Federal regulation that requires two magnetos

[Sac Arrow]

We all know the FAA requires two magnetos on aircraft piston engines. We think, anyway.

What CFR covers this? Google is failing me.

 

[Racerx]

Type certificate?

 

[MauleSkinner]

§ 33.37 Ignition system.​

Each spark ignition engine must have a dual ignition system with at least two spark plugs for each cylinder and two separate electric circuits with separate sources of electrical energy, or have an ignition system of equivalent in-flight reliability.

 

[Roller]

Also just common sense, I would not fly a plane without two independent ign systems, probably by 1500tt most airmen have had a mag check out, or all but checkout on them

 

[Sac Arrow]

§ 33.37 Ignition system.​

Each spark ignition engine must have a dual ignition system with at least two spark plugs for each cylinder and two separate electric circuits with separate sources of electrical energy, or have an ignition system of equivalent in-flight reliability.

Thank you.

 

[Dana]

Note that Part 33 applies only to engines type certificated under FAR part 33 (all new engines). It doesn't apply to older aircraft certificated under CAR3 (for example, there are still many standard category aircraft flying behind single ignition A-40 engines), and it doesn't apply to experimentals.

Also just common sense, I would not fly a plane without two independent ign systems, probably by 1500tt most airmen have had a mag check out, or all but checkout on them

I have owned several aircraft with single ignition, several ultralights and one experimental. But they were all (including the experimental) very light and slow aircraft, meaning a forced landing was more of an inconvenience than a danger. The plane I fly today is heavier, faster, and has dual ignition.

 

[MauleSkinner]

Note that Part 33 applies only to engines type certificated under FAR part 33 (all new engines). It doesn't apply to older aircraft certificated under CAR3 (for example, there are still many standard category aircraft flying behind single ignition A-40 engines), and it doesn't apply to experimentals.

True. But here will be an equivalent reg going back to at least the 1940s. What that is, I don’t know.

 

[Bell206]

It doesn't apply to older aircraft certificated under CAR3 (for example, there are still many standard category aircraft flying behind single ignition A-40 engines), and it doesn't apply to experimentals.

CAR 13 was the previous rule. But the single ignition system was limited to small engines below a certain horsepower. And I think it only lasted to the early 40s. After that all recips regardless HP needed dual ignition systems or equivalent.

 

[Dana]

And I think it only lasted to the early 40s. After that all recips regardless HP needed dual ignition systems or equivalent.

Yes. I was only pointing out that many older aircraft and experimentals are still flying with single ignition. And there's that "or have an ignition system of equivalent in-flight reliability" language...

 

[Bell206]

Yes. I was only pointing out that many older aircraft and experimentals are still flying with single ignition. And there's that "or have an ignition system of equivalent in-flight reliability" language...

Don't know if "many" would still be accurate on the TC'd side, but I believe the "equivalent in-flight reliability" statement was to cover the use of diesels in certified aircraft.

 

[AV8R_87]

I believe the "equivalent in-flight reliability" statement was to cover the use of diesels in certified aircraft.

Not really, the rule says "Each spark ignition engine...".
My hunch is that they believed at some point a better system would be available and they wrote the rule in a smart way.

I don't have actual MTBF numbers for magnetos, but if I find them it would be interesting to compare them to a modern automotive ignition system.

Had a mid 90s car that had about 6000 hours worth of driving. In that time it had a single coil on plug module failure, around the 5000hr mark, and a cam position sensor failure. The first one caused a loss of power, the second one just made it more difficult to start.
Never had a failure that caused an engine stoppage, but I still don't like the idea of flying with only one spark plug per cylinder.

 

[Bell206]

I don't have actual MTBF numbers for magnetos, but if I find them it would be interesting to compare them to a modern automotive ignition system.

If you dig down into those MTBF numbers you'll find a properly maintained magneto is the most reliable component on the engine. And where there is a magneto failure 80% are due to improper mx.

 

[AV8R_87]

If you dig down into those MTBF numbers

Do you have them, and are you willing to share them?
I always assumed an MTBF of 1000 hrs without maintenance, so two of them combined gives 10000 hours between a failure that will cause a power loss.

 

[Dan Thomas]

I always assumed an MTBF of 1000 hrs without maintenance,

It would be longer than that. I have encountered 1700 hour mags that have never been off for inspection, but they were not working up to standards, either. As the points and cam or rubbing blocks wear, the E-gap changes and the spark gets weaker, making for hard starting and poor performance overall.

The chances of failure rise with more years and hours on them. Corrosion can cause failure of the impulse spring, retarding the spark to near TDC where it is useless. Corrosion can fail the ball bearings; saw that on an old Cessna 170 whose mags had obviously been badly neglected. We took a pair of mags off a Piper Warrior that had an engine at maybe 1500 hours since new, but around 35 years old, and they were corroded far beyond economical repair. Truly sick. Still worked, but would likely have failed before the engine was done.

So, the failure can be gradual or sudden. Doing the 500-hour inspections makes failure of any sort far less likely. Doing SBs also removes failure factors. Making sure any ADs are done on time is just plain good sense.

"Good" mag drops on the runup are no guarantee that your mags are in decent condition.

 

[Bell206]

Do you have them, and are you willing to share them?

The detailed ones I saw where in a legal setting but there have been various articles/studies done on mag failures. For example, the Australian ATSB did a series on recip engine failures and issued some Airworthiness Reports which addressed mag failures and why. Their results were very similar to the US results.

 

[Sac Arrow]

It seems to me that mags don't typically receive the same level of service that automotive distributors use to. When I was growing up, replacing points and condenser was nearly as regular as changing oil. And you may have to reset the gap in between replacements.

As far as I can tell, the aviation magnetos are no different than automotive distributors except they have internal generators that deliver current by the pulse, but the distribution and points contact system is the same.

But in our old cars, we could simply flip a couple of clasps and lift the cap off, pull the rotor out, and replace the points and condenser with a screwdriver and the whole thing took maybe five minutes max if you knew what you were doing.

Pulling an aviation magneto is kind of a bigger deal.