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When shutting down, why dont we just pull mixture out and immediatly turn mags off?

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General Aviation

For the C172S and Archer II that I fly, I was taught to shut down with throttle idle and Mixture cut-off..wait for the prop to stop spinning and then turn mags and master off. I feel like the prop/engine would stop spinning as soon as we turn the mags off.  Why do we wait for the engine to "choke" itself dead before turning the mags off.  I would think turning the mags off would shut the engine down. Thoughts?

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3 Answers

  1. Paul Tocknell on Nov 17, 2010

    You are absolutely right.  You could shut off the engine using just the mags.  There are a couple of problems with that technique though.  

    The first problem is that by using the mags to kill your engines leaves fuel in your carburetor.  It doesn’t take much time for fuel to become stale.  The good stuff in fuel tends to evaporate and leave behind pretty crummy stuff that will make it next to impossible to start your engine the next time and can ruin fuel lines and even the carburetor itself.

    The other advantage of using the mixture cut-off method is that it eliminates the possibility of pre-ignition.  Engines and cylinder walls can get so hot at times that they can actually ignite the fuel / air mixture without the need for spark plugs!  That’s not good.  So you go to shut your engine off (using the mags) but it would keep running.  The only way to shut off the engine at that time is with the mixture.  If you just get in the habit of securing your engine with the mixture cut-off you’ll never have to worry about pre-ignition or stale fuel in your carb.


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  2. M Gallagher on Nov 17, 2010

    Actually this is a safety procedure to prevent personnel injury that has continued from aviation’s earliest days.
    If the engine is shutdown using the mag switch, fuel remains in the system, AND if the wiring to one or more of the engine-driven mags was defective, the engine could possibly fire if the prop (engine) is moved – such as during a pre-flight inpection.

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  3. rhw on Aug 02, 2011

    . . .but, I would suggest that the aircraft be shut down periodically with the mag switch.  It proves that the p-leads are connected, as well as the jumper (usually) in the ignition switch.  The run-up proves the p-leads, but not the jumper, or the complete switch. 
    Best procedure for a normal shut-down is to rev up the engine for a few secons if it has been idling, and then reduce to 1000 RPM’s.  Then slowly (1-2 seconds), pull out the mixture cut-off.  You sghould see a slight rise in RPM’s.  This proves that the idle mixture is set to slightly rich of best power, as it should be.

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