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Magneto Failure.

Asked by: 1003 views
Aircraft Systems, FAA Regulations, General Aviation, Private Pilot

Before departure i did the run up checklist in a cessna 172, testing the left magneto i noticed a drop in rpm if more than 125 rpm, y cleaned the magneto as i was tough (rpm to 2000 and leaning the mixture for a minute). Then testing it again it was within the limits. I departed an climbed to 8000 msl and did cruise checklist. On 2300 rpm and a light tailwind i noticed IAS didnt go higher than 80knots neither groundspeed. I tried leaning the mixture but it remained the same. Y ask for deviation to the departure airport and during descent at 1900 rpm engine had roughtness. Landed a tested the magneto and it was rought. Now i am thinking about it, if i did well, or what was the procedure, i said ATC what was happening but i am not sure if it is an emergency or a single incident if It needed to be notify.

2 Answers



  1. Lucas on Jun 18, 2013

    In general filing a NASA form is never a bad idea, but in case I do not see the necessity. You did not declara an emergency for what I understand and in this type of case (even if you had declared) ATC might ask you to file a report if priority was given to you (and again this doesn’t seem to be the case). Also usually this happens a real busy airports (mostly class B) because if they need to hold a bunch of 747s, A340s, etc, they might need a piece of paper explaining why.
    All you didi was correct and in accordance with to the POH of a 172. Some higher performance airplanes the mixture needs to be leaned even during climb to a certain GPH.
    Being careful is never a bad idea on the other hand if you had continued going and trying to troubleshoot a possibly unsolvable problem could have proven dangerous if not fatal.
    So again in my opinion you did everything right.

    Lucas
    pilottrainingsolutions.com

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  2. David Brown on Jul 15, 2013

    Lucas, all Normally Asirated engines need to be leaned in the climb. Just some POH’s are poorly written, in fact most are when it comes to this topic.

    The exception is those with an ACFP or Altitude Compensating Fuel Pump which is an option on some TCM engines and often found in Bonanza’s. These fuel pumps are configured to do the leaning for you. So these engines do require leaning, but they do it themselves!

    Things like the Rotax in LSA’s do it too.

    So no matter whether it is an O-320 or an IO-720 or anything else, including a Briggs and Stratton Lawn Mower engine will need the same leaning to keep the F/A ratio similar to that at rated power at sea level.

    The exceptions to this are turbo charged and turbo normalised engines as they think they are at sea level all the time so it is full rich or around 80dF LOP for them.

    Hope this helps.

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