I find that many pilots don't know that the Mag Switch doresn't actually switch anything off. It is a shorting switch to earth the magnetos and prevent them from producing an ignition pulse that could start the engine if the propellor was turned.

The check simply identifies that the earth connection is in place, once the switch is opened, there is an assumption that the next time it is trurned OFF i.e. the switch is made (ON) it will perform the same function. As with anything mechanical it may not! In general, switches are quite reliable so the major function of the test is to ensure that the magnetos are both connected to the switch and that the switch is connected to earth; this can be achieved by checking L and R without the need to go to OFF which will allow unburnt fuel to be sent into a hot exhaust manifold where it may ignite.

Whopity - all true, but all that's already been said more than once here, not least by me.

I'm sorry but that seems to me to be exactly the same thing, unless in the case of the "live mag" check they are both switched off together, in which case, which one is keeping the engine running is unknown until a "dead cut" is done. Making the "live"check unnecessary.
Please forgive the pedantics.

The confusion obvious on this thread is all to do with the peculiarities of using a rotary key switch to control the mags instead of a sensible toggle switch for each mag. All part of Cessna and Piper's attempts to make spam cans seem like motor cars rather than aeroplanes.

Aeroplanes which are proud to be such and don't feel the need to masquerade as road vehicles (Cubs, Chippys, Lancasters etc) use toggle switches. As all aeroplanes should.

I know of a certain Tiger Moth where the passenger caught those "sensible" toggle switches with his sleeve so they got switched off, putting the aircraft down in a field.

BTW, there was no key on the piston engined RAF aircraft I flew. Just a rotary switch.

So it seems one cannot switch off both, one at a time, with a key?
I'm happy with two toggle switches and no keys, far less confusing.

If the pilot was in the back seat, why didn't he see the downward-pointing switches stick out of the side of the fuselage?

Where the switches are inside and one set is out of sight of the pilot (Chippy, for instance) they are guarded to prevent accidental switching off.

I knew a meat bomber pilot who had the last man out switch off the mags and take the key with him. Had they been toggles he could have just switched them on again instead of having to do a forced landing with no power!

Toggle switches rule! Only cars have key ignition! ;-)

SSS, I've no idea why, but then I wasn't flying the aircraft. I think the pilot sensibly opted to concentrate on putting it down due to the low altitude at which it happened. It was flown out again once the issue was resolved.

I'm unsure why you seem so concerned why some aircraft were designed and built with a different system to the ones you have flown. It's simply a matter of learning a new technique and understanding why it's done the way it is on a new type. As I've said twice, there is no key on the type I flew for some years, just a rotary switch.

Shy, I have extensive experience of both types of mag switch. I much prefer toggles for their unambiguous simplicity. The confusion shown on this thread is all to do with the key-operated type. The only reason spamcans have key ignition is because cars do. There is no other reason than 'style' to fit rotary key-operated mag switches.

In my book, practicality trumps style every time!

Well, you asked this:

(Your question in bold) I wanted to point out why there is a reason to do it on some types.

It's misleading and potentially dangerous to put forward as "fact" something which is an opinion based on personal preferences. I've been caught out a couple of times in the past by pilot hearsay so now I always read up the manufacturer's requirements and advise others to do the same. ;)

I've looked up the Rotax 912, which was one of the engines mentioned by the OP in his initial question.

Does it actually have "traditional" self contained and self generating magnetos? Looking at the parts diagram, although there is a magneto excitation coil, there is an external electronic module and another coil shown, too. "Switching off the ignition" may not actually be earthing the system on this type as per a self contained mag, but rather disconnecting the live side. Which means we might be discussing completely separate issues as if they were one and the same.

Shy, the horizontally opposed Lycoming / Continental is a crude old engine, just like all but the latest generation. Same technology as an old Brit motorbike!

There's nothing magic about them that requires different engine handling to those other types. Toggle mag switches would work perfectly well (and do in the Citabria and Cub, just two such horizontally-powered aeroplanes I have extensive experience of which use toggle switches). Why should a C150 or a PA28 be different?

Not having read all of the thread, I am probably repeating. But I do fly behind a 912UL - 80 hp, not an S.

I concur with the first part, I usually do it during taxi though some pilots religiously stop near the treshold to do it.

But I do not agree with the second part, exactly for the reason stated: cut one ignition, count three, cut the other. Engine stops much smoother.

@ShyTorque: the ignition on a Rotax does not use the traditional magneto's, but the dashboard switches still shortcircuit something or other to ground to disable it. If you want the full detail you'll have to find it, schematics are all over the www.

I'm quite well versed with the Lycoming. I'm an ex-RAF QFI and used to instruct on a UAS equipped with them and I was also the Chief Ground Instructor of same.

Why should they be different? Because the designer/manufacturer decided to make it so. If you don't like key equipped aircraft you don't have to fly them. I don't like them either, despite having learned to fly in them in my youth, but it's irrelevant.

My point is, if the aircraft you're sitting in isn't equipped with your preferred toggle switches but a rotary one instead, you really should follow the manufacturer's procedure, not your own based on experience from other types. The "dead cut" check is done for safety reasons. If you don't do it correctly, someone else could get injured.

JO,

Thanks, from what I've seen on forums, discussing problems with ignition on Rotax engines it appeared that these can fail due to power supply wiring external to the engine driven part. Unlike a traditional magneto, which has only external HT leads and a grounding lead to "switch it off".

If that is your understanding you have either visited the wrong forum or read incorrectly.
On the "back" side of the 912/914 there is a flywheel carrying a magnet - or several, I'm not sure there. The magnet passes under a total of 11 pickup coils, producing a gust of power in each at every revolution.

4 coils drive the 4 spark plugs of the one ignition box, providing both timing information and power
4 more coils do the same for the second box
2 coils supply power to the charging circuit
the last coil gives tachometer information

At least this is my understanding, I'll be glad to learn better if I missed something. But there certainly is (or needs to be) no connection between the ignition system and the battery power circuit.

For as much as I heard, there are not often any issues with the wiring. The ignition boxes OTOH have a poor reputation, coming from motorbike maker Ducati. There must be alternative makes but I failed to find any information, up till now. Looking frantically though, as said the Ducati boxes have poor reputation and are quite expensive - for a microlighter's purse, at least.

Jan, I think you misread my post. I never mentioned the battery circuit and there definitely wasn't a battery shown on the parts list and diagram I looked at.

What does the external "ignition box" from Ducati actually do? This was why I asked the question about the detail of this specific ignition system. Does the "ignition" switch turn off the LT side, rather than earthing it, as it does in a traditional magneto?

Well, that's a matter of wording - you did mention "power supply wiring", didn't you? As the wires from the flywheel coils to the ignition boxes provide both power and timing you may have meant those, but I understood you meant "wiring from the aircraft power circuit" and that's what there is not.

A pointer to the circuit you looked at would be helpful, though.

I haven't seen a "circuit" diagram and haven't mentioned that word until now. It was a parts diagram and list.

I've checked some more; it is a Ducati DCDI ignition (double capacitor discharge ignition), rather more complicated than the traditional self contained magneto type of the Lycoming. I understand there have been some issues with the Rotax's external connectors between the generator coils and the module and with the reliability of the modules themselves.

Sigh. You and I seem to have very different vocabularies. What's the difference between a schematic and a diagram?

If we really are going to squibble on words, there is in a 912/914 no "power supply wiring external to the engine driven part" and those ARE your words literally copied.

That said: yes, the Ducati electronics must be rather complicated, certainly more complicated than an electro-mechanical conception of a hundred years old, give or take. And as I already said, they have a poor reputation regarding reliability. But I heard nothing wrong about the connectors, they look like the watertight connector blocks one finds under every contemporary car's engine hood.

I wouldn't refute that, it wasn't the point I was making. The point was that the rotary key-operated mag switch was introduced by Cessna and Piper in place of toggle switches for no good technical reason (Bellanca didn't do it with the Citabria nor Piper with the Cub, both using the same engine as a spam can). It simply complicates what should be a dead simple process, such as toggles give you.

And they did it purely for stylistic reasons (to ape the motor car). There is no technical advantage, indeed there is a disadvantage as it complicates the shut down process and introduces areas of uncertainty among neophyte pilots where none should exist, as this thread amply demonstrates. To say "just follow the manufacturer's instructions" is stating the bleedin' obvious, but it excuses the issue.

In short, it's a piss poor design; style at the expense of function, style at the cost of introducing unnecessary complication and potential for misunderstanding to trainee pilots who really don't need such distractions.

That is my point!

Crash one:

Yes. Just to clarify: During a 'live mag' check you momentarily turn off both mags together and check that the engine cuts out.

This is only necessary where the 'both off' position of the switches is different from the 'left off/right off' positions, and the engine is stopped by a method other than switching the mags off; eg. leaning the mixture.

SSD:

I agree. But we have to operate the aircraft as they are, and not as we would like them to be.

Like it or not, rotary switches introduce added complication and extra modes of failure. We have to be aware of that and operate them accordingly.

By the way, rotary mag switches are not new, nor were they invented by the Americans.

MJ:ok: