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5 Answers

Magneto safety question

Asked by: 2669 views Student Pilot

Hello, I am not a pilot, but my 17 year old son is a few solo hours away from his private pilot license. I have a safety concern about an incident that happened with his flight school, and I would like to get your opinion about the validity of my concern. A week ago, prior to taking a Piper Archer out for a solo flight, during the runup check, the left magneto showed no drop in RPM. My son decided not to fly. His flight instructor said he would check it out. The instructor flew the plane and stated that this was an issue anymore. The plane flew all week. Today, my son again tried to take the same plane out for a solo, did the runup check, and the same left magneto issue occurred. This time, the instructor repeated the check and the same issue occurred for the instructor. Here is my concern: why was this 37 year old plane not checked out by maintenance after my son first identified this issue a week ago? I am no pilot, but to me, an intermittent issue with a vital mechanical part should be addressed if reported. I understand there are two magnetos, but they are important. I am especially concerned that my 17 year old son, about to go out for a student solo, had to identify this issue twice before it was checked out. In your opinion, should this issue have been checkout out by a mechanic the first time it was reported? Thank you for your advice.

 

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5 Answers



  1. KDS on Aug 31, 2020

    If there is NO drop in RPM during a mag check, there is a problem that needs to be investigated.

    Why the instructor would need to fly it to validate that fact does not meet the common sense test.

    Beyond that, anything else I could add would just be my guessing as to why people and organizations do things the way they do. However, none of my guesses would be complementary to the organization or the instructor.

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  2. John Scarry on Aug 31, 2020

    One possible safety issue if there is no drop on one side would be that the airplane can start without the key being in the ignition. If a student would move the prop—say to attach the tow bar—the plane could start.

    The other issue is that you don’t know if the plugs associated with that magneto are fouled or not. When the ignition switch is moved from one magneto to the other it is supposed to turn one off. If the engine runs rough when on one magneto it is usually an indication that the plugs are fouled so they are not developing full power. It could also mean something more serious worth the magneto or plugs. But if there is no drop then you haven’t tested those plugs.

    Your son was right to not fly.

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  3. John D Collins on Sep 01, 2020

    I think your son did the right thing in reporting it to his instructor and choosing not to fly. The instructor then flew the airplane and according to your report was not able to reproduce the issue. The aircraft flew for another week, presumably each pilot should have done a magneto check and should also have reported an issue if they saw one. Then your son attempted another flight and found the issue persisted. The instructor was able to confirm the issue and had the aircraft checked out.

    That is not enough information for me to form an opinion if the operator or the instructor did anything wrong. If the instructor, operator or mechanic did not check out the airplane, that would be an issue. I would hope that the person checking out the squawk would also determine if there was a hot mag. A single mag check where there was not an apparent mag drop is not determinative in my opinion. Intermittent issues are sometimes very difficult to diagnose. They should be noted, reported and checked.

    Your son sounds like he is sharper than the typical student at his level of experience. Most students will be less observant. Had a mechanic been consulted, they would undoubtedly performed a runup to confirm the squawk. A good mechanic would also have checked for a hot mag and may have opened the cowling to inspect for a broken P lead. If they were unable to confirm the squawk, they would most likely return the airplane to service. My point is that if the squawk can’t be reproduced, the aircraft is not necessarily grounded on the basis of a single report.

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  4. AV8R on Sep 01, 2020

    This is a very interesting situation, and it sounds like the student pilot here is a sharp student. First of all, was there a mag check conducted to the airplane? That right there would tell you if the magnetos were grounding.
    Your son did the right thing in taxiing back to the hangar. On the other hand there are some problems that are hard to duplicate, even for mechanics. I have had the same problem happen to me. And on one occassion the mechanic could not duplicate the problem. 2nd time it happened the magneto’s P lead was broken. They fixed it and returned aircraft to service. If a problem cannot be reproduced I would have no problem flying the airplane, nor having it returned to service. Most flight schools do the same thing. If a problem cannot be duplicated they will return the airplane to service, and I would have no problem flying it. If the problem persisted I would try and write up a squawk and try and be as specific of the problem as I can.

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  5. AviatorTrevor88 on Nov 23, 2020

    I suppose it’s possible that the CFI and other renters did perform a magneto check, and the result was normal. That could be the result of a loose P-lead wire on that magneto. If that was the case, the CFI and other renters did nothing wrong since they didn’t detect anything was wrong.

    Although, that seems like an unlikely explanation. Your assumption that people ignored the lack of RPM drop does seem more plausible.

    The risks of a broken P-lead is that the magneto is essentially stuck “ON”. This means that if someone walked up to the propeller while the engine was off and swung the propeller by hand, there is a chance the engine could start. I wouldn’t say it’s a high-likelihood that it would start if you moved the prop by hand, but it’s high enough that I wouldn’t want to test it out.

    The second risk of a magneto being stuck “ON” is that you could takeoff with a mag that had improper timing (something that could be fixed by a mechanic), and due to improper timing you could have engine pre-ignition or detonation (very bad things for an engine). The pilot could remedy the problem by switching off the bad magneto and returning to land ASAP on the one good magneto. I’d still put that as a low risk of happening, but it’s 100% a reason to not depart.

    Maybe you could query the CFI about “what are the potential dangers of flying with a magneto that won’t drop RPM during the mag check?” See if he knows about the potential for pre-ignition and not being able to remedy that. If he doesn’t know that, that might explain why he thought no loss of RPM was “safe”.

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