Joined: Jan 2011
Thursday, August 11, 2011 5:21 PM
We think of aircraft as having two sets of ignition systems (including spark plugs) for redundancy purposes, and certainly that is a major one. But it is also to provide a more efficient fuel burn which leads to greater horsepower (and spark plug life). We witness this affect when we do our mag checks and have a drop in rpm when we switch a mag off (there is ignition timing reasons for this as well). My new Ford truck has two spark plugs per cylinder, not for redundancy obviously, but for increased horsepower.
Just like we wouldn't check our fuel shutoff valve in flight, or pull our mixture back all the way in flight to test it, I can't see anything to be gained by checking the mags in flight. And I can imagine all kinds of things going wrong. If a defect exists in any component - the mag switch, the magnetos, the spark plugs or the wiring, we could possibly loose the use of one or more spark plugs, which might be the only ones firing on a given cylinder, or possibly loose use of the magneto itself. If this were to occur we are now flying on reduced horsepower, or worse yet a reduced number of cylinders.
We might think that's not really a problem, we could still land the airplane. And of course we know that will happen one way or another for sure. But if we need to execute a "Go Around" with diminished horsepower we might not clear the obstacle at the end of the runway, or get out of ground effect (especially on a hot summer day).
Seems to me the only thing we can't simulate on the ground when trying to diagnose what we think might be a mag related problem, is altitude.
We can certainly simulate rpm and load (we do that with partial rpm each time we do a run up, and full rpm and load when we check the adjustment of our propellor blades).
We can certainly simulate heat. We can do that with a heat gun, and in a very directed way, at each component individually, far more effective then flying.
We can check for problems with the wiring by ohm tests and visual inspection.
WARNING! THESE ARE BEST LEFT TO A MECHANIC DUE TO THE DANGER OF THE PROPELLOR (among other things): But some times electricity can creep out of a perfectly good looking (and tested) wire, or connections at the mag, or the spark plug, or anywhere else. One way to check for that is to take a sufficiently long piece of electrical wire, attach one end to a ground spot on the engine, the other to a long screw driver blade near the handle (alligator clips work good). Then, while the engine is running (stay away from the propellor!), and while holding on to the screw driver handle, run the tip of the screw driver along any ignition related wires and around any connections. If electricity is getting out it will jump to the screw driver blade and immediately go to ground (via the wire) and you will hear the difference in the way the engine sounds. We can check for intermittent internal connections by gently rapping on ignition components with something like a screw driver handle, or perhaps a piece of 1x2 wood. Anything that won't conduct electricity or damage the component. And by jiggling wires.
Moisture, and how to eliminate it has been covered in earlier posts so I won't go into that here.
So what about altitude? About the only thing I can envision that could be altitude related would be anywhere in the mag that sparks were jumping air gaps. Perhaps with the reduced air molecules and humidity at higher altitudes (do we really fly up there?) instead of jumping the gaps it would be easier for the electricity to crawl along the inside of the housing or cover, especially with moisture present. And once it has created a track to follow it get's easier and easier for it to do so. This would be easy enough to detect on the ground by looking for carbon tracing where the electricity crawled. Certainly a possibility, but seems like a major long shot to me (not the carbon tracing but it only happening at altitude), and this should always be checked for as part of any ignition system maintenance.
The above is just my humble opinion but I hope it is of value.