Shut-down magneto check - advice
 

It seems fairly standard practice at the end of a flight to switch mags off one-at-a-time to check each ignition circuit (i.e. Lycoming).

I've also been advised by several instructors that switching off both together, and back on, checks whether either circuit is live with mags off. Sounds reasonable.

However, our Rotax 912 guru has made it clear that this is seriously dangerous practice on a Rotax, with the risk of shearing propeller bolts.

He also commented it was bad practice on a Lycoming (for example) as it would torque-shock both the crank and prop bolts.

I'd really appreciate hearing your opinions on this.

:eek:Switching both off risks cracking the exhaust inside the heat exchangers resulting in fumes inthe carb or in the cabin.

If you switch off the left mag and get a drop, ditto the right, neither mag can be permanently 'live' or you would get no drop when you switch it off. Ergo, no need to switch off both together then on again.

It's that last bit that can do harm. Some aeroplanes (such as the Chipmunk) don't have a lean cut off, so the engine has to be stopped by switching off both mags (and simultaneously going to full throttle). The latter obviates any tendency to 'run on', as incoming vapourised fuel is ignited by glowing carbon in the cylinder.

How many fractured prop bolts have you seen?

Thanks, guys.

Worrab - me none, our Guru experienced 2 on a 912 with the others loose (same engine)

With a Lycoming or Continental engine at idle, briefly switching off both mags will do no harm, and is good practice in my experience. Any power while doing this may result in a damaging backfire in the exhaust, so idle only.

Your propeller bolts will be fine! Anything which "shock loads" your engine, you're going to feel as a shock in the fuselage, so you'll know it's happened. Careless pilots are doing more harm slamming in full power form idle, than could ever be caused by any power change in the idle power range....

I have too little experience with Rotax engines to offer a qualified comment for them. But, for any engine, a potentially damaging action/event will probably be mentioned in a flight manual limitation, or at least cautionary note. Are there any such for your aircraft?

Why is it good practice? It's potentially damaging and there's no reason to do it, so why would you?

The OP specifically referred to the Rotax 912. This engine is stopped by switching both ignition switches off. It does not have a mixture control or idle cut off. With this engine it is a nonsense to briefly switch both ignition circuits off and then back on again to "check that both circuits are off". Why? Because stopping the engine with the switches means that you have automatically checked that the ignition is off. If one circuit was still live, the engine wouldn't stop!!


Yes this does happen. To stop the engine if the switches don't kill it you have to switch off the fuel and wait til the carbs empty, or you can put on full choke.

My experience is Rotax engines:

Why, on shut down, would one then put the mags back on?

We follow the procedures as set out by SSD above. One off/on, the other off/on, then both off. Listen for the mag drop.

Engine is now stopped, time for tea and biscuits.

Why would we flip them off and then hope there is enough enertia in the prop to give the required 300rpm so that the mags will allow the engine to fire up again if we flip them on? And then just switch it off?

Rotax's are like car engines, turn them on and turn them off - no running rich or lean etc.

To be frank, mags off engine stops. Quick as that! And the Rotax mags have a safety feature built in so that you need 300rpm on the prop to fire.

Ask anyone with a Rotax 912 - particularly the 912S - and the biggest problem for your engine mounts and sprag clutch is starting in winter and getting the engine to half fire, cough and splutter and not really get going. The kickback on the 912S is a real engine knackerer.

It also can eat engine mounts on C42s, Eurostars, Europas and weightshifts - from personal experience. With 4,000 hours on 912 powered C42s at Strathaven, we have some idea of what is good.

None of the above applies to legacy engines, no idea about 100ll guzzlers!

I was always told never to switch off both mags when the engine is running.

My understanding is that if you switch off both mags the engine will continue turning on its momentum for several rotations, drawing in fuel and pumping it into the exhaust unburned. This risks a back-fire (i.e. fuel burns in the hot exhaust) with the risk of damage as mentioned above.

All that applies to Lycoming engine found in PA-28s etc. As for more modern fuel injected types I couldn't say if it's the same.

It's not switching off the mags that's the problem, it's switching them off and then on again!

Apart from anything else, that's begging for a massive backfire (explosion in the exhaust) as the unburned fuel/air mixture is ignited by the next blast of hot exhaust gas as the engine re-lights. Try it in your car / on your motorbike some time. Switch off the ignition when driving / riding along, then on again. BANG! Damage to your exhaust baffles at best! (Probably won't work with modern electronic fuel injection as the injectors will shut off when the ignition is switched off. It's carburated and probably the simple non-electronic injection of aero engines where this will happen - Lycoming, Continental, Rotax, de Havilland etc).

And I ask again, WHY would you do it in any aeroplane anyway? What is the reasoning? Surely we don't have pilots doing daft things they haven't thought though?

Quite.

This is the story from the OP:
I think I’d tell those instructors that they should consider what they’re advising from a technical standpoint.

If the “and back on” was left out it would sound better. As it is, no. If the engine stops when both mags are off, you’ve proved they’re working properly. If it carries on running, you have a problem.

Anyway, you should always treat an engine/propellor combination as liable to start at any time without warning. That way you won’t be caught out if by some miracle it happens...

Beats me. It’s not unique in the collection of odd behaviours with no rational explanation exhibited by pilots who should know better. Maybe it’s because there isn’t terribly much in the way of supervision, guidance and for want of a better word, oversight, once people are qualified and fly in GA. Plenty of space for funny little habits to form, especially if you’re an instructor somewhere out of the limelight.

Some time back I was trying to check out an instructor on a Cessna 421, powered by "GITSO" engines, on the way to the run up pad I sugested he do a "dead mag" check, to my horror, and to the great distress of the gear box ,he shut of BOTH mags and then turned them back on, never did finish checking him out, and the gear box needed changing ten hours later, his career as a corporate pilot {not in my aircraft} lasted four trips before he bent a twin rather badly, so now he is back teaching students how to wreck engines, and all the other nonsense procedures which have crept into flight training on this side of the pond.{such as endless checklists in single fixed gear, fixed pitch trainers}

Sorry I could not reply to your first request for reasoning, I was outside clearing snow...

The mag switch closes a circuit to "turn a mag off" - it grounds the mag out. The fact that the "L" and "R" positions of the switch will turn off each mag in turn, is not a test that the switch "off" position is working to turn them both off. There are AD's on Mag switches for incorrect function.

I was trained to do live mag checks, at dead idle, as required, so I do them after some flights. This was vividly reinforced for me one day, while hand propping a friend's 150 (which I also flew regularly). I called "Mags off", he called back "mags off". I swung the prop to pull it through, and it started. Happy I always swing props like it's gonna start. I gave him a rather disgusted look through the prop arc. He held up the keys for me to see.

Twice, I have had to key "Off" an engine, as someone was approaching the prop, and I wanted it stopped fast. And, in a floatplane, sometimes you want the engine off at that moment, not idle cut off from now.

So, after careful reasoning, and some experience, I do live mag checks - at dead idle only, and for the briefest moment. Just enough to be sure it stopped. I've never had a problem doing that. Not every flight, if I know the plane, just time to time, or before water docking.

For aircraft types where the mags are switched by only two toggle switches, I would agree that two individual mag checks sum to the whole check, but for key type switches, I'm not convinced....

Depends how fast your plane is approaching the dock ... if you're on floats you don't have any brakes.

T directly answer the OP's question -you stop a Rotax to minimum revs and then switch off both mags. The engine will stop, if you have reduced the revs as far as possible with less violence than might occur otherwise. With my conventional mag type switch I can go to one mg and then off.

Switching them back on would cause senseless damage - as it would with a Lycon.

The 912S is a fierce little engine which normally stops as though it has seized, slowing it as much as possible reduces the strain on the mounts and prop.

Switching the mags back on - in any sort of engine is asking to shock load it, blow the exhaust to pieces or just plain stress it. Doing that on a Rotax is even worst, a high rpm, high compression engine should never be treated that way.

Step turn, the rotax typically has two separate switches, so turning them off one at a time does check that both mags are earthed.
Our usual procedure is to check individual mags at engine run up, then stop the engine by turning both mags off together.
Rotax engines do sound a bit solid when stopping....

Step, if you do a mag check (left switch off, then on before doing same with right) and you get a drop in rpm when each switch is 'off' (grounding its mag) then you have not got a permanently live mag. If the mag was permanently live (un-grounded even with the switch in the 'off' position) then you'd get no mag drop when you selected that switch to 'off.

This is the same whether you have two toggle switches or a 'left, right, both' key switch.

You'd have thought the manufacturer would have written some sort of manual that tells people how to start and stop the engines!

I'm with Step Turn on this one.

Firstly, different types of engine and engine control/operation here.

Rotax is simple - reduce rpm to idle, off with one ignition switch and rpm decays, off with the other and engine stops. Thus proving the switch is doing its thing.

Lycoming or other (non-Rotax) aero engines that have two independent ignition switches obviously behave like the Rotax.

Lycoming or other (non-Rotax) aero engines with a keyed ignition switch should be treated differently.

For sure, checking the 'left' and 'right' positions will tell one if in that position the switch is earthing out (switching off) one mag or the other.

The 'off' position may have an internal problem of its own therefore allowing one or both mags to be live. Hence, the 'dead cut' check is carried out - but only at idle where it won't cause a problem when the ignition is turned back on.

If someone goes to off accidentally when checking the mags at any rpm other than idle, then you're better off letting the engine stop completely and then restart it.

Personally, when checking mag timing on maintenance checks, I always try moving the key about in the ignition switch in all possible positions (including off) with the mag timing box 'on' to make sure the switch is behaving as it should. I've replaced a number of ignition switches over the years because of an intermittent live mag when the switch was in the 'off' position. I also check to make sure the key can't be removed in any position apart from when 'off' - again, I've found worn keys or switches that fail this check.

A backfire is actually ignition in the induction side.

That sounds logical (the ignited gasses heading back down the intake), but I never heard of a 'front fire'.

'Backfire' has become the accepted term for fuel / air mixture exploding in the exhaust having passed unburned through the cylinder (because someone turned the ignition off then on, or on closing the throttle on a fast running engine when there's an exhaust air leak). The accepted term for it happening in the carb or intake (the induction side) is 'spitting back'.

Those of us who used to maintain our own (carburated) cars back in the day were very familiar with both as we struggled to get ignition timing and fuel mixture correct.

Smarthawk has it absolutely right.


MJ:ok:

Any one read SB's ?
 

There is a Lycoming SB about shutting engines down, the main thrust of it is to prevent plug fowling, unfortunately no one above has even referred to this and the procedures it recommends.

Most of what I read above is just the rehashed flying club folk lore with little technical merit, Smarthawk being the one beacon of technical expertise in this very dismal thread.

SSD,

I was surprised to read that. The T10s I flew in the 60s had a "slow-running cutout", consisting of a ring and a cable on the right side of the cockpit. I had to look up the Pilot's Notes to double-check my memory and I see that it was introduced as Mod H246. Originally T10s were shut down by turning off the ignition switches.

Further reading shows that the RCAF Chipmunks also had a slow-running cutout, but the Canadian civil ones did not.

Was the cut out removed when the T10s were placed on the civil register?

wow, I guess I am dumb. granted I haven't flown a piston anything in over 30 years, but I never did a post flight mag check.

shut down with mixture, prop stops, remove key.

oh well.

lycoming was my main piston, continental too.

I second what Smarthawk wrote :ok:

None of the civvy chipmunks I've flown had the 'Ring pull' cut out, and some of the ex T10s had it removed. Most of them still had it though.


MJ:ok:

A and C,

The OP was about ROTAX engines.

Perhaps being taught wrongly by those experienced on other types?

time for a new thread...
 

Inspired by Step Turn and Piper Classic's references to float planes... and to avoid thread creep, might be interesting to hear from you guys that have or would like to have experience on these aircraft. so I shall not entertain you here with my experience of using idle cutoff as an anchor when approaching the dock.....

Carry on with your mag checks....I've only set an engine on fire once, when overpriming a reluctant 152 when I was a learner....

I can see Smarthawk's point about the 'off' position maybe having a fault, so by all means put mags to 'off' on shutdown. But as I keep saying the damage can be done when mags are switched off then on again while the engine is still turning.

So if you want to check both mags are 'off' in the 'off' position, turn the key to that. But why turn them on again having proved the engine stops firing?

Civvy Chippies I've flown were always switched off by switching off the mags and fully opening the throttle - there no other method (OK, you could turn the fuel off and wait a few minutes for the engine to stop!). In the engine compartment one could see where there used to be cable coming from the RH side of the panel, but I always understood that to have been to cycle the cartridge starter, which was removed when Chippys were civilianised.

Xrayalpha
 

I have no doubt whatsoever that the teaching is wrong, most of what I see taught at flying clubs is rehashed Gypsy procedures and limitations.

Lycoming do publish a shut down procedure I have never seen it carred out by a civil flying club.......... The RAF however do use the procedure writen in the Lycoming SB, I am guessing they are the only people who have bothered to read the technical data.

I have no doubt that there are still instructors reeling out the same rehashed Gypsy stuff for Rotax engines.

I doubt the prop bolts are the issue with the Rotax, I would hazard a guess any damage would result in the reduction gearbox or clutch.

SSD:

On conventional Lycoming/Continental engines with rotary mag switches, you shut down the engine by moving the Mixture to ICO, then, when the engine has stopped, switch off the Mags. This not only prevents 'pre-ignition' ot 'running on' after switching off, but has the advantage that the engine is left in a doubly safe condition, as both the ignition is off, and the idle system is drained of fuel.

You carry out the 'Live Mag' check just before that, with the throttle closed, and the engine idling. The reason you put the mags back to 'on' is to enable the idle system to drain when you put the Mixture to ICO.


MJ:ok:

That's always been my procedure (for Continental and Lycoming), never a problem.

The sense of abuse of aircraft piston engines seems entirely misplaced by some folk. An idling engine is pretty hard to abuse in the short term. What I would really like to see stopped is throttle jamming - in either direction. Pilots sitting on the runway, and jamming it open, or at altitude, jamming it shut. That's where expensive damage is being done! If you handled the control wheel or stick that way, you'd be damaging the airframe, how is the engine any different?

I always do it before shutting down (Lycoming) but only one at a time. One expects slight RPM drops to verify that the switch is grounding each mag. in turn. If the engine stops you have just detected a defective magneto.

I have had the engine stop when doing this at the end of the day after a flight. The magneto had failed and needed to be replaced. It meant that it was possible to get the aircraft fixed in a timely way and not find out about it only when starting the next time.

If it stops firing wnen one mag. is off then just let is stop.

Jim59, will the post flight mag check as you have suggested, tell you if you could have a live mag after the engine has been stopped? The post flight check is often called a "Live Mag Check".

I "third" it.

The "dead cut" mag check technique was taught by the RAF on their Lycoming engined aircraft, which had a single rotary type magneto switch. This type of switch has three earthing contact positions in one switch, rather than two switches with an independent earthing contact in each. Two positions earth out one mag each, the "OFF" position earths them both.

Normal shutdown was from idle "Drop no stop, drop no stop, drop and stop, back to both" Then the fuel cutoff knob was operated to stop the engine from idle.

If a rotary mag switch has failed internally at the "OFF" position (i.e. the earth contact isn't earthing) and you stop the engine using the fuel cut off, you wouldn't know that one or both mags had been left in the ungrounded condition. So, the rotary switch points at "OFF" but both mags could be live.

At idle, the "shock" caused by switching the mags back on again is no worse than that caused by a partially fouled plug shorting out during taxying or during a power check, After all, only one cylinder fires at a time.

Makes perfect sense on an engine with a rotary "mag" switch and that is stopped by pulling the idle cut off. But no sense at all on a Rotax 912 or similar that is stopped by switching off the "mags" and that doesn't have an idle cut off.

Which is why the manufacturer's advice should be sought out and followed, rather than hearsay from folks who have never read it.

Yes. And this briings us nicely back to the original question.

Modern engines and systems have introduced new advantages but also new risks, and modes of failure.

Although a 'One Fits All' procedure may be ideal, and was quite common in the 'old days', different combinations of engines, installations and systems may require different methods of operation.

If you are unsure what you have, or how it should be operated, go back to the official sources of information, and only add to, or introduce variations to the official advice if you are absolutely sure that you are not introducing unknown risks by doing so.


MJ:ok:

Ps. Just to check that we're all using the same terms here: A 'Dead Cut' check is where you switch off the mags/ignitions one a a time to check primarilly for a faulty mag/ignition, when the engine will 'dead cut'
A 'live mag' check, is where both mags/ignitions are turned off to check that neither of them are still 'live' when they are both in the 'off' position.