Magneto Checks
 

When I was learning to fly, I was taught to check the mag. drop after starting the engine. Later on, a different instructor told me that this initial mag check was not to check the mag drop, but to confirm that turning the key actually turned the mags off. The theory being that the engine was not up to temperature and checking the drop at idle was not a valid check of the magneto performance. A full mag check should be carried out at 2000rpm and the revs. should be allowed to stabilise on each mag. prior to moving to the next switch position, rather than a quick, 'Both-Left, Both; Both, Left, Right,Left,Both'.

While doing the mag. checks I have to confess to inadvertantly turning the key too far the wrong way and causing a Dead Cut. Now, my Cessna check list had 'Check for dead cut' as a valid check, which my instructor then scored out. I feel that stopping the engine momentarily, is a Bad Idea, as it must put a shock into the engine as well as releasing a small charge of unburnt fuel into the exhaust system. This could then ignite causing damage to the manifold or a backfire.

I don't feel that engine handling is taught as well as it should be.

Certainly I was taught (by our maintenance man) to give it a time on left, then back to both, then to right, then to both, stopping at each for while. That has more than once revealed an oiled-up plug where I suspect we wouldn't have noticed otherwise.

Turning both mags OFF is likely to wake up the neighbours with the BANG when you turn back on. It would be an absolute no-no for a car, but probably isn't quite as disastrous for an aircraft. Still wouldn't do it.

Turning off the mags totally is so daft that it's unfunny. All sorts of things can get blown to pieces like that.

Mag checks should be done to allow the engine to stabilise at the RPM rather than just a quick drop for the reason Keef has given.
If there is one thing that drives me nuts, it is having people rushing through their checks, the Carb heat being a particular favourite.

The mag check just after start is only to check that you can actually switch the mags off. It isn't a meaningful check of RPM drop since the rev's will be low and the engine isn't at all warm.

The RPM that you check the mags at is different depending on the type you are flying, eg. C152 1700rpm, PA28 2000rpm, HR 200 1800rpm etc..... The POH will have the right numbers in it for the machine you are in.

But if discovering a mag problem or oiled up plug immediately after start up saves you taxying all the way to the hold before you find the problem checking at 2000 rpm then there has been a positive benefit for everyone on the airfield.

If stopping the engine on the mags is so bad for it, what about some of the very old a/c such as the Tiger Moth that doesn't have a mixture control and is always stopped by switching off the mags.

I appreciate that this has to be done after a 30 second "idle" but is there a damage risk here?

Waiting for the a/c to run out of fuel is the only other option to stop something of this vintage!!

Monocock

It's not the switching off that does the damage, it's switching back on while the engine is still turning. The engine will re-start and the unburnt fuel mixture which has been pumped into the exhaust manifold through the cylinders during the period the mags were off will ignite and exlode. This can cause serious damage.

The Chippy is shut down by switching off the mags, but it is done at idle power setting, then the throttle smartly opened to full once the mags are 'off'. The fully open throttle at very low rpm means there is little vacuum in the carb venturi so litttle or no fuel is drawn into the engine (which could cause run-on if there are hot carbon deposits in the cylinders, and in any case would cause oil to be washed of cylinder walls, increasing wear at next startup).

SSD

Owenlars, when the mags checked at start up, then you cannot really tell if you have one fouled up plug. The pre takeoff power check is the the best place to tell if you have any problems such as a fouled plug, since the engine should be coming up to temperature and the great gobs of fuel you chuck in at start up should have disappeared by then.

A fouled plug isn't a problem really. the vast majority of times it can be burnt off by increasing the rpm and leaning the mixture for a few seconds. Get your FI to show you if they haven't already.

A SSD says, switching an engine off by the mags isn't going to harm it, it's the unpredictable nature of what happens when you turn them back on that's the issue. If you have a motorbike. try running the engine up and then hitting the kill switch momentarily, see what happen next! Either a flame out the exhausts with an almighty bang, or a kick back through the carbs leading to singed privates. Now try that in £100,000 a/c!

When gaining my licence, it had always been explained to us both by the various FI's and our powerplants groundschool instructor that the initial 'dead cut' check of the mags after starting was just that - a check to see if the individual mags were 'dead' or not. If the key (or switch) was turned to select either left or right mag and the engine died, we took our hand off it immediately and allowed the engine to stop before calling for an engineer. The check is always carried out at low RPM as this is the situation which would result in the least damage to any components in the event the mag was inadvertantly turned back to both and the engine restarting.

The other mag check at higher RPM is to check that the RPM drop from both mags to single mag operation is within limits, and that the engine would continue to function in the event of losing a mag in flight. As has been said, during this check the RPM should be allowed to stabilise for a few seconds on each mag to check for fouling of the plugs. There should be little or no risk of a dead cut at this stage which could damage the engine as that check has already been carried out.

Say again

Quite right but you can tell if you've got a knackered magneto (I guess most of us check magnetos just before we switch off so we can do something about it before we go flying next, it's the same principle). I, for one, have also identified fouled plug(s) and cleared them(it) by leaning on the way to the fuel pumps. The point is that if you can identify something before you taxi anywhere it's got to be better than taxying to the hold and finding something during your power checks. And of course the power checks (which we all do don't we) are the acid test to show stuff up that you cannot see at idle.

Kolibear

Just reading your second paragraph. I may have misunderstood you but dead cut is not referring to turning the both mags off, instead you are checking briefly that both mags are at least working.

I think engine handling is best learnt in your own share - cos then you get it right to keep down on the maintenence bills!

One more point - don't forget to check each mag before shut down - and watch the RPM to make sure it drops a little (50rpm or so), this shows that the mag will not be live when you shut down the engine, always a bonus if your about to pull it byt he prop!

Bodie

The purpose of the "dead cut" magneto check after start is two fold in the sense that when selecting an individual magneto there should a) be some drop but b) the engine does not stop.

a) confirms that you do not have a live mag

b) confirms that you do not have a completely dead mag!

In the case of a) - ie in the event of ZERO drop - one might then troubleshoot further by closing the throttle completely and turn the magneto switch to OFF to see what happens. If a mag is live then the engine would continue to operate in which case maintenance action would be required since it would not be possible to switch off the mags completely in the event of an emergency which requires a shutdown in the air and also the engine is live should anyone be imprudent enough to pull the propellor through whilst on the ground.

In the case of b) if you have a completely dead mag there is no point in going any further unless you are prepared to risk your neck flying around on one mag!

All these points should have been covered in basic flying training but as an instructor it is not infrequent to discover that some pilots are a little unsure as to WHY these checks are conducted as they are.

Finally, the magneto system is NOT fail safe in the sense that when you switch a mag off you are MAKING a circuit (ie grounding the mag to the engine etc.) This is yet another reason for treating a propellor as live at all times. I recently watched with horror as a private owner pulled the prop through whilst the aircraft was in the hangar!!

Hope this helps

SAS

My POH requires a all-off mag check too, saying it must be done below 1000rpm to prevent exhaust system damage! I've never done it, preferring the traditional half-off, half-off check.

There is a possible failure mode within the mag switch which would be detected by an all-off check and which would not be detected by the traditional check, but it's extremely unlikely.

IO545

It's been bugging me as to why my checklist says to select Mags OFF then Both checking that it would have stopped. Seems to me that checking a drop with LEFT and RIGHT checks the same thing.

How would that failure occur?

Brodie,

I think that you are correct & I have misunderstood the term 'dead cut' - you learn something new everyday here - thanks!

Slow-Rider

Don't ask me..... perhaps if I spent some time examining the internals of the mag switch I might be able to come up with a highly theoretical scenario of a breakage in a particular precise manner which might do that.

Personally I don't believe it, and it's not worth the risk because if you get it wrong the resulting combustion inside the exhaust system sounds dreadfully dangerous.

IO, I assume that because your machine is injected, then the procedure may be different, but I still wouldn't recommend switching the mags off totally and can't really see how it would be totally different anyway.

Mind you, I still hate the idea of magnetos compared to a proper electronic ignition system!

The cynical part of me wonders if the engine manufacturers want us to dead cut, so that their aftermarket parts business gets a boost! How much is an new 540?:{ :uhoh:

If you want to be cynical, pick up a handbook on engine management from the Lycoming disti stand at the next GA exhibition. It basically says: we think you are all idiots so we want you to run engines well rich of peak because that way you can't damage them. It also says things like "our engines are built to the highest standards so there is no point in doing anything to make them vibrate less".

It is apparent that a lot of the instructions/procedures in GA have at best no justification; at worst they are dictated by lawyers.

To be fair, if you look at the standard of instruction when it comes to engine management in GA, Lycoming are stuck between a rock and a hard place... they do sort of say that if you have proper engine instrumentation then you can run LOP but they distance themselves from it as far as they can.

Deakin should be compulsory reading for everybody who has 3 levers, and all of them should have a multi cylinder engine monitor :O

This is what I've always understood the purpose of the dead-cut check to be. However, if you have badly fouled plugs then this can cause the engine to stop as well. I discovered this one when preparing for a night flying sortie. When I did the post-start dead-cut check the engine stopped when I selcted one set of mags. I switched everything off, went back to the clubhouse, and reported the aircraft U/S with a dead mag. One of the instructors went out to check (why do they never trust us?) the aircraft. He confirmed that the engine did stop on one of the mags, but after running at 2000rpm with the mixture leaned out for a few minutes, the problem had gone, showing that it must have been due to fouled plugs.

Brooklands

Why don't we trust you to make decisions that would cost us lots of money? I think you answered the question yourself!!;)

Can anyone give me an idea how long you should lean
the mixture to 'clean' the plugs. Is it just a case of leaning until the revs drop, then full rich again. Or should the engine be run for a couple of minutes leaned.

Many thanks,

TO

Lean it good and hard, on the ground you can't do any damage. After a minute or so enrichen the mixture and check whether you still have misfiring/vibration on the magneto in question. If things don't clear up in a couple of goes you've probably got something a bit more serious possibly a damaged plug, get someone to look at it.
If you do lean on the ground do not forget to enrichen the mixture for take off, unless you are at a high density altitude airport of course. This is to ensure you have sufficient fuel flow to cool the engine whilst it is at full chat.

Errr, nope. You can knacker an engine on the ground if it is at a high power setting and you lean it too much as the engine will be under load unless you remove the prop. I've melted quite a few bike and car engines over time because of running too lean and causing detonation. Normally caused by me mucking about and not really knowing what I was doing when I was a teenager.

As a rule of thumb, Run the engine to around 2000rpm, and then lean until the engine reaches peak EGT (if a gauge is fitted) or until the revs drop and then richen by a bit. Leave it like this for 10-20 seconds and then richen the mixture. Go through the power checks again, most of the time this will sort it.

This is not a definative guide and if you are unsure of the procedure get your FI to show you. They should have at least gone through the theory of this during your training if it never actually happened to you.

Better safe than sorry and if you are ever unsure about the mags, then take it back and get it checked.

Get a flying instructor to show you how to burn the dirt off the plugs !!!!!! ??????????? god help us !!!!!!!!!.

You had better make sure its an instructor who is very long in the tooth ! most of the young ones seem not to have the first idea about how an engine should be used.

If you think I,m being harsh just take a look at some of the technical posts that we have seen on this forum over the last few weeks , a lot of this is the result of there teaching ( or lack of it ).

Perhaps a visit to the engineers would cast a bit more light on the subject and wile you are in the hangar no doubt they can show you a few of the cracked cylinders that they have removed due to inproper leaning of the mixture.

This young instructor knows how to defoul plugs. I don't know any who don't know or don't have to do it on a fairly regular basis.

Since most schools frown on students or PPL's leaning the mixture, I don't know how many cracked cases are down to detonation rather than any other cause.

Whilst I will admit to some instructors not being stellar, the vast majority are competent and don't forget that once you have a licence, the duty falls on to you as PIC to be as knowledgeable as possible. The business of instruction isn't easy on occasion, time pressures can be very high and since most of the theoretical side becomes self study, then blaming FI's for people's lack of understanding is a bit harsh.

I have lost count the number of times students of mine have said "yes I understand." Only to demonstrate very clearly that they haven't the foggiest idea really. Be honest with us and then maybe we'll beat the knowledge gap.

Oh and when an instructor asks you to read certain chapters in the text books before the next lesson, it would be nice if you actually did!
(Rant off!)

Interesting view. I have never done a 'post start' mag off check, but normally do a pre-shutdown mag off check. This is becasue as mentioned before the mags are grounded out by the key switch, to stop them creating the spark. Now if there is a fault with the earthing then switching the key to off won't ground the mags, and the engine won't stop. This is clearly dangerous, as next time the mechanic is playing around, and spins the prop for whatever reason, the engine could spark to life.

The mag check is only momentary, so won't cause any damage to the engine, you just observe that the engine is obviously stopping.

To help avoid fouling, I normally lean on the taxy, go full rich for the run-up, and if the mags are fouled, increase throttle and lean until the engine just starts to sound rough. Give it a few seconds, go full rich and bring the throttle back to specified in POH (2000, 1800 whatever).

I think its crap we're still flying around in aircraft where you can destroy the engine by misuse of the mixture, throttle and prop controls, you don't know how much fuel you have becasue the fuel guages don't really work, you can destroy an engine by shock cooling, and crash becasue some ice forms in the carb, plugs foul up for a past time, and a "new" engine will burn more oil than my 1984 Dihatsu 4Track which has done 130,000 miles!

Give me a FADEC TDi any day :D

EA

Say again

Fair point, my assumption was that if you are at much above 2000 revs on the ground you are either taking off or burning out your brakes. I should have put your caveat in.

1. Anyone tell me why its called a "Dead Cut", if you are not going to switch off completely? (NOT saying you should).
Why isnt it another Mag Check, at a lower rpm? Far less confusing.

2. Anyone tell me why we were tought to bo back to both mags after checking one, before checking the other? Whats the logic?

I understand it with separate switches (as in Super Cub, Tiger etc) but not with a key system. Surely what you are doing is checking each system at certain power settings? Seems unnecessary to go through one system to get the one you want to check and you already know the 2 mags on RPM.

Are these ancient urban myths?

UV

UV the lower plug on a flat engine (as most of them are) faces upward and when you run a cylinder on only one mag, if the non-sparking plug is the lower spark plug it may get slightly fouled with fuel filling up the well between the body and the central electrode. Putting the mags back to BOTH gets all the plugs firing and burns off the excess fuel in the plug. It makes sense to do it for both sets of mags/plugs as:
a/ it is easier to remember
b/ if the upper plug is not firing, it may become fouled also.

If you went from RIGHT MAG OFF straight to LEFT MAG OFF and more than one of the plugs fired by the right mag were fouled then the engine would probably stop.

Say again slowly
 

Can you please tell me why you think that detonation is an issue when it comes to cylinder head cracking and how do you get this detonation when burning the dirt of the plugs ?.

I don't recall that I did. I also didn't say that it was particularily likely that you would get detonation when 'cleaning' the plugs, my statement was that it is not impossible to causes detonation when on the ground if you overlean at a high power setting.
Lots of revs+Load on the engine+too lean mixture=detonation.

SAS
 

I dont dispute the fact that you may get detonation when running lean at high power but why ?.

And how about cleaning the plugs before shutdown when the engine is at full working temperature rather than running it hard after start up when it is not fully walmed ( even if the oil temp is in the "green") ?.

I agree with A and C

My personal airplane is a Grumman AA1B with the low compression 80 octane version of the LYC 0235. I run it at about 1500 RPM for about 20 seconds, do a quick left-both- right- both
mag check to make sure both mags are grounding out, and then reduce power to idle while simultaneously going to mixture ICO.
This will clear any deposits from the plugs that have accumulated on the taxi in. I also lean it on taxi. I have found this procrdure necessary on every small 80 octane engine that is being run on 100 LL gas. 100LL gas despite the low lead designation still has 4 times the amount of lead as the old 80-87 gas ( now virtually impossible to get in North America ) and will quickly foul plugs at normal taxi power settings.

Why? If you read the thread properly, you will see that somebody mentioned that it wasn't possible to get detonation on the ground. I simply pointed out that it was very possible, if unlikely.

Of course I recommend running the engine up a bit before shutdown. Many POH's specifically suggest this, i.e C152 1500rpm for 10s. It does make a big difference, especially on a/c that are poor starters. Often plug fouling is a major cause of starting problems. Just don't do it with the tail pointing at a
hangar door! Open or otherwise.

Come on S A S tell us why High RPM and lean mixture may to be the cause of detonation.

I'm sorry, but what don't you understand? Pre-ignition and detonation can be caused by having an excessively lean mixture, which when compressed may burn in a less than controlled manner. Detonation (or pinking, the tinkling noise you sometimes hear under load in a car engine caused by the pressure waves hitting the cylinder walls) is the uncontrolled explosion of the fuel/air mixture rather than a controlled burn that allows the build up of pressure in the cylinder to be more gradual and less damaging.

High RPM in a light a/c causes a high load on the engine therefore you can get detonation. Piston speeds increase, the charge is heated more when the engine is running at higher RPM due to the cylinder temps being higher, therefore the vapour pressure will be lower, therefore the chance of spontaneous combustion is higher.

That make sense to you? Could you explain to the rest of us how high rpms and loads makes detonation less likely? If that's the case, my B.Eng and M.Sc will have to go back to the cracker factory I got them from.

Count to ten slowly...backwards...take a deep breath.

There. Feel better now?

No need to be so uppitty. Don't assume we all know as much as you; patience is a virtue too.

OK lets give this a thinking about !
 

Lets start with he fact that pre-ignition and detonation are two diferent things , pre-ignition is the ignition of the charge it the cylinder normaly by glowing embers of carbon and the charge burns in the normal way producing a relitivly slow and controled flame front.

Detonation on the other hand is the explosion of the charge in the cylinder in this case the burning of the charge is uncontroled and almost instant.

As you say this produces in cars "pinking" the tinking noise that you hear when the engine is under load but this is were I have to differ with you and it is my opinion that the noise is partly due to the un-even loads on the top of the pistion resulting in the piston skirt hitting the side of the cylinder.

Pre-ignition can result in detonation because if the charge is ignited early the flame front and the piston rising result in the un-burnt charge in the cylinder furthest from the point of ignition detonating due to the pressure / temp rise hence the uneven loads on the pistion.

The nub of what I was driving at was why is this most likely when the mixture is lean ?.
As you may have seen Avgas grades are quoted as 80/87 , 100/130 , 115/145 most of these grades have been phased out and it is unfortunate that 100LL that we all know should be designated 100/130 but those numbers were already taken.

The numbers refer to the octane rating of the fuel , the larger the number the better the detionation resistance characteristic of the fuel.

Why two numbers ? this is because the detionation resistance is dependant on the mixture , the first number is the octane rating at max lean mixture and the second number the octane rating at full rich mixture.

From this you can see that leaning an engine at relitively low RPM ( after all most engines will not make max RPM on the ground due to the prop pitch ) and therefore low piston speed and decreaseing the fuel octane rating by leaning the mixture can result in detonation due to the combination of the lack of detionation resistance in the fuel , the slow progress of the piston up the cylinder relitive to the speed of the flame front that has been set off by a spark that is fixed for cruse RPM , this results in a pressure/ temp rise that detonates the part of the charge furthest from the spark plugs .

I will leave the thermal shock issue on the cylinder heads alone but is must also be a factor when cleaning the plugs by "burning off".

Hmm, maybe my hangover caused a bit of an over reaction, apologies..

Whilst Pre-ignition and detonation are seperate things they can be and often are linked together and one will often cause the other.

My comments about RPM are really based on a fixed pitch a/c since MAP is actually a bigger concern, but you have no direct indication of it without a CSU. Higher RPM actually REDUCES the tendancy to detonate due to reduced time for the pre-flame reactions, but a high load will INCREASE it because of the increase in engine temps and end gas pressure.

The higher cylinder temps caused by running at a leaner mixture are an aid to detonation compared to the much lower temps you would get with an excessively rich mixture. A mixture of around 13.5:1 has been proven to be the ratio most likely to cause detonation. If the mixture is leaned too far, then the slower speed of the flame front can cause all sorts of issues, especially as less heat is converted to mechanical energy and the cylinder temps will rise rapidly.
For geeks only: for every part you lean the mixture by, the octane needs to increase by 2(RON+MON)/ONR
(RON: Research Octane Number, MON: Motor Octane Number, ONR:Octane Number Required)

I hope the noise you hear in the car is not the piston skirts hitting the cylinder liners since you would have a very second hand engine very quickly. I have seen (heard really!) 'pinking' in engines that have a very short 'skirt' and haven't detected an unusual amount of wear on the liners post strip down which is what I would expect.
However ,what you say is absolutely right, and there can be quite alot of movement caused by an off centre detonation, though this is often catastrophic rather than the annoyance that you hear.

The detonation resistance of the fuel is as you say the octane rating, when running on a higher octane you can reduce the squish and increase the compression ratio without fear of damage. Something I have found to my cost when running an old car at 13:1 on normal 95 octane. The mixture leaned out and pop went a very expensive bit of tuning work. New head please!(and a lot of other bits!)

The important numbers aren't just the number given ie: 100 octane but the difference between RON and MON, the 'sensitivity.'
As a rough guide for a carburetted engine I'll reproduce some figures I have for octane ratings required compared to compression ratios and then to thermal efficiency.

Compression Octane Number Brake Thermal Efficiency
Ratio Requirement ( Full Throttle )
5:1 72 -
6:1 81 25 %
7:1 87 28 %
8:1 92 30 %
9:1 96 32 %
10:1 100 33 %
11:1 104 34 %
12:1 108 35 %

I hope this makes sense and I apologise if I have been overly technical in my use of language. This is a hugely complicated subject, and to go into it in great depth would require far more typing than I can be bothered with and enough text to bore 50 people to death with.

I'll just put a quick caveat in. Light a/c engines are so gash that you are unlikely to cause detonation in your normal bog standard O-320. Turbo'd and supercharged engines are a different matter however. We all need to be aware of it because if it does happen, then it can be a very expensive issue!

I'll put an explanation of how the octane ratings are worked out if anybody is remotely interested.
In aviation fuels, the different octane numbers are known as the Lean and Rich ratings, but they have different meanings. Any number above 100 is not an octane number, but a performance number.

The lean mixture rating is an octane rating and it is worked out in the same way as Motor Octane but converted by means of a table. There is very little difference between motor no.s and octane no.s.

The rich mixture rating is a performance number worked out using a supercharged engine and allows us to work out the mixture strength and boost for a specific light knocking setting. The Performance Number indicates the maximum knock-free power obtainable from a fuel compared to iso-octane.(A specific specially distilled fuel that acts as a control) The performance number is always higher than the octane number. The rich rating is an arbitrary figure that has been derived from decades old data.

Interestingly when 100LL was first introduced there were a large number of incidents of plug fouling, over heating and cylinder head cracking. It was so bad that 80/87 had to temporarily re-introduced. It was due to the reduced lead content of the new fuel. (0.53mlTEL/L compared to 1.06mlTEL/L.)

100LL is a different fuel from 100/130 and is it's own designator as well as octane number. It does not have a Rich mixture rating as such and was designed to replace 80/87 AND 100/130.

If anybody is still awake then I hope this may come in useful one day. (Not sure how, but you never know!)

My first post on this subject was casting doubt on the way some instructors teach the tecnical subjects S A S clearly knows what he is talking about but like a lot of what happens in PPL training sweeping generalisations have been used in the first few posts made.

It is a very hard circle to square between over complication and brevaty and this was half my point by pushing this debate along.

A number of times I have been asked to do the last few lesons of a PPL course only to find that the technical knowlage of the student is practicly zero.

If the student is a young and atractive girl the knowlage is usualy below zero , I can only hazard a guess as to what the young flying instructor was thinking about over the course.

It is up to all us instructors to think and speak clearly on these subjects and try to find the correct balance between tecnical accuacy and the level of knowlage that will allow the student to fly safely.