Shut-down magneto check - advice

[Kinger]

A backfire is actually ignition in the induction side.

[Shaggy Sheep Driver]

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.

[Mach Jump]

Smarthawk has it absolutely right.


MJ

[A and C]

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.

[India Four Two]

Some aeroplanes (such as the Chipmunk) don't have a lean cut off

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?

[glendalegoon]

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.

[Big Pistons Forever]

Originally Posted by Mach Jump

Smarthawk has it absolutely right.


MJ

I second what Smarthawk wrote

[Mach Jump]

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

[xrayalpha]

A and C,

The OP was about ROTAX engines.

Perhaps being taught wrongly by those experienced on other types?

[mary meagher]

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....

[Shaggy Sheep Driver]

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.

[A and C]

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.

[Mach Jump]

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

[9 lives]

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.

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?

[Jim59]

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 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.

[9 lives]

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".

[ShyTorque]

I second what Smarthawk wrote

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.

[Heston]

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.

[ShyTorque]

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

[Mach Jump]

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

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.