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.