150 RPM Differential In-Flight Mag Test

Mooney Fan

Mooney Fan

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Mooney Fan
The engine runs fine on both while in cruise. My left mag on the 0320-E2A (no engine monitor) checks out during ground mag checks and is within tolerance (barely) with the right mag. But while conducting an inflight ROP mag check 2450 RPM at 2500', the left mag had a 200 RPM drop vs a 50 RPM drop on the right. Should we expect the mags to be within 50 RPM of each other when doing the test inflight?
 
A basic magneto drop-off check based on engine speed is typically done during the pre-flight inspection to determine the loss of engine speed that occurs when magnetos are switched from BOTH magnetos to one (LEFT or RIGHT) magneto. The maximum drop-off limit is specified in most Lycoming publications as Revolutions Per Minute (RPM) from a stated engine speed.
Although this basic drop-off check, based on just engine speed, is satisfactory under most normal conditions, it does not take into account unusual circumstances of temperature, humidity, and engine wear. In those cases, use the following procedure to measure the magneto drop-off as a ground check based on manifold pressure on Lycoming opposed series aircraft engines equipped with a manifold pressure gage or a constant speed propeller. This procedure will be incorporated in all new editions of Lycoming Operator’s Manuals.

A proper magneto check is important. Additional factors, other than the ignition system, affect magneto drop-off. They are load-power output, propeller pitch, and mixture strength. The important point is that the engine runs smoothly because magneto drop-off is affected by the variables listed above. Make the magneto check in accordance with the following procedures

b) (Fixed pitch propeller). Aircraft that are equipped with fixed pitch propellers, or not equipped with a manifold pressure gage, may check magneto drop-off with the engine operating at approximately 1800 RPM (2000 RPM maximum).

https://www.lycoming.com/sites/default/files/Magneto Drop-Off.pdf

For your just barely ground run up, clean and gap the plugs every 50 hours and lean during taxi.

 
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In 20 years I've never done an in flight magneto test, and I'm not going to start. You do it on the ground to make certain everything is all right before you get in the air. After that I keep going. You might want to check how old those magnetos are. They're really not good for much more than 50 hours in service.
 
steingar said:
In 20 years I've never done an in flight magneto test, and I'm not going to start. You do it on the ground to make certain everything is all right before you get in the air. After that I keep going. You might want to check how old those magnetos are. They're really not good for much more than 500 hours in service.
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FIFY
 
I’ve had this happen and been advised to move the throttle to 2200+ RPMs for 30-60 seconds to burn off excess fouling on the plugs then done an additional mag check and had them be within the 75 RPM range.
 
steingar said:
In 20 years I've never done an in flight magneto test, and I'm not going to start. You do it on the ground to make certain everything is all right before you get in the air. After that I keep going. You might want to check how old those magnetos are. They're really not good for much more than 50 hours in service.
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As a mechanic I've checked them in flight with no issues. Just don't turn them both off then on again; that can get a big bang that damages the exhaust system. If the engine dies when you check the first mag, close the throttle and turn the mag back on, then open the throttle again.

Mag switches are there not only for checking the mags during the runup; they're there so you can shut off a rogue mag in flight. If a mag starts crossfiring or if its plastic distributor gears wear to the point that the distributor shifts and starts sending sparks to the wrong cylinders, you need to be able to shut it off to restore power. There's no sense having two mags if one of them can ruin the good work done by the other, without any means to fix that.
 
Yes, the RPM drop is not as important as the RPM difference between the individual mags. IOW; I would see a 100 rpm drop, on both mags as just fine, but 25 rpm drop on one, and 75 rpm drop on the other as being not so good.

AFM for my Cherokee with O320 states that Max rpm drop per mag is 175 RPM, and no more than 50 rpm difference between them.
I would expect the difference in flight to be the same 50 rpm or less.
 
Update- Installed new plugs no improvement. Ground run up @2000rpm. R-Mag 1950 L-Mag 1875. Going to take a look at L-Mag. It was overhauled 300hrs ago. Timing?
 
Mooney Fan said:
Update- Installed new plugs no improvement. Ground run up @2000rpm. R-Mag 1950 L-Mag 1875. Going to take a look at L-Mag. It was overhauled 300hrs ago. Timing?
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Thanks for the update. what type of mags?
 
Mooney Fan said:
Update- Installed new plugs no improvement. Ground run up @2000rpm. R-Mag 1950 L-Mag 1875. Going to take a look at L-Mag. It was overhauled 300hrs ago. Timing?
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Plugs, point gap, timing should all be checked when there's an issue. Then you go from there.
 
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