More power on one mag?

[traumamed]

I had a strange situation in a recent high-altitude flight in a Turbo Arrow III with a Conti TSIO-360-FB engine. It has less than 2 years/300 hours since OH. Uneventful climb to FL190. Shortly after I settled into cruise, I began getting an intermittent misfire every 30-60 seconds. Engine was running smooth otherwise, and CHTs, EGTs, and TIT were all normal. I suspected a high-altitude misfire since it has been around 100 hours since I've cleaned and gapped my plugs. I enriched and pulled power back from 65% to 55%. There was no change to the condition. I decided to do an in-flight mag check, and sure enough, turning off mag #2 eliminated the misfire. I'm pretty sure mag #2 serves the lower plugs, which supported my plug theory since the lowers tend to wear more quickly.

Here's the weird part. When I turned off mag #2, I actually **gained** power. My MP went up a few tenths of an inch, RPM went up 20-30 rpm, and my TAS increased by nearly 5 kts. CHTs and EGTs went up trivially (maybe 10 degrees and 20 degrees, respectively). I eventually descended down to cruise at 15,000, and turned mag #2 back on. The misfire did not return and the engine continued to run smoothly. However, I lost 20-30 rpm, lost a few kts, and CHTs/EGT went down nominally. I did a ground check after landing, which was totally normal: 50-70rpm drop on each mag, and both within 20 rpm of each other.

Isn't shutting down a mag supposed to reduce power output by 10-15%? What am I missing here? Obviously I'll be pulling the plugs, but should I pull mag #2 as well? Or would checking mag #2 timing be sufficient? TIA.

 

[Magman]

My thought is it is a crossfire in the mag. Are they pressurized mags ?

Air is an insulator and if the pressurized air system became disconnected

or the filter in that system became blocked you would have your condition.

Lack of insulation properties at altitude allows cross firing in the distributor

which reduces power.

 

[traumamed]

My thought is it is a crossfire in the mag. Are they pressurized mags ?

Yes they are P-mags. Bendix S-20 (S6LSC-25P to be specific).

I should have checked the air lines, but I was short on time (and it was really cold lol) after landing. Definitely something to think about. Still seems curious why the power dropped off on two mags even below the altitude where I got the misfire. By crossfire I'm assuming (possibly incorrectly) that you are referring to mag arcing? If so, shouldn't the misfire persist?

 

[Capt. Geoffrey Thorpe]

I would wonder about the timing of the one "bad" mag - too advanced?

 

[traumamed]

I would wonder about the timing of the one "bad" mag - too advanced?

Timing issue was a main concern for me as well, but I'm definitely no A&P and 19th century tech isn't my wheelhouse. I actually wondered the opposite though, that maybe the timing is retarded on that mag since I lose power when it's on but otherwise run smoothly. No pre-ignition or detonation issues are evident.

 

[Capt. Geoffrey Thorpe]

Timing issue was a main concern for me as well, but I'm definitely no A&P and 19th century tech isn't my wheelhouse. I actually wondered the opposite though, that maybe the timing is retarded on that mag since I lose power when it's on but otherwise run smoothly. No pre-ignition or detonation issues are evident.

I would think that if it was retarded, there wouldn't be any improvement by turning it off - the fire is already lit and running a little late. If it were advanced, then getting the combustion started early could cause pressures on the compression stroke to be too high and potentially things like knock. I would also think, that just a couple degrees wouldn't be enough to cause a problem. But, my experience with this sort of thing is automotive.

 

[traumamed]

I would think that if it was retarded, there wouldn't be any improvement by turning it off - the fire is already lit and running a little late. If it were advanced, then getting the combustion started early could cause pressures on the compression stroke to be too high and potentially things like knock. I would also think, that just a couple degrees wouldn't be enough to cause a problem. But, my experience with this sort of thing is automotive.

Ahh good point. I had a one-ignition-system way of thinking going there, didn't I.

 

[MauiJim]

Yes they are P-mags. Bendix S-20 (S6LSC-25P to be specific).

I should have checked the air lines, but I was short on time (and it was really cold lol) after landing. Definitely something to think about. Still seems curious why the power dropped off on two mags even below the altitude where I got the misfire. By crossfire I'm assuming (possibly incorrectly) that you are referring to mag arcing? If so, shouldn't the misfire persist?

Don’t know how difficult it is but physically swapping mags between positions might determine if it’s a mag problem or something else. If both perform correctly in their new positions, you’re done.

 

[Magman]

I’ve met engines with 1 mag that the timing was so advanced that RPM

increased during the Mag Check when the “ Normal “ mag was selected.

I doubt timing is that issue here as it would be apparent at all altitudes.

My belief is you a getting flashover in the Distributor and it may be firing

a cylinder at inopportune times. This “ kickback” could be damaging to

your engine and I would not fly until you find a culprit.


The Coil generates a spark with voltage of about 25,000 that is always

looking for a nice ground. There are at least 3 ways to control this.

1. Pressurize the mag as air is an insulator.

2. Make the mag physically larger to increase the potential air gaps.

Bendix did this with the 1200 Series which is larger than the 20 Series.

3. Not common on GA aircraft is Low-Tension Ignition. Each plug has it’s

own Coil mounted nearby so there is no high voltage in the mag.

Think. DC-7, Constellation etc.


Mags use a “ carbon brush” to transfer the spark from the stationary Coil

to the rotating Distributor Rotor. Accelerated wear can contaminate

the Distributor with dust that facilitates flashover.


Swapping mags without checking the affected one internally would

waste several hours of time and possibly result in engine damage.

Suggest Step 1 is checking pressurization hoses and filter.

 

[brcase]

I had a similar mag failure once. I was on a 2 mile final and the C-152 engine suddenly started doing the classic movie engine failing sound where it would lose power, (mis-fire) for a couple seconds run fine for a couple seconds and then repeat. I declared an emergency. The only time I have actually done that in 9000 hours of flying. I went full power, pulled carb heat, went full rich which made no difference, once with about a mile from the runway I had the runway made and went to idle and landed. Taxied off the runway and did a runup which sounded the same.

I couldn't tell you the parts of a Magento, just generally know how they work. The way the mechanic explained to me (and I understood it) was there is a ring glued into the magneto that had come loose and was slow rotating (vibrating) around causing the timing to slowing be continuously shifting. New/repaired Magento and all was fine. Lesson learned was a Mag check would have produced the results you saw, it would have ran fine on the other magneto (with happened to be a electronic magneto) if I had done a mag check.

One note with electronic magnetos it is good to know how they are powered. I did know in this plane if we turned off the master switch we would lose the electronic magneto.

Brian
CFIIG/ASEL

 

[Dan Thomas]

Listen to Magman. He's right, it's probably flashover inside the mag, where the spark is going places it shouldn't, like the wrong cylinder. That can fire off the mixture as it's coming into a cylinder and actually fire off everything in the induction system, making the engine run really rough and reducing the power.

Turning off a rogue mag like that is why we have magneto switches. They're not just for checking the mags on the runup. They can save your life. Other cases where mags can act up dangerously include worn plastic distributor gearing in the mag, where the distributor rotor gets out of time and starts firing wrong cylinders on every spark. It's one good reason why the manufacturers want those mags off every few hundred hours for internal inspection.

 

[traumamed]

My belief is you a getting flashover in the Distributor and it may be firing a cylinder at inopportune times. This “ kickback” could be damaging to your engine and I would not fly until you find a culprit.

Listen to Magman. He's right, it's probably flashover inside the mag, where the spark is going places it shouldn't, like the wrong cylinder. That can fire off the mixture as it's coming into a cylinder and actually fire off everything in the induction system, making the engine run really rough and reducing the power.

Are we describing a scenario that would ultimately cause detonation here? Or are we talking about a different phenomenon entirely? I had no evidence of detonation on my engine monitor. CHTs actually go down when the affected mag is on, which is counter to what would be expected in a detonation scenario. The engine also runs completely smoothly at or below 15,000 feet, even when just the "problem" mag is selected and the "good" mag is off. It just produces a few % less power on the "problem" mag and on both mags together than when running only on the "non-problem" mag.

I will check the pressurization lines, filters, and plugs. I'm starting to wonder if the high-altitude misfire is something entirely separate from this subtle power difference between the mags, which for all I know may have been present from the day the engine was installed. I had not done an in-flight mag check on this engine at any prior point.

 

[3393RP]

Are we describing a scenario that would ultimately cause detonation here? Or are we talking about a different phenomenon entirely? I had no evidence of detonation on my engine monitor. CHTs actually go down when the affected mag is on, which is counter to what would be expected in a detonation scenario. The engine also runs completely smoothly at or below 15,000 feet, even when just the "problem" mag is selected and the "good" mag is off. It just produces a few % less power on the "problem" mag and on both mags together than when running only on the "non-problem" mag.

I will check the pressurization lines, filters, and plugs. I'm starting to wonder if the high-altitude misfire is something entirely separate from this subtle power difference between the mags, which for all I know may have been present from the day the engine was installed. I had not done an in-flight mag check on this engine at any prior point.

It's not detonation. The fact it occurs above 15,000', where the atmospheric pressure is typically around 8 PSIG, is a relevant clue. That's 54% less than sea level atmospheric pressure on an ISO day. The lower pressure makes it easier for a high voltage discharge to travel through that medium to ground instead of your ignition leads. The insulating property of the air is reduced.

The CHT is reduced because the combustion event isn't efficiently completed, an indication one mag is not firing as it should, i.e. going to ground or another cylinder.

This condition you're experiencing is very likely due to a pressure seal failure in the mag.

 

[dtuuri]

...turning off mag #2 eliminated the misfire. ... When I turned off mag #2, I actually **gained** power. ... I eventually descended down to cruise at 15,000, and turned mag #2 back on. The misfire did not return and the engine continued to run smoothly.

What do you mean "mag #2"?



If you mean the mag closest to BOTH, the "L" one, then you turned off the mag with an impulse coupling for starting purposes. It strikes me as worth checking that for wear and tear. Suppose, rather than advancing the spark to the correct specification after engine start — it overshoots. Then, you'd have extra pressure in the combustion chamber, forcing the breakdown in the distributor which the others are talking about.

 

[455 Bravo Uniform]

Pre-ignition by cross-fire, rather than by hotspot. Combustion mixture fires earlier than it should. Earlier firing puts stress on the compression cycle and burns the mixture earlier, and the actual intended spark for that cylinder finishes off the combustion if it’s still ignitable. I would expect cooler EGTs, not hotter.

If that cross-fire takes away from the intended cylinder, then I could envision hotter EGT on the spark deprived cylinder.

CHTs might be a different story.

Detonation may not be occurring in this instance…

 

[Dan Thomas]

f you mean the mag closest to BOTH, the "L" one, then you turned off the mag with an impulse coupling for starting purposes. It strikes me as worth checking that for wear and tear. Suppose, rather than advancing the spark to the correct specification after engine start — it overshoots. Then, you'd have extra pressure in the combustion chamber, forcing the breakdown in the distributor which the others are talking about.

It can't overshoot. The impulse spring holds the mag in the impulse's max advanced position, against a stop in the coupling, and that's where the mag is set to the engine during timing. You can get an impulse spring failure, but that will seriously retard the spark and cause power loss, but shutting off that mag wouldn't increase the power. The other mag is already sparking as well as it can.

 

[dtuuri]

It can't overshoot. The impulse spring holds the mag in the impulse's max advanced position, against a stop in the coupling, and that's where the mag is set to the engine during timing.

It's been decades since I've seen an impulse coupling, so if you say a worn stop can't advance the timing, I defer to your wisdom.

 

[Magman]

Engine is a Conti which generally have IC’s on both mags.

The stop Dan referred to is robust. A failure there would involve much

more than ignition issues! IC components are heat treated to improve

wear characteristics for longevity. I’ll guess a typical coupling will

3000ish hours. NOT the Spring though!

 

[traumamed]

What do you mean "mag #2"?

I figured someone was going to call me out for not saying "L/R." I no longer have a key. The plane was upgraded to push-button start with rocker switches for magneto control. They are labeled #1 and #2, instead of L and R. I have now become accustomed to that nomenclature. #2 corresponds to what would traditionally be called the right magneto.

Thank you all for the continued discussion. I wish I had an update to report, but I won't have time to take a look at things for another week or so.