Welcome Guest. Sign in or Signup

No rpm drop when checking magneto. What’s wrong?

Posted by on January 30, 2009 4 Comments Category : Flight Instructor Blog

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

P-lead on an magneto


  1. Ben Sinclair on Feb 01, 2009

    Is this an issue that would stop you from flying until it’s resolved, or would you still go up and just make sure to inform the ground crew when you return?

  2. Paul on Feb 01, 2009


    If this is something I caught during the run-up, especially at a remote airport, I would probably still fly back to my home airport. As soon as I returned, I would leave a note in the log record, and notify the flight school and and the aircraft mechanic immediately, preferably by phone. If for some reason you needed to shut off the engine during the flight, you could still do so using the mixture cut-off.

    I usually don’t like giving blanket statements like the one above because every situation is different and there is not one right answer to this question. This is one of those questions that falls under the gray shade of aviation (not a black and white answer) so take that with a heavy dose of salt.


  3. Art Friedman on Mar 12, 2017

    Besides a broken P-lead (which means a hot mag, which in turn means that you shouldn’t move the prop by hand), this could also be caused by significantly different ignition timing between the magnetos. This is also something that requires caution, since incorrect timing can cause engine damage. For this reason, it is something you might want to check before flying home from a remote airport.

  4. Gary on Sep 26, 2017

    I was running an IO-360 Lycoming engine in an RV-8, there is no mag drop for either mag, but in the OFF position, the engine dies. Any ideas

Leave a Reply