Small Mag Drop - What does it mean?

hish747

hish747

Pre-takeoff checklist
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On my airplane with a Lycoming O-320, the mag drops are in the range of 35-50 RPM on both the left and right mag. Does this indicate a potential problem with P-leads the way no mag drop would? Does it indicate a timing issue? I believe if it was a spark plug issue the drop would have been larger, not smaller. Any thoughts would be appreciated.
 
hish747 said:
Any thoughts would be appreciated.
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No drop would indicate a problem. So I don't quite follow your question. When your drop starts to be over 125 or there's a 50 rpm difference between sides then you might start to check things but that would be up to your mechanic.
 
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hish747 said:
On my airplane with a Lycoming O-320, the mag drops are in the range of 35-50 RPM on both the left and right mag. Does this indicate a potential problem with P-leads the way no mag drop would? Does it indicate a timing issue? I believe if it was a spark plug issue the drop would have been larger, not smaller. Any thoughts would be appreciated.
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Same on both mags? If so, I'd say it indicates mags are in good shape.
 
Yes, 35-50 on both mags so between 5-15 between the two. I've never seen such a low spread and my instructor was a little concerned about why it was so low.
 
hish747 said:
Yes, 35-50 on both mags so between 5-15 between the two. I've never seen such a low spread and my instructor was a little concerned about why it was so low.
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If you get a drop from both mags to one mag then that indicates the P-leads are OK. If you are still worried, you can quickly cycle the switch from BOTH to OFF and back to see if you get more than 50RPM drop.

EDIT: Forgot to mention that the BOTH-OFF-BOTH mag check should be done at idle. Thanks @Magman

Otherwise, 50RPM from both to one and 5-15RPM from one to the other seems within spec:
https://www.lycoming.com/sites/default/files/O-320 Operator Manual 60297-30.pdf
 
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How quickly is “ quickly”?

Going from BOTH to OFF to BOTH at an RPM above about 800 can be too exciting!

Rental pilots will check to see if anyone heard the “ Bang”.

I learned of the “ Muffler Removal Technique” with a ‘51 Chevy.
 
Has it always been this way, or is this a new development? To me, it sounds perfectly fine. If the drop is less than 75rpm between the two and the engine runs smoothly, I’m flying!
 
What RPM are you running when you do the check? The more RPM the more shaft load and the larger the drop. The is a lot of difference in propeller load torque (26%) between 1600 and 1800 RPM. The more torque the more pronounced the drop. The main thing is the drop is relatively even between the mags. Hope this helps.
 
The “Mag Check” is really more than that.

You are really comparing the Quality of Combustion in 3 switch positions.

Fuel distribution also is in the picture .

Injected engines with a restricted nozzles will present as a drop on BOTH.

Some engines ( Franklin) are sensitive to RPM when checking.

You check them at something like 800 or 2200 RPM.

I could time your mags so it would RISE when going from BOTH to LEFT.

But that wouldn’t be correct.
 
If you get a drop, that's good. A low mag drop would generally indicate the timing, spacing, etc., are in good shape

The Aztec I fly only drops 25-50 on the right engine.
 
FYI, I do the mag check between 1800 RPM and full rich mixture. The airplane is new to me as I just bought it last month, but it has been doing that since the first flights. It's just that my CFI had me wondering if something was wrong. Thank you all for the reassuring information!
 
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hish747 said:
FYI, I do the mag check between 1000-1200 RPM and full rich mixture. The airplane is new to me as I just bought it last month, but it has been doing that since the first flights. It's just that my CFI had me wondering if something was wrong. Thank you all for the reassuring information!
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Does your POH say to do the mag check at 1000-1200 RPM?
 
I fly a Piper Warrior (-161) behind an O-320-D3G. POH in Normal Procedures instructs mag check at 2000 rpm, 175 rpm max drop and 50 rpm max difference.
 
hish747 said:
On my airplane with a Lycoming O-320, the mag drops are in the range of 35-50 RPM on both the left and right mag. Does this indicate a potential problem with P-leads the way no mag drop would? Does it indicate a timing issue? I believe if it was a spark plug issue the drop would have been larger, not smaller. Any thoughts would be appreciated.
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It means you mags are functioning well, the mag timing is spot on, your spark plugs are clean and gapped properly. If you keep the plugs gapped and cleaned, this is the result you should always have.
 
Clip4 said:
It means you mags are functioning well, the mag timing is spot on, your spark plugs are clean and gapped properly. If you keep the plugs gapped and cleaned, this is the result you should always have.
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Until the mags are weakening because they haven't been internally inspected for too long.

I agree, the low mag drop is indicative of a good, hot spark. A spark isn't just a spark, no matter what some folks think. You can start a campfire much quicker with a tiger torch than with a match.
 
asicer said:
Hmm. I would expect leaning out an over-rich engine to make the mag drop even smaller.
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Actually running the mag check leaned out, to say peak EGTs or even LOP, will better stress-test the ignition system, and increase the mag drop. It makes sense if you think about it, a lean mixture is harder to light.

That is what I normally do on the ground, and will see a symmetric mag drop in the 200 RPM range. Doing my mag check at full rich will show a drop in the range of 50 RPM. At the moment, my mags are freshly IRAN'd, plugs are up to snuff, and timing is set correctly.

All other things being equal, and under the same test conditions, a smaller symmetric mag drop speaks to a healthier iginition system.

* Orest
 
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oskrypuch said:
Actually running the mag check leaned out, to say peak EGTs or even LOP, will better stress-test the ignition system, and increase the mag drop. It makes sense if you think about it, a lean mixture is harder to light.

That is what I normally do on the ground, and will see a symmetric mag drop in the 200 RPM range. Doing my mag check at full rich will show a drop in the range of 50 RPM. At the moment, my mags are freshly IRAN'd, plugs are up to snuff, and timing is set correctly.

All other things being equal, and under the same test conditions, a smaller symmetric mag drop speaks to a healthier iginition system.

* Orest
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One plane that I fly tends to run over-rich at low RPM. I get a symmetric mag drop in the 200RPM range but lean it slightly by untwisting the mixture knob just a little bit and it improves to the 50RPM range.
 
asicer said:
One plane that I fly tends to run over-rich at low RPM. I get a symmetric mag drop in the 200RPM range but lean it slightly by untwisting the mixture knob just a little bit and it improves to the 50RPM range.
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Sure, if you are "over-rich", the fuel will be harder to light as well.

* Orest
 
Dan Thomas said:
Until the mags are weakening because they haven't been internally inspected for too long.

I agree, the low mag drop is indicative of a good, hot spark. A spark isn't just a spark, no matter what some folks think. You can start a campfire much quicker with a tiger torch than with a match.
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That’s purpose for 500 mag hour service.
 
A very high percentage of older general aviation aircraft are running truly decrepit ignition harnesses. I would replace any harness over 10 years old. Most aircraft have considerably less Mag drop with new harnesses.
 
The mixtures has a big effect on mag drop. Too rich and you could see a larger than usual drop …lean the opposite. Mixture affects combustion timing. Mag timing could also be a factor. So it depends.
 
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