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Mombu the Aviation Forum REGISTER NOW! Mark Forums Read ________________________________ [ Search ] Use our Hints: Reply Bookmark and Share IFrame 1 19th November 17:56 Default no RPM drop on mag check dave butler External User ----------------------------------------------------------------------------- Posts: 1 Our Mooney just came out of annual and now the tachometer needle doesn't move at all during the magneto check as you switch through R, L, BOTH. The engine does stop running with the switch in OFF. I asked my my partner who manages the maintenance relationship to look into it and he contacted the IA who did the annual. The answer I got was: "I discussed mag drop problem with <the IA>: he complied with the mag AD during the annual stating that he also timed the mag and set the points and would expect very minimal RPM drop. He also tested the P lead and found it normal. The only confirmation is turning the key off and the engine should stop, I guess that was done. Looks like a non issue." This is a Mooney M20J with the Lycoming IO-360-A3B6D with the "dual magneto", two magnetos in a single housing. I will check the POH to see what it says about RPM limits during the magneto test. I'm not where I can access the POH right now. My question is: is it OK to not have any RPM drop at all when switching from BOTH to R or L? I'm vaguely uncomfortable with it, but I can't come up with any reason it's a-bad-thing. Thanks. Reply With Quote dave butler 2 19th November 17:56 Default no RPM drop on mag check rich badaracco External User ----------------------------------------------------------------------------- Posts: 1 See the following site: http://www.sacskyranch.com/eng101.htm from with any Reply With Quote rich badaracco 3 19th November 17:56 Default no RPM drop on mag check dave butler External User ----------------------------------------------------------------------------- Posts: 1 Thanks. Is there something in particular I should see there? I didn't see anything that got to the heart of my question. The P-leads must be OK since the engine stops when the switch is in OFF. Dave -- Dave Butler, software engineer 919-392-4367 Reply With Quote dave butler 4 19th November 17:56 Default no RPM drop on mag check art External User ----------------------------------------------------------------------------- Posts: 1 Dave, You might try measuring the mag drop using a device like the Proptach Digital Optical Tachometer. http://www.proptach.com Mechanical Tachometers do not have sufficient resolution to measure a small RPM drop. The Proptach has a resolution of 1 RPM and an update rate of 1 second and no hook up required. There should always be a drop on each mag if the engine stops in the OFF position of the switch. Another possible failure mode could be a short between the Right and Left P leads but not to ground. That failure could be in the wiring harness or the switch and would be like having only one mag system. Art Sundeen Reply With Quote art 5 19th November 17:56 Default no RPM drop on mag check monitor point seven External User ----------------------------------------------------------------------------- Posts: 1 I'd look at mag timing. That's the likely culprit if the P-lead checks out. Reply With Quote monitor point seven 6 19th November 17:56 Default no RPM drop on mag check ron rosenfeld External User ----------------------------------------------------------------------------- Posts: 1 "Minimal" drop is different than "tachometer needle doesn't move at all". There are other failure modes that can cause this problem. If the mags are properly timed, and if the ignition switch is shorting the P lead, RPM should drop. You can have a faulty switch that is failing to short the P lead(s). Do you have an EGT gauge for each cylinder? If you do, then in single mag operation, all EGT's should rise. Don't hand turn the prop until this is corrected. Ron (EPM) (N5843Q, Mooney M20E) (CP, ASEL, ASES, IA) Reply With Quote ron rosenfeld 7 19th November 17:57 Default no RPM drop on mag check ron natalie External User ----------------------------------------------------------------------------- Posts: 1 There should be a visible (with a regular tach) drop when on one mag. If not, there is a either a problem with the P-lead (the mag is not getting grounded) or the mags are mistimed. Never believe that the behavior of the mags in the L or R position tell you anything about what will happen when you put the switch in OFF. Believe me, my ignition switch (before it was replaced) could be set to what appears to be off (you could even remove the key) and it not ground the mags (but the normal L-R positions give you mag drops). Of course, then there's always the old Rod Machodo click-click-click-BOOM! mag check his students do. If the engine dies for whatever reason when you turn the mag switch, close the throttle before you turn it back. Reply With Quote ron natalie 8 22nd November 04:24 Default no RPM drop on mag check ron natalie External User ----------------------------------------------------------------------------- Posts: 1 Not OK. It can be a bad switch or the mags mistimed (since we ruled out the P-lead is ok). Reply With Quote ron natalie 9 22nd November 04:25 Default no RPM drop on mag check jsstevens External User ----------------------------------------------------------------------------- Posts: 1 Seems to me (a rank novice, almost) that the RPM drop is the indication of actually switching off one of the mags to allow you to test the other in isolation. If you get no drop, are you really testing the single mag? If you are not, how do you know you have a fully redundant ignition system? That would be my concern. Understand, I am a student with ~12 hours in C-172s and _no_ experience with a Mooney (other than envy). John S. Reply With Quote jsstevens 10 22nd November 04:25 Default no RPM drop on mag check g.r. patterson iii External User ------------------------------------------------------------------------------ Posts: 1 It usually is, but if the timing on both mags is retarded a little (about 10 degrees does it on my O-320), the engine will run at the same speed on either or both mags. This could be the problem, or, as others have said, the switch could be failing to ground the mags in the "L" and "R" positions. George Patterson If a man gets into a fight 3,000 miles away from home, he *had* to have been looking for it. 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