Mag Failure -> Pre-ignion -> Runaway CHT

[exM20K]

So, there I was…. FL190 over the snow last week.  And this happened.

https://apps.savvyaviation.com/flights/shared/flight/10195195/1d78020a-eaa5-4b88-83e4-cd35a4de4f06

left mag failed at 2:48, and #6 CHT went on a moonshot.

what has me (and George at GAMI) stumped is why #6 EGT also rose so dramatically? Typically, pre-ignition gives *lower* EGT.

Left mag replaced, and if AirPower will tell me where it is going, I’ll ask for photos when it is apart.

Lessons learned:

1) don’t mess around with mixture in an event like this.  Pull the power out.

2) mag failures can be more than the usual nothing-burger of an event

3) anecdotally, lots of pressurized and non pressurized mag failures in my circle of acquaintances. This mag had maybe 2-300 hours on it.  Maybe less, will have to check logbooks.

Piston, cylinder, and valves look normal and no metal in the filter, so it’s all good in the hood.

If anyone here has an idea on the rising EGT, I’m all ears.

-dan

 

 

[hazek]

39 minutes ago, exM20K said:

2) mag failures can be more than the usual nothing-burger of an event

Good lesson!

Btw, I see no runup? Mag check before flight could have given us clues about nr 6? EGT increasing and staying constant while CHT climbing would indicate a more complete combustion for some reason. Initially EGT follows the family but CHT starts to depart immediately followed by EGT until you go mixture rich. CHT keeps climbing until you reduce power. After that CHT nr 6 starts to cool off and EGT 6 comes slowly back into family. Also your TIT seems low.

Will be interesting to see what was going on in nr 6!

[exM20K]

Both mags had a 500hr inspection in March 2024, 270 hours ago.  I am even more interested in photos of the disassembled mag now.

-dan

[dkkim73]

Thank you for sharing. Curious also what you learn. 

BTW the mechanic found tiny cracks in my mag filter last annual. Hadn't had any malfunctions up to FL190 but maybe it was a matter of time. Just a data point it's another thing to check in general with pressurized mags. Not an issue here obviously since one cylinder. 

Good point about reducing power. Did you scope that cylinder to look for damage? 

 

[Schllc]

I replaced one mag with a surefly and have been very happy.  I also like knowing i have two different systems.

I have had a mag failure in flight as well, but it did not cause any aberrant temperatures.  I did a mag check and the engine shut down for a brief, not so joyous, moment. 

I replaced both mags, the harness and plugs for all new.  At the time there was a special if you bought the ignition system complete that was only about 1k more than just rebuilt mags.

 

[dkkim73]

2 hours ago, Schllc said:

I replaced both mags, the harness and plugs for all new.  At the time there was a special if you bought the ignition system complete that was only about 1k more than just rebuilt mags.

 

That's a good deal. What vendor offered that? (wondering if it is likely to recur)

OTOH I live only a few miles from Aircraft Magneto Services...

[Schllc]

6 minutes ago, dkkim73 said:

That's a good deal. What vendor offered that? (wondering if it is likely to recur)

OTOH I live only a few miles from Aircraft Magneto Services...

It was 8 years ago, I’m pretty sure it was aircraft spruce, but it could have been the shops vendor. I remember it because my family was with me for the event, it was my first Mooney, and the shop tried to talk me into just rebuilding the failed mag. Logs said, the mags and harness were original.  When I insisted we change it all with new, is when the deal came to light.  The shop got onboard when we found out the price. 

[Schllc]

For what it’s worth, I’m extremely dubious of “overhaul” in the aircraft genre. 
I recently sent in an air conditioning condenser fan in for “overhaul”. 
six weeks and 2400 later I got my motor back with nothing but the brushes changed. 
so $8 worth of parts. When it failed 10 hours later I took it apart and that how I found out. 
I would have thought “overhaul” for a 40 year old motor for 2400 would include commutator, winding,  brushes and bearings.   This is what was done on my table saw motor which cost 1700. 

From now on, anything I buy overhauled or get overhauled will either be because new isn’t an option or be a fraction of what it is new. 

[Ragsf15e]

3 hours ago, Schllc said:

I replaced one mag with a surefly and have been very happy.  I also like knowing i have two different systems.

I have had a mag failure in flight as well, but it did not cause any aberrant temperatures.  I did a mag check and the engine shut down for a brief, not so joyous, moment. 

I replaced both mags, the harness and plugs for all new.  At the time there was a special if you bought the ignition system complete that was only about 1k more than just rebuilt mags.

 

Im going to do a SF next time either of my mags is out as well.  I had one in my F and liked the performance, lack of mx, and different systems just like you described.

[dkkim73]

3 hours ago, Schllc said:

I replaced one mag with a surefly and have been very happy.  I also like knowing i have two different systems.

 

This may be a can of worms, but how do you like the Surefly with the turbo and the other (pressurized) normal mag. I recall there is some subtlety around variable timing which is (I believe?) deactivated with the turbo. 

Also, is there a backup battery for the Surefly or is it on the ship's [normal/essential] bus? 

[Schllc]

It’s a little smoother, but prop went on at the same time. Temps are not notably different. 
regular mags are fixed as well so…. 
uses ships battery but requires a direct line to the battery.   
starting is more like a car now. Hot warm cold. All easy.   I am not ready to go full electric just for modality variety, but if it’s as reliable as it should be I would consider two at some point. 
if they could control the timing like on a modern car there would be much less need for two electric, the ignition would be much more efficient. 
I don’t regret it yet.  That being said I only have about 25 hours since the journey began, and she has been down for a few months. The mechanic is flying it a bit too. 
it’s a little more complicated than changing a mag, but I kept the half of the ignition harness and got a rebuilt mag to keep as a spare. 

[Shadrach]

2 hours ago, Schllc said:

For what it’s worth, I’m extremely dubious of “overhaul” in the aircraft genre. 
I recently sent in an air conditioning condenser fan in for “overhaul”. 
six weeks and 2400 later I got my motor back with nothing but the brushes changed. 
so $8 worth of parts. When it failed 10 hours later I took it apart and that how I found out. 
I would have thought “overhaul” for a 40 year old motor for 2400 would include commutator, winding,  brushes and bearings.   This is what was done on my table saw motor which cost 1700. 

From now on, anything I buy overhauled or get overhauled will either be because new isn’t an option or be a fraction of what it is new. 

The industry is in a paradigm shift.  There are a number of things happening all at once.  Lots of experience is retiring out of the industry. Some are aging out, and some are dismayed with management. There is also a fair amount of consolidation still taking place in the industry. Even local repair stations are being rolled up into larger regional conglomerates.

Things are more expensive than ever, but at least they’ll take longer… 

I would urge everyone to seek out as many options as possible before committing to vendor. I have been surprised by some of the things that I’ve seen in the last year. I recently saw an engine teardown invoice come back at 47% over the initial estimate with no major changes to the scope of the work performed. 

I have an acquaintance who had to send an engine mount back to a well known aerospace welding firm twice because it would not mount up to his AEIO-360.  He and his mechanic tack welded a template at their own expense and sent it back to the overhauler with he overhauled mount. This forced the overhauler to admit that their freshl overhauled mount was out of spec. They covered it under warranty, but took no responsibility for the additional time and expense that their mistake created.

What you experienced with your air conditioner doesn’t surprise me in the least. The industry has always had its fair share of cheats and snake oil salesmen, but I feel like we’re going through especially rough patch when it comes to quality, service and pricing. 

I bought an exhaust header from Acorn last year. I paid over $700 with shipping. Just found an alternative supplier in Florida that has the same part listed on their website for under $350. 

I would urge anyone in the market for subcontractor work to shop around and not take anything for granted. The pricing delta just up and down the East Coast for the same work can vary by more than 50%.

[Ragsf15e]

4 hours ago, dkkim73 said:

This may be a can of worms, but how do you like the Surefly with the turbo and the other (pressurized) normal mag. I recall there is some subtlety around variable timing which is (I believe?) deactivated with the turbo. 

Also, is there a backup battery for the Surefly or is it on the ship's [normal/essential] bus? 

I helped install the one on my F.  It wasn’t hard, but more than just the mag swap.  I had the interior out for something else, but you pretty much need to run a power wire down the left side of the cabin and through the firewall.  It can actually run on very low voltage, so it will take your alternator(s) breaking and your battery being run way down before you’re stuck on just the traditional mag.  As you said, the dip switches will be set to fixed timing, but id say it’s still noticeable.  The timing never “slips” and it should never need maintenance for tbo.  Finally , as @Schllc mentioned, it starts better.  It’s programmed to fire tdc below ~500rpm or something like that and it just works.  You can still get a hot start fiasco if you’re ham fisted, but it’s definitely improved.  I was happy with mine.

[exM20K]

On 11/16/2025 at 10:44 PM, dkkim73 said:

BTW the mechanic found tiny cracks in my mag filter last annual

Tech found the same on mine last week and replaced it. I don’t think that was a contributing factor here, but I have seen MAP annd FF  instability in the past from a disconnected line here.

Piston, cylinder wall, and valves scoped and appear normal, and 4 hours later, no increase in oil consumption.

 

-dan

[exM20K]

13 hours ago, Schllc said:

For what it’s worth, I’m extremely dubious of “overhaul” in the aircraft genre. 
I recently sent in an air conditioning condenser fan in for “overhaul”. 
six weeks and 2400 later I got my motor back with nothing but the brushes changed. 
so $8 worth of parts. When it failed 10 hours later I took it apart and that how I found out. 
I would have thought “overhaul” for a 40 year old motor for 2400 would include commutator, winding,  brushes and bearings.   This is what was done on my table saw motor which cost 1700. 

I’m not sure “new” helps here.  I suspect that low-quality “imported” parts will increasingly be the norm as accessory manufacturers are operated by finance-first leadership teams.  Again, I’m interested to see this one apart and will look for the paperwork when I get back up north. I’m not super-confident about the right mag, which was o/h exchanged at the same time.

-dan

[Schllc]

54 minutes ago, exM20K said:

I’m not sure “new” helps here.  I suspect that low-quality “imported” parts will increasingly be the norm as accessory manufacturers are operated by finance-first leadership teams.  Again, I’m interested to see this one apart and will look for the paperwork when I get back up north. I’m not super-confident about the right mag, which was o/h exchanged at the same time.

-dan

Unfortunately , you are probably right. 
I think I just want to avoid the “judgement” of the Overhauler.  In my example they decided all they needed to do was the brushes.   Worst part was the bearings were bad, not just old and the commutator had a groove over 1/8” deep from using the wrong brushes and bad bearings. 
one would hope new had a more rigid inspection criteria. 
crossing my fingers…

[exM20K]

AirPower says all the exchange mags go into a mixed box of stuff to be returned to the manufacturer, so regrettably, this episode will forever be a mystery.
-dan