Mag grounding check

Discussion in 'Maintenance Bay' started by Brad W, Feb 11, 2020.

  1. Dan Thomas

    Dan Thomas Final Approach

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    172M's came with ACS switches in them. I wish mechanics would get under that panel and see who made the switch. The ACS has an entirely different AD on it.
     
  2. Dan Thomas

    Dan Thomas Final Approach

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    It doesn't even need to be done fast. After all, if you let the engine die, the muffler has as much fuel-air mix in it as possible, and it doesn't go boom when you restart. At runup RPM there's a lot of flame or superheated gases coming out of the cylinders and they will set off raw fuel in the muffler if the mags go to off and on again. Not at idle. I never, ever had an afterfire on an idle mag check in all the years I flew or maintained airplanes.
     
  3. Dan Thomas

    Dan Thomas Final Approach

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    I used to shut my Stromberg down by turning the fuel off. Didn't want the cylinders charged, and since I didn't fly that airplane often I didn't want fuel in the carb drying out and forming varnish in there. It would idle for about 40 seconds on a bowlful of fuel.
     
  4. Huckster79

    Huckster79 Cleared for Takeoff

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    With my old Stromberg carb that has mixture control but not a full cut off the mags are checked at each shut down... it’s the only way to turn her off...
     
  5. Hang 4

    Hang 4 Line Up and Wait

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  6. Brad W

    Brad W Pre-takeoff checklist

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    kinda....one at a time though and you really aren't confirming if the right mag is all the way off when the left mag is still on in that test.... The run-up mag check is more of a check to verify that the mags are working to fire the engine. This discussion is about checking to see if the mags will switch off cold

    I personally don't think you are missing much.

    Seems the big concern is un-burned fuel making it's way in or past ...leading to backfire potential.
    pulling the mixture sees to it there's not enough fuel left in the intake manifold, engine, or exhaust to support combustion.
    but switching the key off would theoretically leave fuel ready to burn in the intake and some of that could be even sucked through...leaving the potential that the engine could restart with a hot mag if someone pulls the prop through...or I suppose even if a hot spot caused
    dieseling

    As far as I have seen, nobody has really addressed the issue here showing that the quick off then on isn't a sure way to check it.
    To really verify it (except for the bad switch issue), it seems to me that it's needed to switch it off and give it time to shut-down

    and I didn't catch anyone suggesting a procedure of what to do if the engine does shut down by switching key off....
    leave it be?
    re-start so that the mixture can be pulled to starve the engine?

    seems to me that in the case of discovering you have a GOOD mag ground and the engine shuts down when you key it off
    1) if an owned aircraft, or otherwise if you can be sure that nobody else is going to be around the aircraft turning the prop for a while, then with the engine already off, just pulling the mixture to idle cutoff and letting it be is probably fine.... the engine will cool and any remaining fuel will probably dissipate.... but how long of a "while" is needed before it's safe?
    2) if it's a rental or uncontrolled, (or probably always) then perhaps it might be a good idea to either
    a) pull mixture to cut-off and crank it through a few props using the starter, or
    b) re-start the engine and kill it with mixture cut-off
    what do you folks think?
     
  7. James331

    James331 Touchdown! Greaser!

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    I think you’re worrying about something that’s not really that much of a issue. Does the POH say to do this?
     
  8. smv

    smv Pre-takeoff checklist

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    The following POHs do not specify checking the mags at shutdown (these are just the ones I have easily at hand):

    C172N
    C182Q
    C182RG
    M20J
    PA28-151
    PA46-350P
    DA20
    DA40
    R-22
    R-44 II

    I am pretty sure I remember the DA20 and DA40 also not mentioning anything about checking the mags at shutdown.

    That said, I do make it a habit to occasionally check the mags before shutdown.
     
  9. Dan Thomas

    Dan Thomas Final Approach

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    That's for the bendix switch. Here's the one for the ACS: https://rgl.faa.gov/Regulatory_and_...B8ABD56539B4684886256A3E00759DBF?OpenDocument

    Take a look at the list of airplanes you can expect to find it in, especially the long, long list of Cessnas. And then realize that just about any airplane with an Off-Right-Left-Both-Start switch could have one of these, since they're commonly used to replace bad Bendix switches.

    Note that there are several things that need to be done on the initial task, including installing a diode on the starter contactor. Found those missing, probably never installed when the switch was installed, or the mechanics just assumed it was a Bendix. Most seem to know nothing about the ACS switch AD.
     
  10. Dan Thomas

    Dan Thomas Final Approach

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    The engine idles at around 600-700 RPM. You switch off and the RPM will drop if everything is fine, and if you turn back on at 300 or 400 RPM it will pick right back up. It's NOT a precision, highly-skilled, carefully-timed operation. Anybody can do it.

    It's always best to starve the engine for shutdown. Any fuel in the cylinders at all is an accident waiting to happen. If it dies with the switch, close the throttle (it should already be closed) and restart, then pull the mixture. After 47 years I think I can assure you that it will be fine.
     
  11. James331

    James331 Touchdown! Greaser!

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    I’d just go off the POH or ...

    [​IMG]
     
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  12. Dan Thomas

    Dan Thomas Final Approach

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    Does the POH say anything about grounding the airplane before fuelling? Or about making sure it's tied down in a high wind? Or shutting the engine off before you let someone out of the airplane? Or not backing the tail into a snowbank? Or not landing the with the brakes on? Some newer POH/AFMs might mention some of these because if they don't someone will do it and sue the OEM when they get hurt or dead. Common sense is expected, or at least it used to be.

    Most of the fleet consists of old airplanes that, in my experience, need a lot more care than they're getting. The number of broken or frayed wires I have found is simply amazing. Have fixed alternator failures caused by ancient wires that finally broke that should have been caught about five annuals previously.

    I once bought and flew home a Citabria in which the trim wouldn't stay put. I found the trim cables wrapped around the top elevator cable, and all three were frayed. This airplane had been through about 15 annuals or 100-hour inspections since those cables had been installed. Later bought another Citabria that had been restored ten years previously, and the same problem was found in it, too.

    Simply amazing, and very concerning.
     
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  13. skyking3286

    skyking3286 Pre-takeoff checklist

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    The other thing that a mag check after a flight discovers is if you have a rough mag that will only show up when the mags have heated up and the coil opens. I had a bad coil that would be undiscovered if I didn't check the mags at the end of the flight. Only showed up after 45 minutes of flying time during the summer when things heated up. The engine was running fine under power, but at idle with the mag check it would run rough on one mag. So it's not just hot mag but HOT mag....
     
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  14. Dan Thomas

    Dan Thomas Final Approach

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    Yup. And mag switches aren't just for checking mags. If a mag goes rogue, such as happens when the plastic distributor gears fail and the rotor start sparking the wrong cylinders at bad times (like on the intake stroke), you lose a lot of power (maybe most of it if the spark sets off the charge in the whole intake system) and might not be able to continue to stay aloft. The mag switch shuts off that bad mag and everything smooths out on the remaining mag and you fly home. If the P-lead has failed to that mag, you cannot shut it off and now you're in trouble.