No rpm drop when checking magneto. What’s wrong?
Here is a very important question that came in from Travis:
I’m training in C172’s from the early-mid 80’s, and I’ve got a mag check question: Occasionally when I do the mag check, I’ll get an RPM drop on one mag, but not on the other. The engine always runs fine, just no RPM drop on one mag where there was on the other. What’s going on here? I’ve never had a problem, but now I’m just curious! Thanks!
Travis, what you are describing sounds like you have a “hot” mag. Basically a magneto that most likely has a broken P-lead (it could also be the mag. switch). The P-lead goes between the start switch and the magneto. The p-lead functions to ground the magneto (turn it off). You need to report this to a A&P mechanic as soon as possible because this is a dangerous situation. If someone happened to turn the propeller by hand, it could conceivably start the engine while sitting on the ground or in the hangar (ouch!).
When conducting a magneto check, you always want to see an RPM drop of some kind. A lack of that drop, indicates the magneto is not shutting off. Another way to check this is while at low RPM (<1000) you can quickly turn all mags off and the engine should behave like it is going to quit, if it keeps on running, this is another sign of a broken magneto wire (or magneto swtich).
P-lead on an magneto