Welcome Guest. Sign in or Signup

3 Answers

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

Asked by: 12803 views
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?

Ace Any FAA Written Test!
Actual FAA Questions / Free Lifetime Updates
The best explanations in the business
Fast, efficient study.
Pass Your Checkride With Confidence!
FAA Practical Test prep that reflects actual checkrides.
Any checkride: Airplane, Helicopter, Glider, etc.
Written and maintained by actual pilot examiners and master CFIs.
The World's Most Trusted eLogbook
Be Organized, Current, Professional, and Safe.
Highly customizable - for student pilots through pros.
Free Transition Service for users of other eLogs.
Our sincere thanks to pilots such as yourself who support AskACFI while helping themselves by using the awesome PC, Mac, iPhone/iPad, and Android aviation apps of our sponsors.

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.

     

    +15 Votes Thumb up 15 Votes Thumb down 0 Votes



  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.

    +10 Votes Thumb up 10 Votes Thumb down 0 Votes



  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.
     

    -2 Votes Thumb up 1 Votes Thumb down 3 Votes


The following terms have been auto-detected the question above and any answers or discussion provided. Click on a term to see its definition from the Dauntless Aviation JargonBuster Glossary.

Answer Question

Our sincere thanks to all who contribute constructively to this forum in answering flight training questions. If you are a flight instructor or represent a flight school / FBO offering flight instruction, you are welcome to include links to your site and related contact information as it pertains to offering local flight instruction in a specific geographic area. Additionally, direct links to FAA and related official government sources of information are welcome. However we thank you for your understanding that links to other sites or text that may be construed as explicit or implicit advertising of other business, sites, or goods/services are not permitted even if such links nominally are relevant to the question asked.