Yeah, my mag runs “change of pants” rough on the ground anyway. Not always mind you, it’s about 50% of the time it’s like that. There’s no GA maintenance up here so maybe I can get her down to Inverness for a look at the mag. So the general consensus is to leave it on ‘both’ if a mag fails? And that it’s ok to perform a mag check mid flight? I’ll give it a go next time I’m up. But maybe not for as long as a minute as someone suggested, just for a few seconds I reckon. Also, pilot DAR, you say to do the check at low power or idle, but do you not get a better idea of any problems while checking them at a higher rpm (about 2000)? Isn’t that the whole idea of the power check? Any more thoughts are appreciated. Oh, and I will check the screws on the mag as well. Just for info, it’s a Lyc O-235-L engine.
Sustained operation on one mag is not great if it can be avoided, but not particularly harmful either.
My TIT (turbine inlet temperature) sensor would strongly disagree 
Are you 100% sure it’s not a persistent plug fouling issue? Most perceived magneto problems are in reality spark plug problems…
(Btw, this is one of my pet peeves… in many german PPL textbooks and PPL lessons, the engine runup is often referred to as “the magneto check”. This however implies that we do this in order to check the mags, which is very inaccurate. It’s a check of the entire igntion system, i.e. not only of the mags but also of the plugs, the harnesses and even the ignition switch…).
If you’re sure you really want to check your magnetos in the air make sure you don’t lose control of the ignition key! Could be a bit embarrassing scrabbling around on the floor trying to retrieve it.
Yes TIT will increase on one mag, as will EGT (which is about the same thing), as unburned fuel burns further downstream in the exhaust. This might require “management”, such as changed mixture, or lower power setting. But, short of a limitation prohibiting operation on one mag (which I have never seen), the engine is designed and certified to operate that way. That said, avoid this type of operation. If the other mag is operating at all, leave it operating, so whatever benefit might be had in keeping TIT/EGT lower is had. If the other mag is not operating at all, your choices are limited….
Now we are in 100% agreement Pilot DAR Some posters suggested to turn off a magneto when it is running a bit rough. 200°F is the difference between what your exhaust system can cope with for some years and what it will ruin it fairly quickly.
A magneto can change its timing and that is nothing to be overly concerned about. They have a plastic case and they are held with a simple clamp that cannot be tightened very strongly due to the plastic. Checking the timing of a mag is trivial (you can buy the kit for 100 € or so) and adjusting it is simple, too (but better get somebody qualified). When it’s off 2-3 degrees, you very often see rough running at low RPM.
And of course bosco is right, fouled plugs can be the reason. It is truly amazing how few pilots know that you should never taxi full rich, in general never operate the engine rich unless required for cooling during takeoff/initial climb.
I wasn’t going to say this because it might spark a separate argument, but since two people have mentioned it I might as well come out with it. I don’t use Avgas. I run on Mogas, mainly for economic reasons…. And since mogas doesn’t have lead in it, it’s unlikely that fouled plugs are the cause of the rough running. Still could be fouled plugs, but it’s less likely. Also, about taxiing with a fully rich mixture, I do so all the time. The main reason being that my mixture control is very inaccurate. The lever is more than half way down before I notice a difference, and then that almost kills the engine. It’s a shame, because if it’s a longer taxi I put the carb heat on because up here there’s a very high risk of carb ice, and apparently taxiing with carb heat on is even more cause for leaning the mix.
The reason why EGTs rise when on half mags is because some fuel is still burning when in the exhaust system, and the EGT probes see this. It’s not a big rise – maybe 50-100F on mine (IO540).
They have a plastic case and they are held with a simple clamp that cannot be tightened very strongly due to the plastic
What mags are those??? The D3000 single shaft dual mag, and every single mag I have seen, has an aluminium case, held by two clamps. There is a torque spec on these two bolts. Also the clamps need to be the correct ones.
The diecast (or sandcast) aluminium case of the D3000 mag is a problem because nobody makes them anymore. All the internal parts are PMAd or otherwise available, but if you crack the case then you have to scrape around for a new mag. When the D3000 parts availability situation originally exploded a few years ago, I paid $2500 for an overhauled one which I keep as a spare.
The lever is more than half way down before I notice a difference, and then that almost kills the engine
Mine is the same but that is correct. If you have a digital fuel flow indicator you can see the fuel flow dropping all the way down, and the point at which one should ground run during taxi is just before the EGTs start to fall again, and the engine will stop pretty soon after that if one continues to lean. The setting is not critical. But I have found a huge difference in the amount of muck in the spark plugs, seen at the 50hr service, if the warmup engine run was done full rich versus maximally leaned. If you do the former, the plugs are full of oil and are hard to clean or to check visually.
Yes, aluminum of course. The torque on the two clamps is very limited and it’s not a very strong fixation. A lot of mangeto cases and clamps were destroyed by over-torquing. The way magnetos are installed, they can gradually shift. The crappy dual mags are much worse of course, as Pilot DAR points out. Luckily I never had any problem with them but I will exchange the ignition system of my aircraft the moment an alternative becomes available!
I don’t use Avgas. I run on Mogas, mainly for economic reasons…. And since mogas doesn’t have lead in it, it’s unlikely that fouled plugs are the cause of the rough running. Still could be fouled plugs, but it’s less likely.
We’re not necessarily talking lead fouling here. We’re talking carbon deposit fouling. Not much to do with the type of fuel.
Also, about taxyng with a fully rich mixture, I do so all the time. The main reason being that my mixture control is very inaccurate. The lever is more than half way down before I notice a difference, and then that almost kills the engine.
A bad excuse, really. Most aircraft behave like that during idle, i.e. when leaning the mixture continously, you don’t notice much for a long time (maybe a minimal rise in RPM) and then, at a certain point, the engine quickly wants to stop. Just set the mixture at a point where engine will just idle smoothly. That will sure be much a leaner mixture than at the full rich setting.
