Magnetos

[3393RP]

In general, I found the education takes place initially but after a time of falling on deaf ears it stops unless an owner brings it up. That same education continuely takes place on PoA, yet every month the same issues pop up and not only by new posters. So long as the owner is the gate keeper to the aircraft there's not much we can do except not sign off things. And we know how well that works.

You, Dan, and others have been trying to educate PoA members about recommended service intervals and their importance for years. The mechanical failure of an ignored magneto in flight can introduce enough debris into the engine to make it quit running. The seriousness of that possibility is obvious.

This poster had no clue of the maintenance status of his Slick magnetos, and one failed after 700 hours and nine years. Doing the math and thus assuming an annual flying average of 77.78 hours per year indicates he flew the mags for 2½ years past the recommended service intervals of 500 hours.

He had to do some "digging" in the engine logbook to determine their hours, and was "surprised" by the failure. I like to believe aircraft owners do more with their aircraft's manuals than remove them from a desk drawer and give them to their mechanic once a year when the annual inspection is due, but obviously that's not true.

Returning from Maine to FL in my 1977 Cessna R172K XP, the left mag went bad. No prior warnings. All of the run ups checked out fine with the typical 50 to 60 RPM drops during the 15 hrs of flights the prior 6 days before the failure. I had to leave the plane in NC awaiting a new Mag.

So I decided to do some digging when I got back to FL and checked the log books. Discovered both the LH & RH were last replaced in 2012 and had accumulated approx 700 hrs before the failure. No indication in the logs that a 500hr inspection had been done.

Since they were the same age, I bit the bullet and had replaced both. I was very surprised that everything checked out normal with the LH Mag until it failed. I thought there would be a slow progression of increasing RPM drops. One flight it’s fine, next flight it drops over 200 RPMs and engine sounds terrible. Is this normally how these Mags fail?

 

[MIFlyer]

It is surprising, particularly for very basic (and critical to safety of flight) things like the mags. I actually see the value though in IRANing them at different times so they're not subject to the same infant mortality, reinstallation and AD/recall issues. That said, if I bought a plane tomorrow with 800 hour mags, I'd be getting them exchanged or IRANd this winter...

 

[RyanB]

It is surprising, particularly for very basic (and critical to safety of flight) things like the mags.

What’s surprising about them?

 

[Dan Thomas]

Now. That indicates that the mag internal timing has shifted due to points cam wear or points erosion. Simply reclocking the mag is treating symptoms, not the cause. The spark will be weakened and sooner or later it will fail you.

Replying to my own post, to add this: https://www.championaerospace.com/assets/technical/SL_Magneto4300_6300_001.pdf

There is internal timing inside the mag that has as big an effect on performance as the mag-to-engine timing. The rotor spins a magnet that runs its flux field through the coil's primary windings, which will then generate electron flow in the winding. That flow continues after the magnet has rotated though the neutral point, due to inductance in the coil keeping the electron flow going as the flux field begins to collapse. The points open at the most effective angle, the angle between neutral and points opening, and that is known as the E-gap. This brings about the biggest collapse of the field that generates the spark voltage in the secondary winding of the coil.

When the points erode, or the cam wears (rubbing block in a Bendix), the points open at a different time, a different angle, than desired, and the spark voltage suffers. Sure, it will spark, and mag drops might be minimal, but it's not ideal, and there comes a day when it won't start or you get a big mag drop. In the meantime, you're getting less power and fuel mileage than if you had a healthy spark.

And that's aside from all the other wear/corrosion inside an old mag, just waiting to cause some serious trouble.

When I was young, we had to replace points and condenser in our cars every 12,000 miles or so, or risk being broken down alongside the road somewhere. At 60 MPH, that's only 200 hours. 200 hours at an RPM similar to an aircraft engine. 400 or 500 hours on an aircraft mag is NOT unreasonable for an inspection. Until we have reliable electronic replacements for mags, we're stuck with this, like it or not, and a pilot will like it a lot less if he loses ignition in flight.

 

[ktup-flyer]

I do mine every 500hrs, or before, if it's close and in for its annual. It was $780 to have both of mine done...cheap insurance

 

[MIFlyer]

What’s surprising about them?

Sorry, it's surprising to me that pilots don't care more about safety of flight issues like mags.

I get that there are a lot of ADs/SDs where you could rationalize the risk being low, but being someone who restores old trucks, I can tell you that old ignition systems need regular inspection and adjustment, so I wouldn't be too lacksadaisical about mags, gears and props. Other things I could see the mental process that gets people to be more negligent about it.

do you agree?

 

[Dan Thomas]

Sorry, it's surprising to me that pilots don't care more about safety of flight issues like mags.

Many owners/pilots never think about the mags. They're used to their cars, which start easily and run for many years with never a hiccup, but fail to realize that airplanes are still far behind that level of technology. Numerous reasons for that, but in any case, those mags run until they don't, and the pilot has one fail in flight and loses power, or it starts misfiring badly and nearly kills the engine by sending sparks to the cylinders at the wrong times, or the airplane won't start at some remote airport when he wants to come home. If the scenario is frightening or inconveniencing enough, he starts paying attention to mag maintenance, especially if the mechanic takes them off and finds that they are beyond economical repair and must be replaced with new mags. Alternators suffer that way, too; either maintain them, or they trash themselves.

Pain. The great educator.

 

[Magman]

Based on OPs comment regarding “ adjustment slots” the aircraft is equipped with Bendix mags. There is an AD regarding the possibility of required replacement of the coils and rotating magnets that was issued circa 1973 . Required compliance time was often a long time off ( at overhaul or similar) . It’s possible low usage aircraft have not complied with it yet. The AD has been revised several but some like this often fall though the cracks and are not done.

Physically checking for compliance is not particularly difficult but swapping rotors is best done by a mag shop. The rotor must be shimmed for correct preload and special tools will be needed.

Often folks get taken in by the “ All ADs c/w” log entry. OLD mags can be fine IF their inner components are updated as needed. Many techs will do a good job of this but will skip repainting the mag.

Yes; unfortunately there are also those that repaint but skip need replacements.

 

[mondtster]

Sorry, it's surprising to me that pilots don't care more about safety of flight issues like mags.

Right or wrong, most maintenance decisions seem to be financially driven to some extent. In the case of magnetos, I suspect that many folks expect to see some warning signs before a magneto fails that would allow them to run to the point of failure rather than inspect/repair them before they get to that point. Unfortunately, many are met with a surprise maintenance event when using that approach.

In my book, running to failure is never a good plan for any aircraft system. If for no other reason, the failure has an opportunity to occur away from home and/or at the worst possible time. Most airports I frequent have zero services and parts would be at least a week out so I prefer to do what I can to avoid break downs.

 

[mondtster]

Now. That indicates that the mag internal timing has shifted due to points cam wear or points erosion. Simply reclocking the mag is treating symptoms, not the cause. The spark will be weakened and sooner or later it will fail you.

I agree for the most part, although the mention of slots suggests bendix magnetos. I have seen some with perfect internal timing that need to be run all the way to the end of the slot to set the ignition timing properly. However, since the OP is asking about magneto servicing I'm betting it may have been a long time since the last time someone looked at them and may be causing the trouble that re-indexing them was attempting to fix.

 

[Magman]

On some Bendix the Drive Gear has to be rotated 180 degrees in order for the slots to provide sufficient adjustment. Easy with an impulse . Not all direct drive have 2 key ways though.

 

[Dan Thomas]

Based on OPs comment regarding “ adjustment slots” the aircraft is equipped with Bendix mags

Yup, most of those are Bendix, but the old Slick 664/667 mags had those slots, too. A 664:

If I recall correctly, the Slick was a redevelopment of the Case magneto. Case had those slots. Bendix was a redevelopment of the Scintilla mags.

Often folks get taken in by the “ All ADs c/w” log entry.

Only lazy mechanics make entries like that. ADs complied with at the time should be all itemized, listing details of what was done to achieve compliance. Some are recurring, and a spreadsheet should be used to track them. Besides that, many mechanics seem to only check what the databases will dig up for them, and that's usually only the airframe, engine and prop ADs, Appliance ADs, and mags are appliances, have to be separately and manually researched. I have very often found outstanding appliance ADs on stuff in the airplane. Most commonly it's ignition switches, often misidentified (if they even got under the panel and looked) as Bendix when it's actually an ACS switch. Separate ADs on both of those, so the ACS AD from 1996 is long outstanding. United altimeters. Vacuum pumps. Carburetors. Mags. Plenty of stuff like that.

 

[Magman]

Dan. I agree with your recall . Case ( aka chug- chug tractor mags) begat Slick 400/600 series mags. I have seen Case mags on Pipers ( J-3s, Vagabonds etc). I don’t ever recall a 400/600 on any Cherokees though. 400 series long gone primarily via AD. 600s of half a century ago have difficulty obtaining parts.

The feds did no one favors with the AD database also. I related in another thread that had my bro enlisting the FSDO in determining revised ADs. Let’s say we are talking of rev 1 and current is rev 4 . Usually the difference is minor. Other times the diff between rev 1 and rev 4 is serious. Btw The feds failed to come up with info.

I’m still keeping the last of my old paper ADs.

 

[3393RP]

I get that there are a lot of ADs/SDs where you could rationalize the risk being low, but being someone who restores old trucks, I can tell you that old ignition systems need regular inspection and adjustment, so I wouldn't be too lacksadaisical about mags, gears and props.

I own a 1966 Ford F100 with a 352 FE V-8. There's something satisfying about doing a "tuneup" on the old beast, as I forget how often the procedure had to be done on my cars and trucks in the 60s, 70s, and 80s.



Changing the points, condenser, rotor, and distributor cap, cleaning or installing new spark plugs, and setting the timing results in an engine that starts easily and runs well.

I'd like to say it also improves fuel mileage to some high teens double digit number, but 13 MPG is about as good as it gets, even with the Warner T-85 three speed overdrive transmission. I'm planning on rebuilding the engine over the winter, maybe that'll kick it up a notch.

 

[Dan Thomas]

I agree for the most part, although the mention of slots suggests bendix magnetos.

Bendix still needs internal timing:



The plastic rubbing block on the points wears, and the points themselves erode, and both cause timing drift. A worn rubbing block (or the plastic cam in the Slicks) retards the timing. Eroded points advance it. Ideally, they should cancel each other out, but they often don't. There is no substitute for proper inspections.

 

[Bell206]

Other times the diff between rev 1 and rev 4 is serious.

You've mentioned this before but I still dont see the issue with revised ADs. Each subsequent new revision deals with the previous AD revision in the text of the AD. In a number of times the text states the original has been removed which means it is no longer valid. It is also the reason 91.417 requires the revision date as the new revision keeps all the necessary compliance requirements as the original. Perhaps an example? Now if your talking about superceded ADs then thats a different ball game and has its own issues for continuity.

 

[mondtster]

Bendix still needs internal timing:

View attachment 110601

The plastic rubbing block on the points wears, and the points themselves erode, and both cause timing drift. A worn rubbing block (or the plastic cam in the Slicks) retards the timing. Eroded points advance it. Ideally, they should cancel each other out, but they often don't. There is no substitute for proper inspections.

I know. And sometimes even right after setting the internal timing per the overhaul manual you’ll run out of slot.

I just went through it again after I overhauled the bendix magnetos on the o-290 in my Cub. As Magman points out, sometimes flipping the drive gear 180 degrees out could fix it. I had to install the one on my cub a tooth off.

Just pointing out that not everything works out perfectly all the time.

 

[Lndwarrior]

If it makes you feel any better, it's within the POH and about 100 to 120 RPM drop on each mag. Going from the problem RPM drop of 500, 100 seemed like "essentially no mag drop". I was making conversation and following up on my original post for the benefit of the forum.

We all understand conversations via the internet fail to convey emotion, but your reply seems "snarky". Thanks for the feedback

Wasn't trying to be snarky.

When an aircraft owner says "essentially no mag drop" it implies, perhaps in this case wrongly, that the op is not familiar with magneto operation. Since "essentially no mag drop" would be an indication of a potential serious problem.

Why someone knowledgeable in magnetos would choose that language instead of "normal mag drop" is unusual and misleading.

The logical assumption from the statement is the person speaking was not familiar with magnetos, was unaware of the risk presented by the statement, and thought it was ok to continue flying with magnetos that had essentially no drop.

If you look at my response from this perspective, it was not snarky but an honest attempt to get you to understand the seriousness of "no mag drop".

I get now that it was just a poor choice of words.

 

[MIFlyer]

I own a 1966 Ford F100 with a 352 FE V-8. There's something satisfying about doing a "tuneup" on the old beast, as I forget how often the procedure had to be done on my cars and trucks in the 60s, 70s, and 80s.



Changing the points, condenser, rotor, and distributor cap, cleaning or installing new spark plugs, and setting the timing results in an engine that starts easily and runs well.

I'd like to say it also improves fuel mileage to some high teens double digit number, but 13 MPG is about as good as it gets, even with the Warner T-85 three speed overdrive transmission. I'm planning on rebuilding the engine over the winter, maybe that'll kick it up a notch.

Yes, I have one (going on 3) international v8s in various stages of re-awakening to rebuilding. One got a new gaskets, plugs, wires, freeze plugs and a new DUI igntion and it did wonders for it. The others I'm still deciding on the exact approach (new points/condensor or convert to electronic. I will likely go electronic).

Speaking of your FE, my next project (after this two year backlog is caught up of trucks) would likely be an FE390 project, unless I don't talk myself out of finally "manning up" and buying an airplane. You're exactly right though.... a true "tune up" to an older ignition/carb system really makes it a totally different vehicle to drive.

 

[plongson]

If anyone follows this thread to the end...I pulled both mags and one is being replaced and one is being overhauled. We'll have essentially new mags. I've learned more on this subject than I ever imagined.

Being the first plane we've owned and the plane being 1962 vintage, we relied HEAVILY on inspections prior to the purchase. What we are finding out...bit by bit...although we like this plane, it was grossly over represented...and we are learning an expensive lesson. Inspections were conducted by an associate of the seller...(hind sight 20/20??)... It was purchased sight unseen in Alabama and we live on the west coast...the list of lessons goes on and on. And this seller allowed the plane to be ferried across the country in this condition! Could have had a tragic ending.

We're in too deep now to turn back and it seems we're seeing the light at the end of the proverbial tunnel. Buying a used old airplane private party is no different than buying an old car...BUYER BEWARE! The unscrupulous are everywhere.

 

[MIFlyer]

If anyone follows this thread to the end...I pulled both mags and one is being replaces and one is being overhauled. We'll have essentially new mags. I've learned more on this subject than I ever imagined.

Being the first plane we've owned and the plane being 1962 vintage, we relied HEAVILY on inspections prior to the purchase. What we are finding out...bit by bit...although we like this plane, it was grossly over represented...and we are learning an expensive lesson. Inspections were conducted by an associate of the seller...(hind sight 20/20??)... It was purchased sight unseen in Alabama and we live on the west coast...the list of lessons goes on and on. And this seller allowed the plane to be ferried across the country in this condition! Could have had a tragic ending.

We're in too deep now to turn back and it seems we're seeing the light at the end of the proverbial tunnel. Buying a used old airplane private party is no different than buying an old car...BUYER BEWARE! The unscrupulous are everywhere.

BUT, you have your very own plane to fly.

 

[Bell206]

we relied HEAVILY on inspections prior to the purchase.

Not to add to your unfortunate circumstance, but this is a good example of the why most buyers should involve the mechanic who will maintain the aircraft after the purchase with the review process prior to the purchase. Airworthiness can be very subjective with neither party technically wrong. Plus there is no single standard for pre-buy "inspections." Unfortunately, this is not such a rare event and the reason I always push this point when I can. But look on the bright side at least your aircraft is repairable unlike some others who had worse luck and had to buy a second aircraft.

 

[Magman]

Glad it will work out ok. I agree with Bells comments regarding the “ gaining Tech”.

With a Prebuy it should be the Buyer that dictates the scope of the process. Many folks are only concerned with Airworthiness squawks or over $1000 or some figure.


While aggravating; the mags are not a particularly costly item to address. Overhaul times and 500 hr servicing are “suggested” by Service Bulletin only. It is the Owners option whether to comply with an SB. That are numerous SBs that are
ignored by the Owners as well. Few folks want to comply with the 400 ( ?) hour inspection of Lycoming Valve Guides as an example.

Suggest you verify Service Bulletin Status on your new mag as well. Many of the dealers are selling mags with outstanding SBs. And Techs are not checking prior to installation.

 

[Pinecone]

Being the first plane we've owned and the plane being 1962 vintage, we relied HEAVILY on inspections prior to the purchase. What we are finding out...bit by bit...although we like this plane, it was grossly over represented...and we are learning an expensive lesson. Inspections were conducted by an associate of the seller...(hind sight 20/20??)... It was purchased sight unseen in Alabama and we live on the west coast...the list of lessons goes on and on. And this seller allowed the plane to be ferried across the country in this condition! Could have had a tragic ending.

This is why the recommendation to have a pre-purchase inspection performed by someone NOT associated with the seller in any way.

Saavy will look over the logs for free and point out any issues they see. If you don't know of a appropriate shop, they have a paid service where they help find one.

I paid an A&P/IA who owns the same make airplane to help me through the trials and tribulations. And he found things in the logs of a couple of planes I considered that raised red flags.

 

[Eagle_]

Yup. That impulse spring keeps the mag at the desired spark advance point. It only does its impulse thing during start, by retarding the spark to near TDC and snapping the mag rotor rapidly at that point to generate a hot spark. The rest of the time it has to hold the rotor in the advanced position, and if it breaks, the timing goes to TDC and power goes to nothing.

If improper knowledge of the impulse coupling still leads to widespread problems, why don’t we all use retard breaker mags by now? Or an EIS system…

 

[Dan Thomas]

If improper knowledge of the impulse coupling still leads to widespread problems, why don’t we all use retard breaker mags by now? Or an EIS system…

The impulse coupling works flawlessly when the magneto is maintained properly.

Period.

But many mags are not removed until they fail. That's crazy, in an airplane. Airplanes that make short flights, or that the owner is in the habit of ground-running without flying it, get condensates in the crankcase that cause corrosion of that high-carbon steel impulse spring. ANY corrosion is bad on such stuff; the tiny pits create stress risers that bring on sudden failure. There is no such thing as a detectable gradual impulse spring failure.

Magnetos also have points and condensers and distributors that need periodic inspection and replacement as well; the retard breaker mag is no different in that regard. Fail to maintain it, and it will eventually quit on you.

EIS is great, but just try getting it certified in an affordable fashion. It is airframe-power dependent, so the airframe's charging and distribution systems have to be reliable. And how often do we get stories about failed alternators or problems with electric circuits on POA? All the time. That's because alternators aren't being removed for inspection, either, and older airplanes aren't getting decent annuals that get the mechanic under the panel to check the bus bar and its breakers and switches. Evidence of impending failure is often easy to spot there. The alternator's wiring and regulator are often ignored as well. So an EIS would be MORE likely to fail if the airplane is treated that way.

We often get the defense that "the alternator in my car is the original and the car has 200,000 miles on it and it's still OK." Yes, it does, and it might last a while yet, but if it quits you coast to the side of the road. You don't end up crashing because it failed in IMC at night more than an hour from an airport. Alternators in cars and airplanes are both geared or belted to the engine's crankshaft so that the alternator reaches its redline at the engine's redline RPM. So your car, with its 6500-RPM redline, cruises at 2100 on the highway; the alternator is at 32% of its redline. In your airplane, the engines redline is, say, 2700 RPM, with the alternator at redline if your engine is at 2700, but you cruise at 2500 or so, and so the alternator is at almost 93% of its redline. What do you think that does to its brushes and slip rings and rotor bearings?

Like I say, a car is not an airplane, and that alternator in the airplane is spinning at three times the speed it does in a car. Basically the same alternator. A totally different operating environment, requiring a totally different mindset about maintenance. Magnetos of any sort are not EIS, which has no moving parts at all, and so it needs care. Treat the airplane like a car and it will eventually get expensive or scary or dangerous.

 

[eman1200]

I don't know why but Magnetos sounds like it would be a great name for a cereal......"Start your day off right with a big bowl of MAGNETOS". "MAGNETOS, they're not just for breakfast anymore". c'mon, who's with me???!?

 

[Dan Thomas]

I don't know why but Magnetos sounds like it would be a great name for a cereal......"Start your day off right with a big bowl of MAGNETOS". "MAGNETOS, they're not just for breakfast anymore". c'mon, who's with me???!?

Might work. Almost nobody knows what a magneto is anymore, so it might sound like something super-advanced and high-tech.

A few years ago Toyota had a TV ad for their pickups. One of the trucks was towing a trailer across a really narrow bridge while a deep-voiced narrator gushed on about its advanced systems and its "big ol' leaf springs" on front and rear axles. Made me laugh. My 1951 International pickup, like almost all pickups of that vintage, had those "big ol' leaf springs" on both axles.

A lot of marketing relies on people's ignorance.

 

[asicer]

I don't know why but Magnetos sounds like it would be a great name for a cereal......"Start your day off right with a big bowl of MAGNETOS". "MAGNETOS, they're not just for breakfast anymore". c'mon, who's with me???!?

You can put this on the cereal box:

 

[Capt. Geoffrey Thorpe]

If improper knowledge of the impulse coupling still leads to widespread problems, why don’t we all use retard breaker mags by now? Or an EIS system…

 

[flyingbrit]

Alternators in cars and airplanes are both geared or belted to the engine's crankshaft so that the alternator reaches its redline at the engine's redline RPM.

Do you have any evidence of this? Everything I have read says that both airplanes and cars have alternators geared at around 3:1, i.e. the alternator turns at three times crankshaft speed. Here is an excerpt from an article on aircraft:

The face gear turns at the same RPM as your crankshaft and the drive hub gear ratio is 3:1 meaning the alternator is screaming along in excess of 7,500 RPM when you’re at max RPM on the engine.
See: https://www.ramaircraft.com/alternatorhub

Here is an excerpt from an article on cars:

An alternator has a normal range of operation. Most alternators need to spin at about 2,400 rpm at idle, have their maximum output above 6,000 rpm, and should never exceed 18,000 rpm.
So, how do you know how fast your alternator is spinning? The first step is to understand how the pulley ratio affects the alternator. Typical original-equipment ratios are around 3 to 1, which simply means the crank pulley is three times the diameter of the alternator's pulley. Ratios less than 3 to 1 are typically looked at as being underdriven.
See:https://www.motortrend.com/how-to/0...r has a normal,should never exceed 18,000 rpm.

 

[Dan Thomas]

Do you have any evidence of this? Everything I have read says that both airplanes and cars have alternators geared at around 3:1, i.e. the alternator turns at three times crankshaft speed. Here is an excerpt from an article on aircraft:

The face gear turns at the same RPM as your crankshaft and the drive hub gear ratio is 3:1 meaning the alternator is screaming along in excess of 7,500 RPM when you’re at max RPM on the engine.
See: https://www.ramaircraft.com/alternatorhub

Think about what you just said. If they are both geared at 3:1, that car's alternator is way beyond most alternator redlines when the engine is at 6500 or whatever.

I just replaced the alternator in my Hyundai Sonata. That engine redlines at 6500, and the alternator has a 10,000 RPM redline, as per the test info that it came with, and so the gearing (belt and pulleys) is set for that sort of thing. It's not 3:1. It's close to 2:1. Alternators will produce power at less than 2400 RPM too. I've tested that.

The fact remains that the airplane's alternator is revving much higher than the car's, when both are in cruise, simply because the airplane's engine is much closer to redline. It takes a lot of power to fly. And that applies even if they're both geared 3:1. So the airplane's alternator wears out faster.

I have had alternators apart maybe a couple hundred times, mostly on aircraft but also on cars and trucks, and the airplane's bearings and brushes wear out faster. That's all there is to it.