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7 Answers


Asked by: 6469 views ,
Aircraft Systems, Student Pilot

How I couldn't find anything about this subject at askacfi, I'd like to clarify a doubt about why to proceed at a magnetos malfunction. Here goes a short patch extracted from an AFM (in this case, C152): "A sudden engine roughness or misfiring is usually evidence of magneto problems. Switching from BOTH to either L or R ignition switch position will identify which magneto is malfunctioning. Select different power settings and enrichen the mixture to determine if continued operation on BOTH magnetos is practicable. If not, switch to the good magneto and proceed to the nearest airport for repairs." Well, I see a recommendation about in case of magnetos malfunctioning, after identify the “good one”, switch to the same. I really could'n't  understand the AFM " If not, switch to the good magneto". So, in a magneto malfunction, is it possible to fly with one magneto and depending on the situation, without the bad one? So, "Why on Earth"  I cannot fly in both? What's the technical problem to fly with both magnetos even one is not working properly? Is the bad one can damage the good one? If yes, why? Thanks in Advance all for helping me improve my aeronautical knowledge! Carlos SBJD

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7 Answers

  1. Brian on Sep 04, 2013

    The big issue here is timing. If they are both on proper timing, but one is weak or going out, then there wouldn’t be a problem. However, if the malfunctioning magneto is causing problems because it has somehow gone off time, that could cause engine failure despite the other magneto working properly. The question is, without more advanced training, how do you know the difference?

    For me, as a pilot with limited in depth understanding of the operations of a magneto, I’ll fly on the good one. Why? The airplane flies just fine on one magneto, why gamble with something I’m ill prepared to diagnose? Furthermore, even if I was marginally (say 99 percent) certain of the problem, what’s wrong with flying on one magneto if I’m 100 percent certain it is operating as it is supposed to?

    In other words, there are uncertainties with flying on a malfunctioning magneto. There are no uncertainties with operating on one good magneto. Simply realize you no longer have a backup and find a place that is safe for landing as soon as practical/possible. That’s what I’d do. Unless of course I had an engineer/mechanic on board with a deeper understanding anyhow.

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  2. Brent on Sep 05, 2013

    According to your scenario, the problem initially manifest itself as “engine roughness or misfiring” when operating on both magnetos. Why would you want to fly a plane with the engine misfiring when you could switch the good magneto only and fly with a smoothly running engine?

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  3. Cadu on Sep 05, 2013

    Hy Brent. This is exaclty my doubt. I never had a problem with mag in flight. So, losting one of them, and switch to the good one, the engine will operate normally just with a loss of power? I mean, without roughness ?


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  4. Brent on Sep 05, 2013

    Well, the scenario that the AFM is describing is when you are unable to achieve smooth operation on both, but you are able to achieve it with one magneto. There could be any number of other failure modes that wouldn’t result in you being able to achieve smooth operation with one magneto (for example: carburetor icing).

    If you think about it, you already have experience running the engine on one magneto, since you do presumably do a magneto check during every runup. As you said, running on one magneto and an otherwise healthy engine results in slightly lower RPM, but not much else in the way of noticeable effects.

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  5. Brian Curry on Sep 05, 2013

    It is perfectly acceptable to fly on one magneto, but not ideal. We have two magnetos for two different reasons:

    1) Redundancy

    2) More complete fuel burn (higher power output). When doing a Mag check on the ground, the RPM drop is the lost of power that results from only one magneto firing. The difference in the two rpm drops is a representation of the different timing between them.

    A rough running engine that runs smoother on on mag then with two is probably experiencing significantly different timing between the magnetos. Isolating to the magneto that gives the smoother engine operation is wisest, as mis-timed spark plug firing can cause worse problems.

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  6. Cadu on Sep 06, 2013

    Great guys. Thanks everyone.

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  7. kakwa steven on Feb 15, 2017

    Want to know about magneto

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