Hi all, hope everyone’s been getting some flying in this summer. I have, and it’s thrown up some problems/questions!
The first of which concerns magneto failure. Say for example your left mag packs up. Do you switch to the right one, or do you keep the key at ‘both’? I can’t seem to find an answer online and I don’t remember it being covered in the PPL. The reason I ask is because I’ve had a rough running mag lately. I notice it especially when on low power settings, like on approach. I just want to be prepared and do the right thing when it happens! Sorry, ‘if’ it happens 
That kinda brings me on to my second question. Can I do a mag check mid-flight, on cruise power? I didn’t ever want to do it because… it just doesn’t feel right. Is it possible though, without risk?
That’s all for now, cheers!
If you know you have a dodgy mag and are getting rough running why not get the aircraft in to an engineer and get it fixed?
Keep it on both unless that is not possible. If you run on one mag, your fuel will burn after leaving the combustion chamber and you destroy your exhaust system, cowling, etc.
Most likely one of your mags has moved a bit and is out of timing. This is more notable at low RPM. Get it adjusted, that takes half an hour max.
Can I do a mag check mid-flight, on cruise power? I didn’t ever want to do it because… it just doesn’t feel right. Is it possible though, without risk?
Yes, and it is recommended to do this every now and then as it tells you a lot more than a ground mag check. It’s easy for a duff mag to generate a spark at aerodrome altitude but much harder at enroute altitude. Do the test only for a few seconds due to heat.
Switching off one ignition circuit in flight is not a risk I would lightly take. No problem if there’s no problem of course – but imagine the worst case scenario: one circuit is dead, you switch off the other, engine stops, will not start again, and there you go for a forced landing. And yes, I can perfectly imagine one circuit would die during flight without my ever perceiving so – the engine will run rough at low revs, on one circuit, but at cruise flight power setting the difference might well go unnoticed.
one circuit is dead, you switch off the other, engine stops, will not start again, and there you go for a forced landing
The Lycontosaurus engine won’t stop with both mags off. It will continue to turn, just without power. The moment you turn on the mag, it will produce power again. No problem at all.
I would not depart with a dodgy mag.
I would also not just tweak a mag for the timing if it worked before – it could have an internal fault. It needs to be inspected.
It is fine to fly on just one mag. Yes the EGTs are a little higher, so you should not fly at high power.
A mag check during flight is fine, and is actually recommended – at a high altitude – to check from time to time that the ignition system insulation is good. To do this, you must be mentally prepared to close the throttle completely if switching to a particular mag cuts out the engine or makes it rough, before switching back to either a known good mag or to BOTH. Then, after you have switched back to a known good status, re-open the throttle. If you fail to do this you could blow up the exhaust system, because a rotating (i.e. pumping air) engine with no ignition is going to fill the exhaust with unburnt fuel vapour.
A bad mag is an emergency and I would land ASAP. This is because the problem could be in the accessory gearbox, and then you are heading for a camshaft problem which is really going to ruin your whole day
This is the in flight Lean of Peak mag test protocol recommended by Savvy. I have done it many times on the Continental 550 N/A in my SR22:
In-Flight Lean Mag Check
The in-flight lean mag check is a test of ignition system performance, and is used to help diagnose
problems with magnetos, ignition harnesses, spark plugs, ignition timing, etc.
Perform the in-flight lean mag check procedure by setting up the airplane in normal cruise on autopilot. Then perform the following procedure.
? Lean to an aggressively lean cruise mixture. The leaner, the better for this test. The test
will be much more discriminating and meaningful if you lean to a lean-of-peak-EGT (LOP) mixture setting. Preferably lean as far LOP as you can without experiencing significant engine roughness.
? For a key or rotary style magneto switch (as found in most singles), select BOTH-LEFT-
BOTH-RIGHT-BOTH, leaving the mag switch in each of these positions for one full minute.
? For individual magneto toggle switches (as found in most twins), turn off the LEFT
ENGINE/LEFT MAGNETO switch for one full minute, then turn it back on and wait for
one full minute. Repeat this procedure with each of the other three magneto switches in sequence. (LE/LM, LE/RM, RE/LM, RE/RM.)
NOTE: It is normal for engines to run a bit rougher on one magneto than they do on two, but they should not run change-of-underwear rough. Please report on the level of perceived
roughness during phase of the test.
NOTE: For turbocharged engines, TIT may rise to above red-line during single-magneto operation. This is normal and will not do any harm if the exceedences do not exceed a few minutes’ duration.
Just my opinion. Mags are tested before each flight, and should not be tested in flight – unless you suspect something is wrong. If you find that it runs rough or not at all one one man, then that mag should be turned off. The reason it should be turned off, is you don’t know what is wrong with it, only that something in that ignition system is broken. It could be overheating or shurtcut, too advanced timing and any number of reasons that could cause further dangers.
@achimha: thanks for correction, I was thinking of my own (geared) Rotax. For a direct drive, as most forumites here will fly, I don’t doubt you are right. And for those flying geared Austro/Thielert there’s no issue either…
Doing a mag check in flight is no problem. Sustained operation on one mag is not great if it can be avoided, but not particularly harmful either. If you have a mag problem you first discover in flight, test to understand it: At low power or better idle, switch to each mag. If the engine runs rough on one mag, but runs, expect a plug problem, or leads insulation breaking down if you’re at high altitude. If that’s what you find, fly it home on both, and get it fixed. If you find rough running at altitude which you attribute to leads insulation breaking down, its probably going to be the same left and right, unless the leads were replaced last at different times, which is unlikely.
If, during your mag check, the engine ceases running entirely on one mag, that mag is U/S, and I would fly to the closest maintenance on the remaining mag. Once on the ground, have a look at the mags under the cowl. It has happened to me a few times that the three simple screws which hold the leads cap to the mag have loosened, and the cap is out of position – a very simple fix, and easy to check before the next flight.
Have a look back through the logs, most mags have a 500 hour life, or overhaul interval – are they getting close to that?
If your Lycoming 540 engine has the dreaded single dual mag, do not take any chances with it, I had one of those going bad in a 182RG once, and it turned out that two teeth were missing from the main drive gear – that was about to become very bad! Get the mag overhauled if in doubt.
