought running the engine full power on the ground was a really bad idea, since it does not cool off properly, even with cowl flaps fully open. If I understand correctly from what you write above, above 5000 ft DA the engine cannot produce enough power, even at full throttle, to damage itself? Or do you lean for max RPM at a lower power setting?

Runup is generally done at an rpm far lower than the prop governor will be regulating, e.g., 1800 rpm or so.    So if you put it at 1800 or 1700 or whatever rpm you do your runup at, and then lean, the rpm will increase.    When you get to max rpm, that's where the mixture can be, or a bit richer.   Set the rpm where you want it and then do the mag check at that mixture setting.

 

[Vance Harral]

I know the "Normal Procedures" section of many POHs prescribe full throttle when leaning for best power in a normally aspirated airplane at high density altitudes.  But let's apply some sanity here...

Go ahead and perform that full-throttle leaning operation at high DA, once, which won't hurt your engine as others have noted.  Then make a finger smudge mark or use a grease pencil or a post-it note or count turns or whatever, to mark where the mixture knob wound up.  Then immediately repeat the leaning procedure, but use a lower "run-up RPM" instead of full throttle: 2000 or 1700 or whatever.  Note the difference in mixture position between the two methods.  For the vast majority of normally-aspirated GA airplanes, the difference will be small to non-existent.

Then go look at a power-developed-vs-mixture chart, maybe like this one.  Observe that it's very important to not run full rich at high DA, because developed power drops off significantly; but observe also that once you've leaned anywhere near best power, the curve is very flat.  This means small differences in mixture setting around the best power point make very little difference in developed power.  Certainly not significant enough to be the difference in whether you hit that tree off the end of the runway.  So even if the mixture setting you get at 1700 is slightly different than what you got at full throttle, it doesn't matter.

If these observations hold true for you and your airplane, it's rational to conclude you're not giving up anything meaningful by leaning for best power at less than full throttle.  It's also rational to conclude you don't need to take minutes-on-end to carefully dial that mixture vernier to eek out the last 10 RPM of indication.  Just set the mixture at or near full rich, quickly bring the engine to runup RPM; adjust the mixture in a timely manner for a decent RPM increase (doesn't take more than a few seconds), and get on with your mag and prop checks.  Doing so saves prop wear, paint nicks, gas, and noise complaints from the neighbors, with no compromise in safety.

This approach is appropriate for the vast majority of the piston GA fleet.  To be clear, there are some exceptions involving pressure carburetors and other interesting mixture animals.  But for your basic 172 or Cherokee or non-turbo Mooney, there's just no convincing evidence that it's important to use full throttle when leaning for best power at high DA.

[PT20J]

6 hours ago, AndreiC said:

This relates to a question I wanted to ask -- how do you lean to max power (or max rpm)? I thought running the engine full power on the ground was a really bad idea, since it does not cool off properly, even with cowl flaps fully open. If I understand correctly from what you write above, above 5000 ft DA the engine cannot produce enough power, even at full throttle, to damage itself? Or do you lean for max RPM at a lower power setting?

As you can see from the above posts, there are a lot of ways to do this. The important point is that the maximum power available from a normally aspirated engine decreases with density altitude. If the DA is high enough to greatly affect performance, you're not going to hurt the engine by leaning it because it's not putting out enough power to need the extra fuel that full rich provides. Another point is that the power vs mixture curve is relatively flat when ROP so you don't have to be overly precise. I just open the throttle full and pull the mixture back until the power drops off and then push it back in until the power recovers. That's close enough and only takes about 5 seconds.