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Safety Tip: When Mags Go Bad

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  • Safety Tip: When Mags Go Bad

    Lost of newer pilots who are out shopping for a plane look at the time since last major overhaul and the suggested TBO. Thinking, "Hey this engine obnly has 900 hours on it, now I can fly for another 1,000 before having to worry about anything."

    But there are two little items in your engine that are made of plastic,,,, and if they break or quit working in flight you are in for some interesting times.
    I speak of your MAGNETOS.

    Yes you have two of them, but oddly enough a local air 135 operator had two occasions in the 2 years when both mags failed.

    I look at mine very year...and I just caught one that had gone bad , yet it was still working.

    The mag in question had about 550 hours on it and as said it still worked fine and tested fine... But a visual inspection showed that the carbon brush on the end of the rotor had been fried from arcing. In other words it SHOULD HAVE NOT BEEN WORKING AT ALL.

    And this mag looked fine during another inspection 50 tach hours previously.

    One photo shows the old assembly on the left with the new assembly on the right. The other photo shows the bad Rotor assembly with the damaged shaft. There is supposed to be a spring-loaded carbon brush on the end of that shaft. It is now welded into the end of the shaft.

    Years ago I had a mag fail in flight. It turned out that one of the P-lead connector had been damaged by somebody who was cleaning planes. It worked OK until the heat and vibration made it connect only at odd intervals.

    So long story short,,,, while an engine may go for a long time without a complete overhaul, the Mags are just plastic parts that should be changed a lot more often.
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  • #2
    Another thing that gets me is people buy planes and brag about how low of engine time it has. The fine print is the engine was last overhauled 35 years ago. Engines have a calendar and a tach time before manufacturer recommended overhaul.

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    • #3
      Particularly in a place like Alaska with wide temp swings and lost of moisture. Condensation plays havoc with cams and other parts.
      I ran a 0-320 Lyc to 3500 hours by topping it with new jugs, new mags and constant progressive PMS. But that plane flew almost every day for 3-4 years. I sold it to a real-estate broker in Nevada who flew it for another 400 hours around the desert before he finally had it over-hauled.
      On the other hand there are lots of ramp queens sitting around the airports up here that may only have 300 hours on the engine, yet they have not flown in 3-4 years.
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      • #4
        Good stuff. Thanks for sharing.

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        • #5
          I had a IO-360 with 3,642 hours on it when it was finally overhauled back in May. Same thing, flew the heck out of that plane, put new jugs on every now and then but was a good strong engine. Only overhauled it to quiet that little voice in the back of my head reminding me every time I went over the inlet. At overhaul it got a new cam and new lifters cause they were beyond tolerances.
          Phillips 20-50 changed and inspected every 50 hours.

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          • #6
            I used to be a Aero Shell straight-weight fan, but once I tried that Phillips 20-50 I was hooked.
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            • #7
              I wonder if any of you has been plagued by Cessna's SLICK mags, as I have in the past. I finally determined that, every time I bought a Cessna product, I immediately changed from the SLICKS to BENDIX with heavy harness. Never had any trouble with the BENDIX, and checked them often as a matter of course.

              Float Pilot is correct. Check the mags frequently. By the way, all my oil changes came at 40-hour intervals. Differential pressures remained forever at the numbers they had been when the plane(s) was purchased.

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              • #8
                The offending Mag in this case was a Slick 4300 series. By Mattel, they're swell!


                xxxxxxxxxxxxxxxx

                As for oil,
                with spam cans or other planes with oil filters I change every 40-50 hours.

                But when there is only a screen, like with my little 90 horse Cub, I change every 20-25 hours.
                My C-90-8F used to leak or blow out a quart every 8 to 10 hours when I ran Aero-Shell.
                But ever since I went over to Phillips 20-50 it will go 20 hours before it starts to burn or blow any oil.

                After 20 hours it starts to need another quart every 4 hours... Unless I change to all new oil and then it goes another 20 hours before any disappears.
                So something must be changing due to carbon or general viscosity break-down. I figure oil is a lot cheaper than a new motor.
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                • #9
                  Yeah thanks for sharing..

                  If I rem right mag shld have timing checked every 100-hrs.

                  My 180B got stuck in McKenzie, BC last June when the Bendix failed. Only thing available at that place was a guy with some Slicks with abt 70-hrs on them..$3k later and flying Ha. No probs since tho.

                  Guess I aught to get both the Bendix Mags rebuilt. Too bad you can't use the same harness on Slick & Bendix Mags..

                  I read one cld actually mix them if necessary..any one done that?

                  RR

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                  • #10
                    Speaking of fresh engines and low time and bad mags, I fall into the 200 hour 3 year old 0-360 that just got rebuilt AGAIN due to metal in the oil. No idea of were the metal came from. Total tear down, split case, looks like it was just built 10 hours ago. So now its back together again and about to be hung on the plane in a day or so.
                    So, like has been said before, just because its a low time engine dosnt mean you can slack on the maintance of mags, oil, etc....

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                    • #11
                      Originally posted by Float Pilot View Post
                      The offending Mag in this case was a Slick 4300 series. By Mattel, they're swell!


                      xxxxxxxxxxxxxxxx

                      As for oil,
                      with spam cans or other planes with oil filters I change every 40-50 hours.

                      But when there is only a screen, like with my little 90 horse Cub, I change every 20-25 hours.
                      My C-90-8F used to leak or blow out a quart every 8 to 10 hours when I ran Aero-Shell.
                      But ever since I went over to Phillips 20-50 it will go 20 hours before it starts to burn or blow any oil.

                      After 20 hours it starts to need another quart every 4 hours... Unless I change to all new oil and then it goes another 20 hours before any disappears.
                      So something must be changing due to carbon or general viscosity break-down. I figure oil is a lot cheaper than a new motor.
                      Very interesting . . . . . . . . . .

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                      • #12
                        Originally posted by Float Pilot View Post
                        The offending Mag in this case was a Slick 4300 series. By Mattel, they're swell!


                        xxxxxxxxxxxxxxxx

                        As for oil,
                        with spam cans or other planes with oil filters I change every 40-50 hours.

                        But when there is only a screen, like with my little 90 horse Cub, I change every 20-25 hours.
                        My C-90-8F used to leak or blow out a quart every 8 to 10 hours when I ran Aero-Shell.
                        But ever since I went over to Phillips 20-50 it will go 20 hours before it starts to burn or blow any oil.

                        After 20 hours it starts to need another quart every 4 hours... Unless I change to all new oil and then it goes another 20 hours before any disappears.
                        So something must be changing due to carbon or general viscosity break-down. I figure oil is a lot cheaper than a new motor.
                        FP - Interesting as Griz says..

                        I was gonna stick with AS 100 but reckon I aught to try the Phillips 20-50 just for a hoot. Cldnt hurt huh?

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                        • #13
                          The CAP went to some sort of oil which now slips my mind. (for the 182) It comes in a gold bottle and you can't buy it in Homer. Anyway the oil looks like regular golden honey colored oil at first, but after a couple of hours it turns a dark blue. We got some on the floor and the stains made me think we had a fuel leak. Whatever it is, the 182 seems to like it. We have a mandatory 50 hour or calendar date change and it only would burn up a 2-3 quarts of the stuff during that 50 hour tach period.

                          As for the Phillips Oil, I only tried it because I won a case of it during a poker run a couple years back. But the little 90 horse is the only engine I have ever tried with the stuff.
                          I just checked my oil burn logs and it looks like it was actually going for only 18 hours before it needed a quart during last fall (Nov) before I pulled her out of the water. She has not flown since, as she is in pieces /annual/overhaul.
                          Since I have good compression I always figured most of my loss was via the breather line.
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                          Guest Cabin, Flight Reviews, Aerial Tours

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                          • #14
                            Originally posted by Float Pilot View Post
                            The CAP went to some sort of oil which now slips my mind. (for the 182) It comes in a gold bottle and you can't buy it in Homer. Anyway the oil looks like regular golden honey colored oil at first, but after a couple of hours it turns a dark blue. We got some on the floor and the stains made me think we had a fuel leak. Whatever it is, the 182 seems to like it. We have a mandatory 50 hour or calendar date change and it only would burn up a 2-3 quarts of the stuff during that 50 hour tach period.

                            As for the Phillips Oil, I only tried it because I won a case of it during a poker run a couple years back. But the little 90 horse is the only engine I have ever tried with the stuff.
                            I just checked my oil burn logs and it looks like it was actually going for only 18 hours before it needed a quart during last fall (Nov) before I pulled her out of the water. She has not flown since, as she is in pieces /annual/overhaul.
                            Since I have good compression I always figured most of my loss was via the breather line.
                            FP - I stink that oil is Exxon Aviation Oil Elite 20W-50. See pic

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                            • #15
                              Never had any problems with mags even though I have had several overhaulled ... Two things that I have encountered were 1.) fine wire irridum plugs that misfire at operating rpm and temp but are fine when at idle and cold 2.) the other gremlen that we chased and caused the overhaul of a mag was a bad condenser that would work OK at idle and mag check but then act badly after you had been flying it for about an hour. These kinds of problems can be the most mysterious things to diagnose, the condenser tested OK until it was rattled on the test equipment.

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