Magnetos & EGT

[OZZI_PPL]

Took a 172 out the other day that had just had a full engine overhaul. Seemed to run ok but a bit rough during the taxi. Did the runups at 1700rpm, and the left mag was running rough and dropping the rpm back to 4 or 500 rpm. I obviously went and swapped it for a better one :)

Just wondered what the correlation is between EGT and Magentos. The reason I ask is when I switched over to the left mag the EGT dropped all the way back almost to zero. Cant really figure out why.

Ideas?

Ozzi

[kiwi chick]

I'm just hazarding a guess here, because:

a) It's been ages since I did CPL Air Tech, and
b) I'm a chick

But isn't the fact that one mag is not working a big hint here? And the fact that the engine is now running at, err, slightly lower RPM, got a lot to do with what comes out the pipe at the back? And hence therefore the temperature of said hole at the back?

I may be way off base here, and if so, give me the lashings I deserve for trying to be a smart ass. ;)

[sternone]

In my opinion probably because there was only one or two cilinders working on that bad mag so that means much less heat produced to exhaust

The total rpm or exhaust temp should also be less than normal when mags on both with the bad one in action

[A and C]

If the RPM dropped 4-500 RPM then there was no ignition in one cylinder, it is most likely that this is due to plug fowling.

The most likely soloution to the problem is to change that plug.
What would be in the back of my mind would be why the plug on a newly overhauled engine fowl?

Could you have a piston ring problem due to incorrect break in procdures ? This would cause high oil consumption and be ther reason for the plug fowling

[Sedbergh]

Assuming one cylinder is not firing, it's merely pumping a lot of cold air and avgas vapour into the exhaust. Hence lower overall EGT

[Shunter]

I'm amazed this isn't taught during the PPL course! If you get a big mag drop it's likely down to plug fouling and is usually quick to sort. Give it some power, like you do during power checks, then lean it out as far as it will go without going lumpy. Leave it for a few seconds, then check the mags again. For all but the worst fouling that will usually clear it. If it's happening repeatedly it probably needs a plug or 2 changing.

Some POH's will specifically tell you to lean on the ground to prevent this kind of problem. I know my Cessna POH does...

[IFollowRailways]

In a standard setup, EGT is usually measured on only one cylinder. A probe is placed in the exhaust tube on that cylinder only - The other cylinders are not monitored.

When you do mag checks, If the cylinder with the probe is the one with the bad plug or magneto and therefore not firing, then indicated EGT drops to near zero even though the other cylinders may be firing perfectly.

If you have a multi channel engine monitor/analyser then it is easy to see which cylinder is misfiring as it will be the cold cylinder!

[A and C]

THe reason that leaning the engine at high power is not part of the PPL is that it is not a very good idea.

Most engine manufactures don't recoment "leaning to peak" at above 75% power, this is to prevent damage to the cylinder heads & valves.

You can run the engine up to full power for a few seconds to get the cylinder temp up to "burn off" light fowling but don't lean at high power.

The POH may well recomend leaning on the ground but NOT at power settings above 75%.

Leaning on the ground is to prevent plug fowling due to lead in the fuel (the chemical that vaporizes the lead won't work at low combustion temp) so the lean mixture on the ground increases the combustion temp but due to the low power settting the total energy released will not damage the engine with high temp at the cylinder head.
The way that most pilots shut down the engine is the reason for most plug fowling.

[sternone]

I'm amazed this isn't taught during the PPL course

Eugh, in my Belgium JAR-FCL course books it's said how to do that...

[IO540]

If you have a multicylinder engine monitor (e.g. an EDM700) and during the mag check one of the cylinders' EGT falls to zero, then you have a duff plug.

Invariably it is the bottom plug so the bottom cowling has to come off to change it ;)

[mm_flynn]

When I was taught the lean to clean technique it was, advance power to say 50% (about 2000 RPM in a fixed pitch or with the props full forward) and lean for peak. This is nowhere near going to wide open throttle and full RPM!

On my engine 2300 rpm and 23 MP leaned to best power is 65%. General wisdom is that below 65% you can set any mixture you like and not damage the engine. Obviously no cooling airflow will mean operating at this level can overheat the cylinders on the ground, but for the purpose of cleaning plugs 20 seconds isn't going to heat the engine too much (although watch the gauges!).

On a multi channel analyzer when you switch to one mag there should be a pretty uniform rise in EGTs across all cylinders and then a return to the starting values when you go back to both.
A duff plug will show a plunge in EGT, No change in EGT may imply the mag ground switch is duff.

[Shunter]

The reason that leaning the engine at high power is not part of the PPL is that it is not a very good idea.

Most engine manufactures don't recoment "leaning to peak" at above 75% power, this is to prevent damage to the cylinder heads & valves.Err, the power-check settings are below 75% power, certainly in my 177, and I would strongly suspect in a 172 aswell. If you're trying to suggest I nail engines to full-bore for power-checks and clearing fouled plugs, you're incorrect.

I still maintain that knowledge of such techniques should be more commonly taught than it is. Due to the British climate, very little is taught in terms of leaning at all. If you try going somewhere with a hot climate and high density altitudes you're going to look rather retarded on a regular basis when your lack of mixture understanding fouls your plugs up all the time and you have no idea what to do about it.

[Pilot DAR]

I agree with those suggesting that you have a non-firing plug.

Operating the engine at full power on the ground for short periods of time, and leaning to clear a fouled plug, is occasionally done without causing harm. Such practice has turned an unacceptable mag drop into acceptable for me on a few occasions.

You may notice that during operation on only one mag, the EGT is actually higher that it would be on both mags. I can't say that I know exactly why, though I've always suspected that the overly rich mixture is now being burned later is it exits the combustion chamber, and so the exhaust gasses have not cooled as much by the time that they reach the EGT probe.

Alos be aware that the precie positioning of the probe in the exhaust pipe can greatly affect it's accuracy. EGT manufacturers seem to recommend that the tip of the probe be in the center of the pipe.

I hope this helps

[mm_flynn]

You may notice that during operation on only one mag, the EGT is actually higher that it would be on both mags. I can't say that I know exactly why, though I've always suspected that the overly rich mixture is now being burned later is it exits the combustion chamber, and so the exhaust gasses have not cooled as much by the time that they reach the EGT probe.
Close, 'overly rich' is not important, but, with only one plug firing the flame propagation is slower and the combustion continues longer into the cycle. Therefore, the exhaust gas hasn't cooled as much when the exhaust valve opens. Hence higher EGT.

[Pilot DAR]

Yes, Same thought, but you expressed it better!

[glazer]

If you have EGT measurement on all cylinders and they all drop to zero when you switch the mag this almost certainly indicates magneto trouble or the connections from the magneto. If only one EGT drops then it is most likely a plug.

If on the other hand you have a single EGT gauge then it will be connected to a single cylinder exhaust. So assuming you have 4 cylinders there is a 3/4 chance it is the magneto that is the problem, and a 1/4 chance it is a plug.

[OZZI_PPL]

I'm amazed this isn't taught during the PPL course! If you get a big mag drop it's likely down to plug fouling and is usually quick to sort. Give it some power, like you do during power checks, then lean it out as far as it will go without going lumpy.


Have been taught that and tried it. Happens fairly often, normally only a few seconds at a higher RPM, about 2000, clears it up fine. Didn't do squat this time. Always lean on taxi too as normally have to taxi a fair distance.

Ozzi

[BackPacker]

If you have EGT measurement on all cylinders and they all drop to zero when you switch the mag this almost certainly indicates magneto trouble or the connections from the magneto. If only one EGT drops then it is most likely a plug.

If on the other hand you have a single EGT gauge then it will be connected to a single cylinder exhaust. So assuming you have 4 cylinders there is a 3/4 chance it is the magneto that is the problem, and a 1/4 chance it is a plug.

Glazer, aren't you missing the obvious here? If you have a duff magneto (opposed to having a duff plug) and you switch to this magneto, won't the engine stop altogether?

So if the engine keeps on running (albeit with a large RPM drop and rough running) on the left mag, it's definitely not the left mag that's at fault. Has to be a (left) spark plug, and if the EGT drops to zero, all you have to do is trace the EGT sensor wire to the exhaust to see what mag it is.

In fact, I think the way engineers determine which plug is at fault, is by switching to the magneto that causes the large drop/rough running, let the engine run on that magneto for a few seconds to a minute, shut the lot down without switching to "both" and then feel with their hands which cylinder is cold.

[glazer]

Yes it is true that if the mag is completely dead then switching to it will kill the engine. But a faulty magneto (not completely dead) will give the effect of dropping all EGT's at the same time but may well keep the engine running roughly. If just one EGT drops then this is indicative usually of a plug problem. In this case, he has a strong drop in EGT on a single probe (note not completely to zero) and it does not seem to cure with leaning. There is a good chance that in this case we are dealing with a faulty mag rather than a plug fault. Of course, it could be that several plugs are faulty in this aircraft and this would of course cause a large EGT drop. With a single EGT sensor it is difficult to be sure.
By the way, the guys at Advanced Pilot in the USA advise carrying out mag checks during flight as the best way to diagnose these faults, especially if you have EGT and CHT measurements for each cylinder. For those interested in engine management and what actually happens inside a typical aircraft engine I strongly recommend the on-line course by Advanced Pilot at http://www.advancedpilot.com/. I have no commercial interest in this by the way!!