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7 Answers

Idle Engine

Asked by: 9684 views
General Aviation

It is my understanding that "idle" is when the engine generates enough power to run, but not enough to perform useful work.

Therefore, when is the throttle in idle position or at "idle speed"?  Does it differ with every airplane, or does it  mean simply closed throttle?

The reason I am asking is b/c when I see "idle check closed" on the checklist for engine run-up, I was taught to close the throttle to ensure the engine doesn't stop.  Other than that, I'm not exactly sure what I am checking for and why.

 

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7 Answers



  1. John D. Collins on Feb 20, 2011

    Each engine has a specified idle RPM.  On my Bonanza with an IO520, the idle RPM is specified as 600. In addition to checking for smooth operation and ready for takeoff, you should verify that the RPM is the correct value.  If you have the idle RPM set too high, say 800 RPM in my Bonanza, you will use up considerable more distance in the landing roll and won’t be able to come any where near POH landing distance performance.

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  2. Kent Shook on Feb 21, 2011

    The other thing you should do when performing the idle check is to turn on carb heat. That will further reduce the RPM, and you want to be sure that the engine will still run in that configuration, as that is what you have just prior to landing.

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  3. GUMPS on Feb 21, 2011

    Thank you for your responses.
    However, am I at “idle” when the throttle is fully closed? 
    Also, where do I find the specified idle RPM value for the aircraft I am renting?  I didn’t see any specific value listed in the AFM.

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  4. John D. Collins on Feb 21, 2011

    Yes, when you have the throttle fully closed (throttle all the way out at the rear stop on most aircraft), the engine should be at idle.  The idle RPM can be usually found in the engine or aircraft manufacturers maintenance manuals.  For most engines this is 600 to 650 RPM and this is likely to be good enough.  If you want a specific number for your aircraft, you can ask a local mechanic for the idle RPM for a specific engine.
     
    Idling engines are normally adjusted to run rich for easier starting.  There is a test that you can do at engine shutdown that will give you an indication if the idle mixture is properly adjusted.  Bring the engine to idle, note the RPM, then slowly lean the engine towards cutoff.  You should see a 25 to 50 RPM rise before the engine starts to slow down as you lean it to cutoff.
     

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  5. GUMPS on Feb 21, 2011

    “Mixture – Idol Cut off” has always seemed like an odd choice of wording to me.
    It seems as if it should read “close throttle, cut off mixture.”  🙂
    Thank you for confirming that closed throttle = idle

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  6. Heather McNevin on Mar 12, 2011

    When you close the throttle, you are not only looking for the engine to not stop, but to run smoothly.  Idle is checked at annual to make sure it is within proper parameters.  They also use a box that measures RPM to ensure that the guage is functioning properly.  Another thing I check at idle at the end of a flight is the grounding wire.  This is an Airworthiness Directive, I believe every 100 flight hours.  At idle, with the avionics off, just before I shut off the engine with mixture mags and master, I pull the throttle to idle and shut the mags to OFF and then back to BOTH.  The engine should momentarily stop.  If it doesnt, the grounding wire is faulty.  This is important because if someone moves the prop with a faulty grounding wire, the engine could turn over and injure the person with the prop.Good luck with your training!  I’m so glad you are questioning why you do things a certain way.  That is the mark of a good pilot not to just follow procedures but to know why those procedures are the way they are.  Keep it up!

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  7. Jake Razing on Oct 12, 2012

    How long must an engine in idle to avoid damage?

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