left (start) magneto

[Roland Donnell]

Cherokee 180 C with OEM magnetos. If the engine will not start on the left mag but functions normally in all other aspects i.e. drop out check at idle (pre/post flight) and run up then that would be a impulse coupling component failure. Correct?

 

[Dana]

Even without an impulse coupling an engine should start; the impulse coupling just makes it easier by delaying the spark and making it a bit stronger, and reduces the chance of kickback.

 

[sansoneservices]

As you stated the LH magneto should have the impulse coupling for start-up. The RH mag kicks in once the engine is running. The magneto starting SWITCH disables the RH mag for starting.

 

[sansoneservices]

Cherokee 180 C with OEM magnetos. If the engine will not start on the left mag but functions normally in all other aspects i.e. drop out check at idle (pre/post flight) and run up then that would be a impulse coupling component failure. Correct?

FYI: some owners install an impulse coupled mag on the RH location for better starting. They must remove a wire "jumper" on the mag switch that grounds the p lead for the RH mag (for start only)

 

[Ryan Klems]

As you stated the LH magneto should have the impulse coupling for start-up. The RH mag kicks in once the engine is running. The magneto starting SWITCH disables the RH mag for starting.

This is true on the switches with a start position (and the grounding jumper installed) but on the 180C (assuming this is still a factory set up), the mag switch is just off/left/right/both, with no start position, and there is a separate push button start. So you would manually do that by starting on the left mag. The later models (when they switch to the throttle quadrant) went to the push and later twist to start switches and work like that.

This should definitely be investigated ASAP, as aside from the hard starting, depending on the mechanism of failure, can result in a whole lot of badness if something ends up in the gearing. There have been a number of ADs around this over the years. You mention they are “OEM mags”, so likely old Bendix mags? When was the last time the mags were off and inspected ?

 

[Roland Donnell]

For the purpose of flying the plane to the shop location is there a "badness" type failure that would affect more than the magneto or is it self contained and just stop functioning.

 

[Magman]

The Impulse Coupling can disintegrate and wreck the engine.

If you hear clicking this is not likely.

2 things that can cause this:

1. The Cam or Cam Follower ( varies with mfg) is worn which retards the

Internal Timing ( E- Gap ) and also retards timing to the Engine.

With precautions the Click should occur no later than Top Dead Center.

Reluctant to tell you more.


2. The Impulse Coupling is a chunk of steel on the end of a magnet.

Occasionally the Flyweights in the IC become magnetized and will not

engage. It may be necessary to remove the magneto to de-gauss the

Coupling. Several places may do this. Any place that does Magna- flux

Inspection is one.

 

[Domenick]

Not sure it's clear in the context of this thread, but the "clicking" Magman mentions is a snap or loud click you hear when pulling the prop through one rotation by hand. Make sure the ignition and master are OFF.

 

[Roland Donnell]

Thanks guys. Appreciate the guidance. Just a mystery on why the left mag will not provide a spark under cranking speed (impluse or not) but seems to function correctly otherwise (after starting on right). Hesitate to start again until the mag is r/r as I understand that a kick back has the potential to crack the crankcase.

 

[Racerx]

Id make sure the switch is wired correctly. Off/R/L both. Mine wouldn't start when switched to left mag but would fire right up on the right.

 

[Dan Thomas]

Even without an impulse coupling an engine should start; the impulse coupling just makes it easier by delaying the spark and making it a bit stronger, and reduces the chance of kickback.

No, that impulse coupling also snaps the mag over rapidly so the magnet and coil can generate enough primary current for a spark at starting RPM.

I suspect that those mags have not been off and opened up for a long time. Eroded points, and worn cam (Slick) or rubbing block (Bendix) change the E-gap, a critical setting inside the mag, and the spark gets weak even with a working impulse coupling.

Bendix/TCM specify 400 hour inspection intervals. Slick/Champion spec 500. Most mags are not getting that care.

 

[Dan Thomas]

The Impulse Coupling can disintegrate and wreck the engine.

If you hear clicking this is not likely.

2 things that can cause this:

1. The Cam or Cam Follower ( varies with mfg) is worn which retards the

Internal Timing ( E- Gap ) and also retards timing to the Engine.

With precautions the Click should occur no later than Top Dead Center.

A missing stop pin will do it, too. If that pin falls out it can get into the accessory case gearing and trash the engine.

 

[Chip Sylverne]

Thanks guys. Appreciate the guidance. Just a mystery on why the left mag will not provide a spark under cranking speed (impluse or not) but seems to function correctly otherwise (after starting on right). Hesitate to start again until the mag is r/r as I understand that a kick back has the potential to crack the crankcase.

A bit of of info about how the impulse coupling works. The magneto is a fixed timing device. The spark fires at a set degree of 25 deg ( or whatever your engine spec is) before the piston reaches top dead center every time, regardless of rpm. Works ok once the engine is running, but not so good when starting, because the fuel mixture explodes and the power stroke begins before the piston is at the top, so the starter is working against the piston, and risks kickback.

The impulse coupling is device that device that uses springs, pawls and magnets that spin and create the spark, thus allowing the magneto to fire when the piston is approximately top dead center. Once the engine starts, centrifugal force takes the impulse coupling out of play.

At least that's how I understand it, but I have a Shower of Sparks system so what do I know. Will say this though, for what the going rate for an OH i$, you might take a hard look at a Surefly.

 

[Magman]

“ Not provide a spark under cranking speed” is a comment I fail to

understand. How was this determined? No start or cowl off?

You may have a spark but not at the right time.

A common issue is a broken IC Spring. This would drastically retard the

timing and be apparent during a mag check. A Bendix may present as a

dead mag but sometimes a Slick will just show a large drop. Slicks

sometimes have the broken spring jam and the timing reduction is not as

bad.


My take is anytime a mag is being serviced the Spring should be REPLACED.

They will inspect fine and work but fail suddenly with no warning.

This is another “ Low time is not good scenario”. Low “operating time” is

high “rust time”. Like other engine parts ; operating the engine provides

the Spring with a hot oil bath to inhibit corrosion. Springs are not tolerant

of rust and may fail in a short time when not flying or limited to ground

runs.

 

[sansoneservices]

No, that impulse coupling also snaps the mag over rapidly so the magnet and coil can generate enough primary current for a spark at starting RPM.

I suspect that those mags have not been off and opened up for a long time. Eroded points, and worn cam (Slick) or rubbing block (Bendix) change the E-gap, a critical setting inside the mag, and the spark gets weak even with a working impulse coupling.

Bendix/TCM specify 400 hour inspection intervals. Slick/Champion spec 500. Most mags are not getting that care.

This is correct ∆∆.. impulse coupling retards the spark to TDC for starting without kickback AND snap/swings the magnet fast enough to generate the secondary spark energy

 

[Dan Thomas]

The impulse coupling is device that device that uses springs, pawls and magnets that spin and create the spark, thus allowing the magneto to fire when the piston is approximately top dead center.

The impulse doesn't have magnets. The magneto has the magnet, and the impulse simply stops the mag's rotation while the coupling continues to rotate, winding up that spring, and the pawl releases at or near TDC to snap that magneto's rotor (which has the magnet) fast enough to create the spark.

In the flight school we had the impulse mag fail to start the engine on a 172, before my time as director of maintenance there. That mag had too many uninspected hours on it, and the e-gap had shifted enough to weaken the spark that the mag's impulse couldn't compensate, and the engine wouldn't start. Once running, it ran fine and mags drops were fairly normal. So, listen: Mag drops don't tell the whole story.
A weakened spark might still run the engine, but starting difficulties or lower power output is the penalty. A spark isn't just a spark; that engine needs a good hot spark to perform as needed.

Springs are not tolerant

of rust and may fail in a short time when not flying or limited to ground

runs.

And ground runs add much moisture to the crankcase and rust that spring. A rusty spring is pitted, and pits lead to cracking and breaking. A broken spring takes the mag to TDC, so that the engine's power drops off some. This is bad, but even worse is a broken spring in a Bendix dual mag, where the one spring drives both magnetos. A broken spring there means a forced landing.

 

[Dana]

No, that impulse coupling also snaps the mag over rapidly so the magnet and coil can generate enough primary current for a spark at starting RPM.

That's the "making it a bit stronger" part. OK, maybe more than just "a bit". But I can certainly see a weak magneto not generating enough spark at low speed (how fast does an impulse coupling turn a mag, equivalent to what rpm?) but run OK at higher speed.

 

[Magman]

“Coming-in Speed” is the Minimum MAGNETO RPM that will produce a

consistent spark. For a Bendix/Continental mag this is 150 Magneto RPM

but the IC may engage intermittently up to 450 Mag RPM. On a 4 cylinder

Mag RPM would be the same as Engine RPM if I figured right. Not true of 6

cylinder mags.


An IC mag should produce good spark regardless of rotation speed. When

the IC snaps the magnet is likely rotating faster than the 150 RPM Minimum

Coming- in Speed.


Another factor here is the Lag Angle or the amount of retard built into

the IC. Generally; slow cranking engines will have more retard than faster

cranking ones. The higher the cranking speed the more the Starting Timing

is retarded due to the action of the IC. Early 152’s were notorious for

hard starting until a Slow Speed Starter and Lower Lag Angle mags were

Installed. This was because Spark was actually taking place after

Top Dead Center.

 

[Dan Thomas]

That's the "making it a bit stronger" part. OK, maybe more than just "a bit". But I can certainly see a weak magneto not generating enough spark at low speed (how fast does an impulse coupling turn a mag, equivalent to what rpm?) but run OK at higher speed.

Neither the Bendix nor Slick manuals give the impulse speed, but in working on many of these things, and working that impulse, I'd guess that the snap would be equivalent to about 1000 RPM. It's fast.

Now, in cold weather where the entire engine has not been preheated, the oil in the impulse spring is stiff and sticky and will slow the impulse's action. That makes the engine nearly impossible to start. Just as well, since the engine oil is too thick to be pumped rapidly into the system. The pump can hardly suck oil into itself. Engines have been trashed this way, especially where the owner hasn't switched to a lighter grade for winter, or to a multigrade.

The Bendix Magneto manual has an excellent description of magneto operating principles. 21 pages of it, in section 1.
http://aeroelectric.com/Mfgr_Data/Magnetos/Continental_Motors/IGN-51_Magneto_TM.pdf

Download it. One never knows when this stuff might be taken down.

Slick (Champion) manual: https://www.championaerospace.com/pdfs/techdocs/F1100/L-1363J_corrected 20210420.pdf

 

[Pinecone]

Id make sure the switch is wired correctly. Off/R/L both. Mine wouldn't start when switched to left mag but would fire right up on the right.

THIS ^^^^^^^

If the wires are switched, it will start on Right but not on Left. Make sure Right is Right and Left is left. Or just start in Both.

If you have a combined mag switch and start switch, this can manifest itself by not firing in the Start position, but firing and starting when you release the switch to Both.

 

[Dana]

If the wires are switched, it will start on Right but not on Left. Make sure Right is Right and Left is left. Or just start in Both.

Starting on Both can cause a kickback if both mags aren't impulse coupled

 

[asicer]

Starting on Both can cause a kickback if both mags aren't impulse coupled

...or one IC mag combined with an electronic ignition that doesn't fire below 50RPM.