Primarily for general aviation discussion, but other aviation topics are also welcome.

Do you do a magneto dead-cut check?

Always
52
51%
Sometimes
9
9%
Never
40
40%
By Jowloo
#592252
I experienced the flabbergasted reaction in SA too. But there seemed to be lots of little differences to practice and procedure over there so I took it in my stride.

On my PPL in the UK I was taught to check the mags operation by checking the drop on each mag in turn. Only in SA did they turn both off and then back on as well. I don't recall the engine having to be at idle, as idling is not something I wouldn't normally do in the shut down process. Usually it's just 1200 RPM, thottle closed, fully lean and stop.

Performing a complete dead-cut (i.e. completely off, then back on) has always resulted in a backfire when I've tried it in the UK. I don't like it and was contemplating reverting to my UK training to avoid it. Having read this discussion I am considering trying it, but at idle speed. Is that what people are collectively advising?

BTW I'm talking about the C172.
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By Irv Lee
FLYER Club Member  FLYER Club Member
#592287
well if in 10,000 flights you get a backfire on 9999 of them, and on one you find the engine keeps running, I'd assume you would have other problems caused by the backfires well before you got to that one flight where the engine kept running.

Dead cut check
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By Adrian
#592291
I voted "sometimes".

On the rare occasions that I want to use a DR400 from my local French aéroclub and have a check flight, the instructors insist that I use the checklist and insist that I move the mag switch to "off" soon after engine start, and again just before shut down.

It's not something I do in my aeroplane, but also it doesn't seem to cause the club aeroplanes any particular engine / exhaust troubles.
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By Grey Beard
#592332
But couldn't the 'dead cut' AKA 'live cut' actually make things less safe? I always understood that by letting the engine die by pulling the mixture out, you were ensuring that there was no combustible fuel still in the engine. Stopping the engine by grounding the mags would not do this - in fact it will increase the amount of unburned fuel hanging around (hence the backfire potential). As a live mag-problem could be an intermittent fault, (perhaps caused by a broken grounding lead that makes sufficient contact while the engine is vibrating and/or warm, but not when the engine is really stopped), turning over the prop by hand may result in a nasty surprise. (And BTW, how many pull the prop over a few times before starting?)
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By KNT754G
#592623
[quote]The trick is to understand WHY the checks are done. The two checks have two different purposes:
- The dead-cut just after start is to ensure that the bendix has disengaged (engine would not cut if the bendix failed to disengage), to avoid wrecking the starter motor.
- The dead-cut just before shutdown is to ensure that the magnetos both actually ground when the ignition is switched off. It's therefore part of a safety check to ensure a "dead" prop after shut-down.[/quote]

Sorry but that is TOTALLY missing the point!

Switching off both mags just after start will NOT tell you if the bendix has disengaged.
The starter warning light will tell you if the starter relay is still energised, NOT whether the bendix is engaged, switching off both mags will result in the engine stopping (unless you switch them back on again quickly with attendant risk of backfire) regardless of whether the bendix gear is engaged if the relay is not energised.

Has anyone actually tried to stop an engine from 1200 RpM by switching off the mags?
I have had a couple of students lately inadvertently switch both mags OFF either just after start or just before deliberate shutdown. The engine keeps running (extremely lumpily) for some time due to the fact that it is seriously hot in there and fuel is still being supplied via the rich mixture control and could easily be interpreted by the inexperienced as a magneto still live.

I go with Irv's Baseball Bat theory. NEVER switch both mags off with the engine running.
If you accidentally do so (because the labelling on some switches leaves a lot to be desired) then leave them off and immediately pullthe mixture to ICO in order to minimize the unburnt fuel in the engine.

Then restart properly, run for a minute and continue from where you left off.
If that happened to be engine shutdown then shut it down properly this time.
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By Grey Beard
#592773
The starter warning light will tell you if the starter relay is still energised, NOT whether the bendix is engaged, ...

I keep my headphones off until after engine start so I can hear the whining crunching sound that I expect a stuck starter would make.
By bookworm
#592793
Adrian wrote:It's not something I do in my aeroplane, but also it doesn't seem to cause the club aeroplanes any particular engine / exhaust troubles.


It's well known that any club aeroplane engine that doesn't meet TBO is always due to a student having the temerity to touch the mixture control during flight. It has nothing to do with frying the engine with climbs at Vx, shock cooling from repeated engine failure drills or the abuse it suffers from live mag checks. :twisted:
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By Ridders
FLYER Club Member  FLYER Club Member
#592808
As the CAA NZ state in that pres - "For something as basic to piston-engine operation as magneto checks, it is surprising how much misunderstanding
exists." Yup - Ive learnt something about the terminology used, for sure. :)
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By OCB
#592967
Had a few instructors, not one has covered dead-cut/live cut. Did do it in groundschool though, but it was more of a warning re: always assume prop is live.

Quite a bit off topic - I once had a stuck on starter in the middle of Glasgow during rush hour in a Triumph Acclaim. Thing wouldn't disengage, even with the ignition OFF. Yes, direct line during starter engaged - bypassing the ignition switch.

It taught me not to assume that turning the ignition switch off would kill the engine (auto/boat/aviation). The only remedy was very quickly running to the boot, getting my trusty 1/2 inch spanner and yanking the earth lead from the battery (no fuel cut alas!). Dogs abuse from my fellow Clydeside commuters, but I reckoned a couple of minutes abuse whilst Short Handled Ball-Pin Hammer Mk1 took care of things was preferable to Rusting Triumph with Engine on Fire :oops:

Given the vintage of the aircraft I (attempt to) fly - being paranoid about the engine is maybe too high on my priorities. Even to the point of arguing with my "747 pilot during the week" instructor...I still can't go from idle revs to full power on TO in the "confident" manner I'm meant to. I still hold back a bit to make sure the lump isn't choking as it advances.
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By KNT754G
#593156
Even to the point of arguing with my "747 pilot during the week" instructor...I still can't go from idle revs to full power on TO in the "confident" manner I'm meant to. I still hold back a bit to make sure the lump isn't choking as it advances.


I always tell my studes to take a MINIMUM of three seconds to get from idle to full power or vice versa.

Bigger more powerful engines need even longer.

Risk of bent crankshafts if accelerated too quick, risk of cracked (shock cooled) cylinders/heads if retarded too quick.
By PeteM
#593173
At the end of the day the majority of the requirement boils down to the rotary switch which the majority of production machines use. There have been Ads on the switch not earthing in the off position but working correctly in the others.

If you have individual mag switches the whole arguement is irrelevant. The longer term issue is whether the potential for blowing your exhaust to pieces is worth the pretty small risk of not earthing one or both mags in the off position. On aircraft which have a mixture cutoff the lack of earthing is hardly a show stopper.

But on my aircraft which has a Stromberg carb there is no idle cutoff on the mixture - so everytime it stops the earthing of the mags is tested. My continental never runs on inspite of the rich mix, my old Gipsy did - but the technique is to let the engine idle to cool down and then to advance the throttle wide open and so cool the hot spots - worked every time.