Magneto Question

[yoshiman]

I have a question hopefully someone can help clear up. I have started my flight training and was wondering about the magneto check. When I switch to the right, does that mean the left mag is not working? And vice versa? Also can someone explain how they work and if they fail why the engine stops? Thanks I was just confused on this and thought it was important to clear up.

[Uncle Ginsters]

Yoshiman,
I'll try to keep it simple, but my apologies if i fail! And i hope you're not just looking for an easy answer to an FIs question!?!

When I switch to the right, does that mean the left mag is not working?

Yes - with "Left" selected, the right mag is isolated, and vice-versa.

Also can someone explain how they work

When a magnet spins in a coil, it induces a current. The magneto is a just that - a magnet spinning, driven by the engine, in a coil, thus inducing a current. That current is then fed to the spark plugs in each cylinder to give ignition.

if they fail why the engine stops?

Most piston engines have 2 mags. This gives redundancy. Each mag will feed one spark plug in each cylinder. So, if one mag fails, there should be at least one spark plug still functioning in each cylinder.
However, in the unlikely event that you lose both mags, there will be no spark in the cylinders, thus no ignition and no power!

Hope this helps - its brief but if you need more detail, let me know.

Uncle G

[foxmoth]

This is probably better in the Tech log forum. For how a Mag works you are probably best going into the books, but basically it generates an electrical charge by rotating a magnet inside a coil to give a spark which is supplied to the spark plugs (obviously more complex than this). Magnetos are by their nature always live and when you switch to "Left" you are actually earthing (in effect turning off) the right magneto, this means off course that if the wiring to the switch is defective they will normally fail in the "live" position - the danger here of course being that this leaves a live propellor and is why props should always be treated as live. Normally of course you have two sets of plugs which are supplied by the seperate magnetos and if one magneto fails you will still have a working engine, though not quite as efficient - this is what is happening during the mag check, only one set of plugs is being supplied. Should both fail of course no spark plugs are being supplied and the engine will stop. One other major point to note here is that magnetos are independant of the aircraft electrical supply, so if the Alternator and battery are not working the engine will still run.

[yoshiman]

Thank you both for your responses and I am not just trying to get an easy answer for my FI. It was just something that the books did not clarify too well. Also, what happens if I switch to left or right and I see no drop in rpm? Sorry if these are poor questions.

[foxmoth]

If you switch from both to "L" or "R" and get no drop it normally means that you have a broken wire on one mag so it is permenantly live, usually here you will get a drop on one side as that mag earths and is switched off but no drop on the other side because it does not - it is possible to get no drop either side but this would be very unusual. The thing you are more likely to see is a larger than normal drop, this is usually caused by a plug not working, either through oiling up or another problem. Any of these problems should be resolved before getting airborne.

[B2N2]

A quick google search:

Not airplane, but good anyway:
http://www.foxvalleykart.com/timing1.html

Airplane related:
http://selair.selkirk.bc.ca/systems1...20systems.html

http://www.sacskyranch.com/aircraft_...leshooting.htm

Some magneto related accidents from the NTSB:
http://www.ntsb.gov/recs/letters/1992/A92_22_24.pdf

Hope it helps.