Over the Airwaves
What happens when you turn the key?
It’s a ritual that every piston engine pilot runs through sometime between start-up and takeoff. We run up the RPM, then switch the ignition key from “BOTH,” then to “RIGHT,” then back to “BOTH,” then to “LEFT,” then back to “BOTH.” All we are really doing, of course, is confirming that each of our two independent ignition systems are performing properly. As illustrated below, the heart of each of these two independent electrical systems is a remarkably simple electrical generator called a magneto. Unlike conventional generators that produce continuous electrical current, the magneto is designed to create a periodic high-voltage pulse that is carried to the spark plugs at precisely the right moment via high tension leads.
The entire process begins by turning the ignition key to “START.” This causes the starter motor to rotate the engine which, in turn, causes each of the magnetos to turn which, in turn, produces and sends a high current pulse to the spark plugs. The resultant “spark” ignites the fuel/air mixture and, voila!, the engine begins to run. Having two independently operating ignition systems provides two important benefits. First, of course, is redundancy. If one system fails, the other system enables the engine to continue running. The second benefit comes from improved fuel/air burn efficiency in each cylinder. Having two simultaneous “sparks” is roughly equivalent to using two matches, each at either end of the wood pile, to light your fireplace.
What are we looking for when checking the mags?
Checking the mags before each flight tells us that each independent ignition system is functioning properly. But what if . . . ????
1. What if we do not see an RPM drop when we switch mags?
This tells us that we likely have a “hot” mag. In other words, one or the other of the two mags is NOT turning off when we switch from “BOTH” to either “RIGHT” or “LEFT.” A broken “P lead” or grounding wire is the likely culprit. Get it fixed!
2. What happens if the engine quits when we switch mags?
This tells us that one or the other mag has failed. This is a definite “no go” condition.
3. What happens if we see an excessive RPM drop when we switch mags combined with engine roughness? This tells us that one or more spark plugs is likely fouled and is either not firing or firing intermittently. Again, this is a definite “no go” condition.
The ignition system is what enables our engines to keep turning. Checking the mags after every engine start-up and before takeoff is the key to safe flight. ALWAYS do this!
by Bob Miller