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.