Magnetos

[mgh 89]

I do agree that the primary purpose was probably to allow even fuel combustion, but it's now commonly excepted as a fail safe, too.

I believe twin magnetos were introduced around the start of World War One.

[Matthew Kane 1,343]

It's not all redundancy: Two magnetos with two separate sparks plugs set up symmetrical flame fronts inside the cylinder after mixture ignition, thus reducing the onset of detonation and allowing a higher manifold pressure setting and correspnding higher BHP output during take-off, and a leaner, more fuel efficient mixture during cruise for added range.Cheers,- jahman. Director, Department of Redundancy Department.

[Guest jahman]

In engineering, redundancy means duplicating critical components of a system to increase its reliability and hence safety. It's to ensure that a single failure cannot on its own create an unacceptable risk. Pre-take off checks include switching to left and right magnetos and checking that the rpm drop is within acceptable limits. If it is isn't you don't take off. Can you givwe me an example of a take off chart for a piston engined aircraft that distinguishwes between one and tow magnetos?It's you who is wrong - not the POHs. As was said years ago a little leaning is a dangerous thing.

With aircraft magnetos, redundancy is not assured. Think of an old Seneca taking-off at max gross weight on a hot day and you lose an engine at 500 ft AGL. Now tell me the second magneto in your remaining engine is redundant! If you lose one magneto one that engine you won't maintain altitude and you will crash. So much for your idea of redundancy.Now the idea that either I am wrong or the POH is wrong, wherever did you get that from? What I said is the performance charts in the POH are based on two magneto performance, which they are.Cheers,- jahman.

[JRBarrett 3,113]

Redundancy is not assured in certain engines, in which the dual magnetos are contained wthin a single housing (side-by-side), both driven by a common shaft. If the drive shaft were to shear, you would lose both mags. Such an arrangement isn't common, with the usual system being two discrete mags, each driven by its own shaft.Jim Barrett

[mgh 89]

[

With aircraft magnetos, redundancy is not assured. Think of an old Seneca taking-off at max gross weight on a hot day and you lose an engine at 500 ft AGL. Now tell me the second magneto in your remaining engine is redundant! If you lose one magneto one that engine you won't maintain altitude and you will crash. So much for your idea of redundancy.Now the idea that either I am wrong or the POH is wrong, wherever did you get that from? What I said is the performance charts in the POH are based on two magneto performance, which they are.Cheers,- jahman.

What power does a Seneca need to maintain level flight with one engine out?What power can a single engine deliver with both magnetos working?What power does a single engine deliver with one magneto working?

[RyanB 0]

Redundancy is not assured in certain engines, in which the dual magnetos are contained wthin a single housing (side-by-side), both driven by a common shaft. If the drive shaft were to shear, you would lose both mags. Such an arrangement isn't common, with the usual system being two discrete mags, each driven by its own shaft.Jim Barrett

Funny thing, I tore one off and witnessed a shaft that sheered about 4 months ago :(. The first thought is "UH OH" (Followed by an urge to flail ones arms and run in circles) Thankfully the airplane had two seperate magnetos, and redundancy was assured. The better 99% of airplanes use seperate magnetos (Vs. the two in ones).

[ZachLW 37]

Funny thing, I tore one off and witnessed a shaft that sheered about 4 months ago :(. The first thought is "UH OH" (Followed by an urge to flail ones arms and run in circles) Thankfully the airplane had two seperate magnetos, and redundancy was assured. The better 99% of airplanes use seperate magnetos (Vs. the two in ones).

Yes, the two in ones kind of defeated the purpose. Take for instance the Skylane 182RG I used for the commercial. What a peace of junk that part is. I really wasn't expecting this thread to get this deep. :( It's fun though.Jahman, I respect and value your input, as always, but you need to stop arguing and just go with what everyone and I says this time --for simplicities' sake--. :( But seriously, you're coming up with exceptions. Doesn't mean it's the rule! I like that someone cares about these systems like I do, though. You're a purist for sure!

[Guest jahman]

Funny thing, I tore one off and witnessed a shaft that sheered about 4 months ago :(. The first thought is "UH OH" (Followed by an urge to flail ones arms and run in circles) Thankfully the airplane had two seperate magnetos, and redundancy was assured. The better 99% of airplanes use seperate magnetos (Vs. the two in ones).

Indeed, with two magnetos in some situations you have redundancy and in others you don't (as I explained previously). So nothing is "assured": Redundancy depends on the situation. There is a flight envelope inside of which a second magneto will be redundant and offer safety, but move outside that envelope and the role of the second magneto will change from being redundant to being essential. Fairly simple, really.Cheers,- jahman.

[RyanB 0]

Indeed, with two magnetos in some situations you have redundancy and in others you don't (as I explained previously). So nothing is "assured": Redundancy depends on the situation. There is a flight envelope inside of which a second magneto will be redundant and offer safety, but move outside that envelope and the role of the second magneto will change from being redundant to being essential. Fairly simple, really.Cheers,- jahman.

For its primary purpose: Continuing to operate in the face of a magneto failure. The system is redundant. It's not designed to undergo normal operation with one magneto, but it can and will when need be (Abnormal operations, for which many situations there's not a procedure for, but sizeable chunk of the POH goes out the window and you have to use common sense).The idea of redundancy can also be witnessed in the positioning of spark plugs and how they fit with the harness and magneto:Each magneto has a lead for each cylinder, on one side two leads go to the top spark plugs, on the opposing side, on the bottom. It's the same for the second magneto, and in the event of Magneto Failure. You'll always have two cylinders igniting at the upper side vs the lower.I don't see your "Kicks" in explaining how a system that is redundant, moves to being essential after a failure. The point of redundancy is not to maintain redundancy post failure. (Otherwise we could have ten magnetos, and an elaborate ignition scheme, and even then by your logic it fails to be redundant) You have to decide what is and is not in context.Context:There are two Magnetos on this airplane. The reason for two, is that if one fails - the second can continue to ignite and operate the engine. The system is redundant.To say the system is not redundant, because other things will change in a small way is to take the word redundant and move it out of context.

[Guest jahman]

It's not a question of kicks, RyanB, rather one of clarity of understanding, in that redundancy of magnetos is not a black or white situation, but has shades of grey, a partial redundancy, if you will.Alternatively, you can distinguish between redundant in terms of the installation ("the engine has 2 working magnetos", the meaning preferred by the manufacturer) vs. redundant in terms of ultimate functionality ("the engine will perform with two magnetos, but at a lowr level" which *should* be the preferred meaning for the pilot.)If you think I'm splitting hairs, I can quite assure you I'm not: I once had to pass a check-ride with an Air Force coronel in a C-150 in the middle of summer: The fella was fairly heavy set and I'm not exactly on the light side myself (the cockpit seemed somewhat tighter than I had remembered from way back), plus the engine had one of the cylinders with compression problems with less BHPs than than I would have liked, but eventually the prop RPM'd its way "up there". The combination of summer heat (density altitude), weight and a less than performant engine was evident after take-off: 200 fpm at 200 ft AGL over some nice houses with gardens, swimming pools and barbies (both kinds). Well, mate, I can assure you at this point having 2 working magnetos was not redundant, it was essential. Catch my drift?So semantics are important, especially if you are an instructor: You may know that WRT magnetos redundant actually means partially redundant to, say, only 95% max BHP, but if you don't explain the subtleties of magneto redundancy to your students, they just might understand redundant as "fully redundant" and some day perhaps suffer consequences to be sorry for.Cheers,- jahman.

[RyanB 0]

It's not a question of kicks, RyanB, rather one of clarity of understanding, in that redundancy of magnetos is not a black or white situation, but has shades of grey, a partial redundancy, if you will.Alternatively, you can distinguish between redundant in terms of the installation ("the engine has 2 working magnetos", the meaning preferred by the manufacturer) vs. redundant in terms of ultimate functionality ("the engine will perform with two magnetos, but at a lowr level" which *should* be the preferred meaning for the pilot.)If you think I'm splitting hairs, I can quite assure you I'm not: I once had to pass a check-ride with an Air Force coronel in a C-150 in the middle of summer: The fella was fairly heavy set and I'm not exactly on the light side myself (the cockpit seemed somewhat tighter than I had remembered from way back), plus the engine had one of the cylinders with compression problems with less BHPs than than I would have liked, but eventually the prop RPM'd its way "up there". The combination of summer heat (density altitude), weight and a less than performant engine was evident after take-off: 200 fpm at 200 ft AGL over some nice houses with gardens, swimming pools and barbies (both kinds). Well, mate, I can assure you at this point having 2 working magnetos was not redundant, it was essential. Catch my drift?So semantics are important, especially if you are an instructor: You may know that WRT magnetos redundant actually means partially redundant to, say, only 95% max BHP, but if you don't explain the subtleties of magneto redundancy to your students, they just might understand redundant as "fully redundant" and some day perhaps suffer consequences to be sorry for.Cheers,- jahman.

Having experienced worse, the situation is certainly exceptional. In fact I refuse to fly in conditions where I figure I'm going to get less than 300FPM on departure, and such an accident (caused by your described flight) would easily and rightfully be put on pilot error:High density altitude.low compression on a cylinderHigh weight.It's not a recipe that fills the warm and fuzzy, and that recipe has in the past, and will in the future - kill pilots. The correct way to avoid the situation is to understand what the environmental and mechanical needs of the airplane are when it operated normally, and then buff it clean to allow yourself a safety cushion which would leave plenty of room for something like a magneto failure. You shouldn't even have to think about one of the magnetos failing if you thought of more important things before it.I'd say the teaching method should be don't take off in such a circumstance. (This moots the point of the magneto becoming non-redundant). While what you say is true, and I agree that in such a situation the ignition system will "Lose" its redundancy..... in practice you shouldn't ever find yourself in such a situation that the loss of a magneto puts you on final for a field. In that same situation you'd probably want to worry less about the magneto, and more about the weather. While an instructor could teach it, It wouldn't be high on my personal list.

[Guest jahman]

Having experienced worse, the situation is certainly exceptional.

Yeah, I've experienced worse too.

In fact I refuse to fly in conditions where I figure I'm going to get less than 300FPM on departure, and such an accident (caused by your described flight) would easily and rightfully be put on pilot error:...

You've got to be kidding, right? Do you think I would get into an aircraft that I knew would do 200 FPM (or even 300 FPM) after getting out of ground effect? As a matter of fact, I wouldn't get into an aircraft that did less than 500 fpm (unless in a towed or self-launching glider; different story).

It's not a recipe that fills the warm and fuzzy, and that recipe has in the past, and will in the future - kill pilots. The correct way to avoid the situation is to understand what the environmental and mechanical needs of the airplane are when it operated normally, and then buff it clean to allow yourself a safety cushion which would leave plenty of room for something like a magneto failure. You shouldn't even have to think about one of the magnetos failing if you thought of more important things before it.

What you say in NTSB-speak is all very nice and dandy until one of your piston rings fails seconds after take-off, or one of your plugs shorts due to a lead deposit, again right after take-off. So no data recorder on your putt-putt's engine so an NTSB smartie concludes "pilot error". Right. Oh, and add the heavy-set check-ride Air Force coronel sitting in your right seat for good measure. Telling him: "Sorry, you're too heavy for the check-ride" is not the answer to a trick question. We all know about density altitude, etc., etc., but the truth is sometimes you are at the limit (but not beyond), and that's when minor failures can bite you. Mag failure is one of them: If you are at the limit (GWT, old engine with one hour left until overhaul, density altitude, whatever, did those take-off performance charts, checked tire pressures, etc., then one mag fails and poof! you don't get off the runway, or you don't clear that obstacle and you still crash. Then (if you survive) you say: "But I did everything by the book" and you did too, but that one mag failure put you short of required performance and you bought the farm. So mags redundancy? Yeah, most of the time, but not this one time.

I'd say the teaching method should be don't take off in such a circumstance. (This moots the point of the magneto becoming non-redundant).

Sure, what we know and don't use in our decision-making process, we pay for in consequences. But the point about mags redundancy isn't mooted if another failure, unrelated to the magnetos like a ring failure, puts you out of the "magneto redundancy" comfort zone after take-off. The proper teaching method is to not believe that just because you have two mags you are safe. You need to internalize other ways of quickly establishing that engine BHPs meet current flight requirements other than throttle position and mags switch position, e.g. like the turns on the tach (for a fixed-pitch prop putt-putter).

While what you say is true, and I agree that in such a situation the ignition system will "Lose" its redundancy..... in practice you shouldn't ever find yourself in such a situation that the loss of a magneto puts you on final for a field. In that same situation you'd probably want to worry less about the magneto, and more about the weather. While an instructor could teach it, It wouldn't be high on my personal list.

OK, I'm glad we understand each other now, but as I said before, sometimes you just are doing everything by the book and you still are at the edge of your performance, where losing a magneto can be critical. Further, all the performance charts in the POH are for new aircraft. If you fly in older aircraft with overhauled engines, some chips to the prop blades and bumps and bugs on the wing leading edges (a.k.a. "The Real World"), do you still really trust those take-off performance charts? How much do you derate performance by for safety?"In theory there is no difference between theory and practice. In practice there is.” - Yogi Bear.Cheers,- jahman.

[mgh 89]

[What power does a Seneca need to maintain level flight with one engine out?What power can a single engine deliver with both magnetos working?What power does a single engine deliver with one magneto working?

Answer was there none!

[ZachLW 37]

So semantics are important, especially if you are an instructor: You may know that WRT magnetos redundant actually means partially redundant to, say, only 95% max BHP, but if you don't explain the subtleties of magneto redundancy to your students, they just might understand redundant as "fully redundant" and some day perhaps suffer consequences to be sorry for.- jahman.

Semantics are important to a point, but then you just start confusing the hell out of (certain) students. There's also a point at which arguing over a tiny word meaning becomes excessive and down right nit picky. Not every student is smart enough/interested enough to hear a soap box about the word redundancy and it's meaning/exceptions/rules. Notice the argument, as you admitted, has been semantics. Your argument took off opposing common knowledge, but has come back 'round supporting it. I'm confused (not really)!

[n4gix 4,949]

As was said years ago a little leaning is a dangerous thing.

And here I've always thought it was "a little learning is a dangerous thing." Although, come to think of it, I guess too little leaning could be dangerous too! :(