View Full Version : Magneto Checks on Shutdown

Angle of Attack
18th Jan 2000, 18:24
I was taught and have always taught that prior to shutdown students should do a quick Magneto dead-cut check. ie quickly switching mag's to off and back to both determining that the mag's are not live in the off position. This is a safety check to reduce the possibility of a prop kicking back when handled. However I've noticed a number of students checking the "L" and "R" positions prior to shutdown and not the deadcut check. Is there any useful reason for doing this, apart from checking both mag's are working, which should be checked prior to takeoff on the next flight anyway. Any comments?

18th Jan 2000, 19:08
Both checks accomplish exactly the same task. I think it all depends on who your instructors instructors instrutor was ...if you get what I mean.

I didn't like the dead-cut because on charter ops your passengers don't generally like the abrupt change in engine noise.

Wee Weasley Welshman
18th Jan 2000, 22:16
UAS, ATC was always a deadcut check but Civilian world to me has always been check drop on both before shutdown. Don't know the history of why the difference but they achieve the same thing.

I think the deadcut has more potential to damage the engine/ignition is done in-expertly by a student.


18th Jan 2000, 23:36
Could it be that this is what the manufacturers check list requires? If you switch the mags off switching them on again can damage the engine.

Try asking your students why they are doing this check, most say they are looking for a drop of 50!!!!

rolling circle
18th Jan 2000, 23:41
The reason why there are differences is that, there is a general lack of understanding of how the magneto switch is wired. In the case of most puddlejumpers the magnetos are selected by a rotary switch, usually controlled by a key. In a few types (e.g. Chipmunk, Seneca) they are selected individually by toggle or rocker switches.

In the latter case, a check of each magneto switch in turn will confirm, by a drop in RPM, that the appropriate magneto is earthed when the switch is selected off. Nothing is gained by selecting both switches off together. You know that both magnetos will be safe.

However, in the case of the rotary switch, confirming an RPM drop at each of the 'L' and 'R' positions does not prove that the magnetos will be earthed when the switch is set to the 'OFF' position. On the back of the switch are connected three earthing wires, one earths the right magneto when 'L' is selected, one earths the left magneto when 'R' is selected and the third earths both magnetos when 'OFF' is selected. If the 'OFF' wire is disconnected or broken then a pre-shutdown check of 'L' and 'R' will indicate that each magneto can be switched off but, when the switch is selected to 'OFF', both magnetos will be live.

It must be remembered that a check of the 'OFF' position of the rotary switch must be carried out ONLY with the throttle fully closed to avoid unnecessary strain being imposed on the cylinders and crankshaft.

You can see that, contrary to Turbine's post, both checks do not accomplish exactly the same task. One confirms that the magnetos will both be earthed when the rotary switch is selected 'OFF', the other doesn't. Also, remember that, even if a 'dead cut' check is carried out, the system is fail-unsafe and the prop must always be treated as live. Think about that the next time you are checking the alternator belt - now there's a pointless and dangerous check!

[This message has been edited by rolling circle (edited 18 January 2000).]

18th Jan 2000, 23:52
How can checking that each mag works independently confirm that they both shut off when the key is turned to off?

Just prior to shutdown you're running close to idle, so the rpm drop when you switch to a single magneto may not be noticeable anyway. You might check R, check L, cut the mixture, turn the key to off, and walk away not knowing that the mags aren't both off.

It's true that if something has gone wrong with that switch, it might show up in the L/R check, but do you want to bet your right arm on it?

climbs like a dog
19th Jan 2000, 14:36
I have been taught by a number of instructors, including my FIC instructor, to only do a L/R drop-no stop check. My understanding is that the dead cut is a pointless check and can damage the engine. You are confirming that the magnetoes are earthing correctly by the drop-no stop check.

Time to check the manufacturers handbook me-thinks.

0 to 2000ft in 10 minutes

[This message has been edited by climbs like a dog (edited 19 January 2000).]

19th Jan 2000, 23:18
Rolling Circle is absolutely correct. A check with both mags OFF where individual switches are fitted is unnecessary, in aircraft fitted with a rotary switch it is eminently sensible.

I'm afraid c.l.a.d. that you have been sadly misinformed, and by so many people too! - by carrying out the check you suggest you are confirming only that the right mag is earthed when L is selected and vice versa, the state of the mags when OFF is selected has not been tested and you have no idea, as you walk away from the aeroplane, whether the mags are live or not.

Furthermore, if done properly, there is no danger of damaging the engine. The check has been carried out prior to shutdown in Bulldogs for over 20 years with no ill effects.

Irish Steve
19th Jan 2000, 23:36
Originally posted by rolling circle:
Think about that the next time you are checking the alternator belt - now there's a pointless and dangerous check!

Well, you may think so, but by checking the belt, with due awareness of the implications and appropriate safety precautions, it's possible to spot things like a lose bolt in the mounting bracket that has abraded the belt, or the result of a stress weakened clamp that has broken and allowed the belt to go slack. Both of which are infinitely preferrable to having an electrical failure in flight!

And yes, I've seen both, found one of them myself, and been able to be very happy that I spotted it before it became a flight safety issue. On the ground, it was a squawk, in the air, at night, in busy airspace, it could have been a very different matter indeed. Yes, be aware of the implications of being close to props even when they're supposed to be safe, but don't throw away a check because you've never found anything wrong!

"Irish" Steve

[This message has been edited by Irish Steve (edited 19 January 2000).]

22nd Jan 2000, 01:22
The check is called a live mag check ,you are ensuring the p lead (mag ground wire)is properlygrounded and the magnetto's can be turned of on the ground.This check prevents accidental ignition of any fumes in the engine thus preventing injury to anybody in the vicinity of the prop. The first check after start-up is the dead mag check,after all no point in taxiing an airplane witha dead mag. The next mag check is done during run-up at a power seting from the POh usally 1700 in cessna's; from both select left this will run the engine on the left mag only you should see a drop of about 50 rpm proving that you have isolated the right mag the rpm should not drop more than 125 on the 150/172' see poh for larger engine's.if the drop is more than 125 you have a problem.Could be fouled plugs/bad mags.Next Select Both then go to the right mag if is >50<125 sounds good back to both .you have now proved the mags are working to known specification,also you know you can isolate the left or right mags should one start miss firing in flight and a very scary vibration starts.do not be scared fiddle with evething in flight if the engine sounds bad. There you go the mystery of the mag checks why and how .Do you know if your mags have an impulse coupling or the shower of sparks . blue skies or fly low.

22nd Jan 2000, 01:29
Whoops sorry should of read post before posting .I meant to say ;to ensure the engine can be turned off on the ground.Safety first ,spelling second to last in the sack race.

rolling circle
22nd Jan 2000, 03:13
Sorry, 2R, don't understand your banter. Any chance of having it in English?

22nd Jan 2000, 06:41
Original question was "why quick mag check before shutdown"
DO it slow and you will hear why it is recommended to do it quick.I will type real slow as I know you will be reading this slow .
The mag check that is done before shutdown is a safety check to ensure the mags are NOT LIVE when the mag switche's are in the OFF position. By turning the mags off the engine will normally not get a spark.
If there is a broken P-lead the off postion will not ground out the mags and the engine will have live mags.A very dangerous situation as the engine can start by turning the prop. Even with the mixture out there can be enough vapour to ignite and hurt someone .

climbs like a dog
24th Jan 2000, 14:33
Well you live and learn. Looks like it's L/R/Off for CLAD from now on.

0 to 2000ft in 10 minutes

25th Jan 2000, 05:35
Rolling Circle is correct. I my early days as an instructor, I found many a u/s ignition switch in the PA28s of the day.


25th Jan 2000, 12:00
The Propeller in every case when not moving should ALWAYS be regarded as live. If you have to turn or pull it through be very very careful...One day it could spring into life.
That's what I told my students

26th Jan 2000, 15:19
Part of the problem is terminology confusion, too. I used to get around this by calling the mags OFF check (rotary switch) the NO LIVE MAG check, and the Left-Right check the NO DEAD CUT check. With throttle closed (500-600 RPM or so) the same check (BOTH-R-L-OFF-BOTH) was conducted after startup and before shutdown, this helped to reinforce the difference between these two (really the same) and the MAG DROP check, which of course, early students tended to do - and at 1700 RPM or whatever the mags were turned to OFF. As an instructor, I always anticipated this possibility with early students and had my left hand near the mixture control, and watched the student's hand like a hawk - you could tell by the hesitant way they operated the key that the old OFF job was on the cards. Turning the key to OFF will not damage anything - apart from making the next start more difficult with an empty carburettor -but turning it back ON again might, of course. So quickly pulling the mixture to ICO was the answer, to prevent the student turning the key back to BOTH. Of course it meant a bit of embarrassment at the holding point, but obviously better than the possible alternative.


28th Jan 2000, 12:58
I believe the final decision rests with the flight manual or POH. I know for certain that the PA28 manual recommends letting the engine stop if it has inadvertantly switched to off. This would indicate to me that switching it to OFF deliberately would not be a wise thing to do. Check the specific engine manufacturers' handling notes to decide this one.

28th Jan 2000, 14:33
Are you all Flying Instructors. It is very worrying if you are.

You should all know the difference between a Dead Cut check and a Mag check by now.

Turbine, it is not the same thing at all. I hope that you are flying turbines and not pistons!

rolling circle
29th Jan 2000, 02:47
I refer to my original post:

"The reason why there are differences is that there is a general lack of understanding of how the magneto switch is wired."

Having read the rubbish that has been written since, I can say only - Quod Erat Demonstrandum!!

30th Jan 2000, 01:46
Hail Bruvvers!

AoA posed a salient and worthwhile topic of discussion for all 'tiny piston' drivers - reespecccct. Lets not get too personal (AutoB and Rolling R!) just coz sum of the Bruvvers is confuzed!

If you look real careful at all this stuff written above then the answer will come bonk you on the nog.

Rolling C gave the correct answer which Watford, ChickenT (and me) agree with. 2R added some bits and DeadH made a good point about the terminology being confusing - it sure is.

All the jazz on alternator belts and mixture is fluff. Let me try and get a grip.

Old aerodonk has two sparky things in each cylinder. Each sparky thing made to spark by its own magneto, so that when one mag goes futt, cylinder still has a sparky thing that is working. This is the dual ignition redundancy thing we know and love.

Magneto is a simple generator and when roted will always make electric. Snagette - don't always want electric (such as when shut down!) so must be able to send electric away from sparkeys when not wanted. Light bulb - send it to earth (airframe/engine mount) by piddly little copper wire called p-lead. Good. But monkey in cockpit (me!) must be able to CONTROL when electric goes to earth and when it goes to sparkeys. Light bulb - provide a switch - only one would do!

Snagette - monkey not only needs to be able to CONTROL magnetos on/off but be sure them is workin fine. See, sparkeys must go fizz together at same time in squeeze-bang bit of suck-squeeze-bang-blow...ie, monkey must be able to check PERFORMANCE of ignition.

Light bulb - give monkey two switches, let him control (switch on/off) each magneto. Lotz of extra added bonuses at no extra cost now:

- monkey can check that each mag will go off (electric to earth thru p-lead and not to sparkey) so will be safe when stopped(remember, mag ALWAYS makes electric). CONTROL.

- monkey can check that sparkeys go fizz at correct time at representative advance angle in squeeze-bang cycle by switching on/off at representative rpms (usually around 1700-2000). Small drop in rpms expected, but big drop might mean timing duff. PERFORMANCE.

Fuzz - the switch. Easy solution, 2 of, one for each mag. Complication - Mr Ezekial J Abbernachher III wants keys to his plane like he has keys to his Caddy. Clever Mr Cessna/Piper etc do this by having one 4-way switch: Off-L-R-Both (could have 5-way with 'start' pos, but stik with 4 for now). Snag - now need to be able to test that switch works in OFF posn as well as L-R-B; can safest to this from cockpit with engine running at LOW rpm (small fuel/air charge so little bang, spark retarded & moment of inertia 'I' of rotating system of pistons & crankshaft dominate ) by switching ign temp to 'off' posn.

More fuzz - Bulldog drivers - I think that their airlordfurhers didn't believe the young shiitehawks should have to carry keys to their mount so paid doubly extra to have the same 4-way switch modded to do away mit da key (similar philosophy to not physically checking the fuel qty wot woz ably put in by groundie - just a difference in ops, thats all ).

Back to the point. Putting it into praccy.

Start engine. May have to start on L if impulse coupling on L-mag but this is fuzz to question asked. When donk going want to check that both mags work fine as a) no point going mit just one and b) certainly don't want to switch entire system off & on again (as wot would happen with duff mag during run-up) at hi pwr with spark advanced all that way back and lotz of bang about to happen. Could be v.costly. So at opportune time after start me just go B-L-R-B. You is lookin for drop in rpm from B to R, that rpm stays dropped between R & L (subtle), then goes back up when back to B. At no time should donk stop coz this is bad (duff mag) and we stays put. Call this check whatever you like - 1st ign check, drop-no-stop, Ivan the check, whatever; must comprendo whats happening. Note that holy tradition passed down from our forefathers suggests that we don't go to the OFF posn here - but sky wouldn't fall in if we did.
Bruvvers, we have just tested that we have control of our ign system.

Run-up/Test Engine. Now want to test PERFORMANCE of ign system (ie, it workin' cool). Now me go B to R, note amount of drop, then back to B. Next me go B to L, note amount of drop, then back to B. Me now compare to drops with each other & maxi allowed from book of words. Me confident that no BIG BANG due whole system go futt coz mags checked ok at startup.
DH - sorry blokey, but theres no point hovr'n over the Mix incase Bloggs (or 'Bruce the Lesser') accidental go to OFF during run-up...damage will already have been well & truly done.

Shut-down. All being well ign still ok after arduous aviation committed (ie still have CONTROL of ign) but want to be sure donk don't start when wife accidental hangs handbag on fan-blade. So test OFF posn. Now me go B-R-L-OFF-L-R-B bein sure not to hang out at OFF for too long. Engine should show signs of wanting to sleep at OFF, but get extra bonus for free - can also test control of ign again! Whizzo.

Hope you not bored like fart in breeze.


PS. YMML - beware book of words with Cessna/Piper etc on cover when it comes to donks. ALWAYS a good instinct to go for book of words for advice, but donk luvvingly nurtured from birth by Mr Lycoming/Continental etc. Donks in planes get changed, but motor-maker is always authority. How many Mr Lyc/Mr Cont books you seen floatin at your flyclub?

1st Feb 2000, 04:25

You write like an idiot, but it makes good sense to me. Shame I'm not instructing any more, so that I could pass that lot on. Mag checks are one of those monkey see, monkey do things for a lot of people and it's only when somebody asks why, that you stop and think. Good one. Keep the questions coming.

1st Feb 2000, 11:15
Way to go flange! Only reason why I taught go to OFF during the check done just after start was that in the long run it helped the student not to turn the switch to OFF during the run-up, by making the whole magneto testing procedure simpler to remember, and isolating the performance check done at run-up as unique. As you say, going to OFF at 500 RPM is not going to hurt anything.

[This message has been edited by deadhead (edited 01 February 2000).]

1st Feb 2000, 11:36
The dead/live check just prior to shutdown as described by Rolling Circle is the same as was carried out in the Bulldog. Is there any reason why it isn't usually done in the PA28?? What do OATS and the other schools do?? Or is it simply that the average pilot can't cope with BOTH, L, BOTH, R, BOTH, OFF, BOTH....ICO...prop stops, OFF and key out?? I think we did this check at 1000 rpm in the Bulldog rather than the usual 1200 rpm to avoid shocking the engine??

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