The International Cessna^® 170 Association

If we assume only a 75 mile average speed of a Cessna 170 Plain, A, or B on an average 1 to 1.5 hour flight. That comes to 37,500 miles traveled in 500 hours. If you drove your old points operated ignition system car engine for 37,500 miles without a tune up your car would not be running properly. You would most likely have a tune up performed. Just resetting the timing would not be all that was performed. You would likely get new plugs, rotor, points and distributor cap. Possibly even plug wires.
Would you do less for your aircraft engine? Talking with aircraft owners for over 41 years has shown that many would like too. You cannot believe the issues that are found in magnetos that are taken apart for inspection at 500 hours. Letting AD’s be the only reason you would choose to look at a magneto at a minimum of 500 hours is just a bad idea. To a small degree you can’t compare car ignition systems to that of an airplane. Cars operate in a much dirtier environment. They do not have shielded ignition systems. Massive electrode plugs only last about 500 hours anyway. But the magneto is just a self powered distributor and the operating principles beyond the rotating magnet and impulse coupling are the same. 500 hour inspections should be considered a minimum for a magneto.
Concerning the AD’s on Bendix magneto's

AD 78-09-07 R3 was superseded by AD 96-12-07 was Superseded by AD 2005-12-06. AD 78-09-07 R3 & AD 96-12-07 required the repetitive inspection of the Impulse couplings installed on the magnetos at 500 hour intervals and the possible replacement of riveted impulse couplings with Snap ring couplings. AD 2005-12-06 superseded the older AD’s and removed the requirement for the 500 hour inspection of any Bendix series magneto installed on any engine except those installed on Lycoming AEIO-540, HIO-540, IO-540, 0-540, and TIO-540 series engines. So that it does not apply to any Bendix magneto installed on an O-300 or C145 engine. Read the AD’s and service bulletins.
My opinion: Interesting thing here is that if your engine only had one impulse coupling only the magneto with the impulse coupling required any work. I think AD 2005-12-06 was one of the worst ideas the FAA has come up with for a couple of reasons. It was the only thing that caused people to perform a 500 hour inspection on their magnetos. Although none of the AD’s require a 500 Hour inspection of the magneto. (Only the impulse coupling) the magnetos often received a 500 Hour inspection at the time they were removed for the impulse coupling inspection because it made sense to perform it while they were removed. The new AD removed that leverage. The old AD’s even once the Snap ring Impulse coupling was installed was not a terminating action for the 500 hour repetitive inspection? I think the only reason the older AD’s were revised is that it is assumed all riveted impulse couplings possibly installed on Bendix magnetos have been replaced. This does not suppose that an engine could have been sitting around for years without the work being performed. And now it does not apply. The old AD also allowed the continued operation of riveted impulse coupling magnetos if they passed inspection. I have found Bendix impulse couplings that would not pass the required inspection both riveted and snap ring type installed on many different engines other than the engines listed in the new AD. I have seen the riveted impulse couplings damage engines (Stopped them from running) by breaking the gears because the broken rivet parts fell into the gear train. (There was a time before snap ring impulse couplings). I think AD 2005-12-06 should never have been written. I also think there should be an AD on any impulse driven magneto short of an AD requiring 500 hour repetitive inspections on the internals of any magneto. This would only force good practice.

AD 73-07-04 was superseded by AD 94-01-03 R2 = R 94-01-03 R2 TELEDYNE CONTINENTAL MOTORS: Amendment 39-9271. Docket 93-ANE-44. Revises AD 94-01-03 R1, Amendment 39-9006, which revises AD 94-01-03, Amendment 39-8785. Supersedes AD 73-07-04, Amendment 39-1731 (Bendix AD). This AD could have replaced coils & rotating magnets in certain magnetos and had to be accomplished at the next 100 hour inspection, the next annual inspection, the next progressive inspection, or the next 100 hours time in service (TIS) after the effective date of this AD, whichever occurs first. The effective date of this AD was June 28, 1995. (If it has not been accomplished your airplane has been sitting a long time or someone is really negligent).
AD 94-06-09 still applies and if it has not been performed by now your airplane has been in violation of the AD for a while. It was required within 10 hours of the effective date of the AD. May 20, 1994. It is still applicable but. It was a onetime AD once accomplished. Read the AD
AD 94-01-03 R2 is still applicable. If AD 94-01-03 or 94-01-03 R1 were accomplished no further action is required. In our case This AD’s may have replaced both the coil and rotating magnet in the S-20 series magnetos. It is still applicable however. In any case once accomplished it is a terminating action and no further work is required. The AD is not repetitive. Read the AD

the riveted flyweight was TCMs attempt to dodge the AD when they bought Bendix electrical... it didn't work, the FAA stuck them with it anyway. legally it's a moot point now since the AD is no longer in effect (unless you run certain 6 cyl Lycomings). but ya... you should look at them once in a while. I also agree... both types of flyweight pivots wear on different engines, and a broken one will cause havoc.

the other AD not on your list: 82/20/01, soft flyweights has a 300 hr interval. any mag with more than 300 hrs was exempt. the soft flyweight would beat itself flat in about 100 hrs, so if it made 300, it's ok

last time the flat side magnets were made was 1957... you can see the magnet through the bottom vent hole, and you can also see the coil through a gap in the points housing... anything other than opaque blood red is n/g

he sintered iron bushing AD only applied to the -21 mags & you have to pull the coupling to check it. the -25s have a different style bush

My mechanic says that I will need my mags looked at next annual
Is this still true?
My c170 runs like a top!!
Iowa

Jim, that’s an excellent review that I hope will provoke readers to dig into their records and inspect/repair any subject magnetos. Thanks for the review.

iowa wrote:My mechanic says that I will need my mags looked at next annual
Is this still true?
My c170 runs like a top!!
Iowa

I will say that if your mechanic says you should look at them, he probably has a good reason. LIke they haven't been looked at in 500 hours and it is time to inspect/adjust them internally.

But as has been pointed out, if he thinks there is an AD due such as those mentioned, he is likely wrong. I still hear mechanics site these ADs that don't exist anymore. Maybe it is easier to site an AD than convince a customer to pay for preventive maintenance.

So, I take it that its not required but recommended
Are there signs that the mags need checking or is it just arbitrarily set at 500 hr
There's two of them,
So it seems that if one fails
Then its time to have them OH'd?

I have a dent on my plane
Which also could go either way
As far as getting repaired
Structurally its makes no difference
But would cost thousands
Iowa

As one who has had mags fail with less than 500 hrs on them. Why in the hell would one even question or consider running them to failure. Rebuild overhaul replace at 500 hrs and hopefully keep on sparking and flying. It’s hard to fathom the cheapskates who want to scrimp on maintenance and balk at spending a few thousand to be able to go home at the end of a flight to your loved ones.
Don’t get me wrong I want to save a buck where I can, but this is the heart of your engine. If you can’t afford to have them done at 500 hrs you probably don’t need to own a airplane.

Just my opinion

JP

I believe many people do not overly concern themselves with magnetos because they primarily only consider their ability to provide an ignition-spark at the appropriate moment. This view is thought to rely upon the fact that Dual mags provide adequate safety-against an inflight emergency….after-all…”If one fails…. that’s why I’ve got TWO of them!…Right..??”

This limited point of view can lead to serious in-flight troubles…. Because…. magnetos are MECHANICAL devices that when the mechanicals fail…they can create extensive engine damage and in-flight engine failure. Their electrical components are far less likely to be a problem than their far-greater vulnerability to Mechanical failure. That mechanical failure can drop parts into the engine gear-train and end up with catastrophic in-flight failure…or, at the least a very scary engine roughness… and can be VERY Expensive.

Even if that worst-case scenario doesn’t occur… I can relate how one less insidious can lead to dangerous conditons…. Because it HAPPENED TO ME.

It was in a TWin.. A Cessna 402… while enroute the left engine suddenly ran rough. Not a failure…but very rough. So I did a magneto check…by switching off one mag….and it got very SMOOTH…. which led me to believe the problem existed in the magneto I switched OFF. I completed the flight and called the maintenance department while my pax conducted their business. Another airplane was brought to me and the troubled airplane was flown home by the mx dept.

Turns out the bad mag was caused by a nylon distributor gear in the magneto…. it had stripped teeth on the gear which operated the “rotor”….the device that points the spark toward the proper cylinder/spark plug at the moment of coil discharge. The mx dept replaced that broken nylon gear inside the mag and returned the aircraft to service.

I warned them that I felt that solution was too simplistic and felt the engine should receive a “top overhaul” at the least. (I told them this not because I’m such a brilliant engine technician…but because I had seen this sort of thing in a past life in which a stripped dist-gear in a mag had blown the head off a cylinder.) The airplane was returned to service anyway…and less than 50 hours later that engine blew a cylinder completely off the engine with another pilot flying, and fortunately no pax aboard…he landed on an airport almost directly beneath him

How does this happen? When the gear rotates the “rotor” to send the spark to each cylinder in proper rotation according to firing-order…then all is well. But if the dist-gear strips…and the rotor stops turning….the magneto is STILL being DRIVEN by the engine…and the coil/points/condenser is STILL MAKING SPARKS at the rate of 6 times every two engine revolutions…. which at the cruise RPM of approx 2400 means that ONE CYLINDER …the one the stopped-rotor is pointed toward….is getting 1200 sparks every minute at the WRONG POINT in it’s cycles of intake/compression/power/exhaust. It will be getting many sparks when its piston is still moving UP in the cylinder while compressing fuel/air mixtures…. much too soon…too advanced for the cylinder design. This shocks that cylinder head, piston, connecting-rod, crankshaft, accessory-case-gear-train… etc etc …. You get the picture. An opportunity for catastrophic failure 1200 times per minute every minute.

The answer? Regular magneto inspections and maintenance. 100 hours/Annually is not too often, IMO.

I've also lost a mag in flight. Didn't really diagnose it while in flight, I had a rpm drop and thought I had a case of carb ice though I shouldn't have had ice and heat didn't fix it. I was concerned enough about what I didn't know was going on with the loss of power to request an immediate deviation through a restricted area directly to the closest airport. This was a restricted area I'd just been denied clearance through so I knew I'd probably face some scrutiny from ATC.

Once safe on the ground the engine seemed to run fine. We parked as this airport was our destination for the day and went off visiting friends. Next day bright and early we went to leave and that is when we found the dead mag. How VERY inconvenient. Turned out the pigtail broke off the capacitor. Would have been much nicer to find this at home with tools and the ability to source parts. Luck was on our side. There was a maintenance tech at the field and when we inquired about another capacitor he pointed us to a pile of used mags and said he had no inventory of what was exactly in the pile. He then said he was going on break in the next hangar and wished us a good flight home, winked and left. We got lucky and found a flat tip screw driver, the only tool we needed and off we went for home. Would have been real expensive loosing work stuck at Ft. Cambell waiting for parts.

I had lost the r/h mag on a flight and didn’t know it. I had done a very long flight at 13,000ft and throttled back to 1500rpm for a slow long decent .as I throttled back there was a couple of “pops” from the engine so the carby heat went on. All engine parameters appeared to be normal after that. Landing was a non event and taxied up to the hangar. I normally do a mag check at 1000rpm before shutdown. R/h mag dead. Opened the cowls and the plate that holds the HT leads to the mag was swimming in the breeze. The 4 screws that hold it to the mag had departed . Replaced the missing screws [course thread] and star washers ,engine ran fine .no rev drops etc. checked l/h mags screws all good.

While discussing in-flight mag-failure, Here’s another indication of a failed mag some pilots may not have thought about or seen …. If you have an Exhaust Temp Gauge (EGT) and have leaned your engine to a customary EGT-reading….then later notice the EGT is curiously running higher than normal…. Don’t automatically assume the engine only needs to be re-leaned. If a magneto fails the EGT indications wil INcrease due to the retarded-ignition-timing-effect resulting from running on only one magneto.

Note: A quick “mag check” will reveal the failed magneto…. BUT you might wish to consider a couple of things before running an in-flight magneto check.
1-Switching the only operating magneto back “ON” after running the check may ignite accumulated fuel/air mixture in the exhaust with resulting damage to the exhaust/muffler system.
2-The sudden stop/run experienced will abuse the crankshaft counterweights.
3-The sudden silence when you shut off the only operating magneto might startle/wake up the pilot.

(While on that subject…. As a CFI I’ve found it useful to caution clients that if during run-up the mags are accidentally switched OFF…. It’s probably best to allow the engine to die completely…then re-start due to the possibilities of reasons 1 & 2 above. It is unlikely that a check of magneto-switch to the OFF position while at low-idle will cause a problem…. But I prefer to avoid the risk. In order to check a magneto switch operation … I often do so during a re-fueling stop by using the magneto to kill the engine on a quick-turn-around (instead of using the mixture.)

If what you say above is true
Why isn't it mandatory to have the mags
Inspected or overhauled every 500 hours?
Iowa

https://www.airmarkoverhaul.com/afforda ... on-repair/

https://cessnaowner.org/500-hour-magneto/

https://www.championaerospace.com/asset ... B1-86C.pdf

https://www.wmtonline.eu/en/our-product ... nesses/faq

http://www.tcmlink.com/pdf2/SB643C.pdf