What's the purpose of the idle check?

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DMD3.

Pre-takeoff checklist
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DMD3.
During the engine run up, what is the purpose of the idle check? I've actually been told that it's not good to run the engine up, then pull it all the way back to idle and that it's best to do the idle check before running the engine up.

Also, some aircraft checklists do not even have the idle check on their checklists, so is it ok to skip it altogether? :dunno:
 
I don't recall ever doing that except idle before fuel cutoff during shutdown, nor seeing that in a checklist. Only thing I can think of not going to idle before takeoff might be to prevent fouling the plugs. Someone else?
 
Make sure it won't gulp out at idle.

With a flying plane with a windmilling prop I would not be too concerned, but in something like a seaplane where you're are taxing up the dock / ramp / beach at VERY low idle (sometimes only one mag) it's good to know your engine won't quit until you want it to quit, especially seeing this is where most dents occur.

I've also had some work done and found my idle was set a little high.
 
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some features of the magneto are more "stressed" at lower rpms.....so a mag check at idle will bear that out.

I do them at shut down.....it's more a maintenance thing than a flight performance thing.
 
Checkout_my_Six said:
some features of the magneto are more "stressed" at lower rpms.....so a mag check at idle will bear that out.

I do them at shut down.....it's more a maintenance thing than a flight performance thing.
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The CFI never actually had me to a mag check with the engine idled, he simply just had me pull the throttle to idle for a few seconds to make sure the engine still ran smoothly at that rpm.
 
If you've ever done a go around or touch and go only to have the engine sputter when you go from idle to full power, you'll appreciate the idle check after your run up to ensure the throttle rigging on your rented spam can is correct.
 
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wilkersk said:
If you've ever done a go around or touch and go only to have the engine QUIT when you go from idle to full power, you'll appreciate the idle check after your run up to ensure the throttle rigging on your rented spam can is correct.
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TIFT4U:yes:
 
wilkersk said:
If you've ever done a go around or touch and go only to have the engine sputter when you go from idle to full power, you'll appreciate the idle check after your run up to ensure the throttle rigging on your rented spam can is correct.
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This is extremely common with Stromberg carbs. They have no accelerator pump. When going from idle to full throttle, I has to be done over a few seconds and smoothly or it will sputter, especially when cold. It is when fuel delivery is being transferred from idle circuit to full flow circuit internally.


Jim R
Collierville, TN

N7155H--1946 Piper J-3 Cub
N3368K--1946 Globe GC-1B Swift
N4WJ--1994 Van's RV-4
 
My assumption was that if the engine so t idle and you pull it out while landing it removes the go around option or to be able to add any power to arrest decent. You always want to keep your options open!
 
DMD3. said:
it's best to do the idle check before running the engine up
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Doing the idle check before the runup when the engine probably is still cold almost guarantees that it will quit, at least in the winter in a 152.
 
I was taught to pull it the throttle all the way out after the run-up to make sure it didn't die on me ('74 172M). Made sense at the time, not sure the reasoning behind though. Thanks for the thread
 
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The purpose of the idle check, as I was taught, is to make sure the engine will continue to run at idle with the carb heat applied. Sometimes at idle with the carb heat applied an engine will die. Or stumble or something. So my routine on runup is to set the power at whatever runup RPM is recommended, check the mags, check the prop, check the engine instruments, apply carb heat to check for an RPM drop, pull power to idle to see that it still runs smoothly, turn carb heat off, resume normal idle RPM.
 
Although I've never heard of an idle check it would be a crude way to check the magneto system points for clean break operation. If the points are uneven and don't break cleanly, the magnetic energy of the rotor gets dissipated at the points rather than at the spark plugs.

From working in a MN auto repair garage 60 years ago, the boss noted that's why distributor points would fail on the first major cold snap in winter. The slower cranking speed (especially on those 6 V systems) weakened the spark if the points were bad.

Electronic ignition systems fixed that for more modern cars - but of course not for aircraft.:(
 
Palmpilot said:
That first document is 24 pages long. Is there anything in particular in it that you wanted to draw our attention to?
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I think it's the long list of 172 maintenance issues referencing an AD regarding the stability of mixture and throttle controls at idle. They need readjustment shortly after maintenance often enough to support a regular idle check.

DMD3. AKA NOT Palmpilot said:
I've actually been told that it's not good to run the engine up, then pull it all the way back to idle and that it's best to do the idle check before running the engine up.
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Sounds like absolute nonsense to me. You'll want your engine warmed up before doing the idle check and you're not going to damage the engine by idling for a couple seconds after the runup. Your biggest danger while idling full rich is of fouling the plugs, which isn't going to be made better by checking on a cold engine.

Simple things you can verify with the idle check:

  • Throttle continuity and smooth operation of the control.
  • Engine decelerates smoothly and keeps running at idle.
  • Idle RPM is within range so you won't be floating down the runway.
 
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FYI, I did NOT write your second quote.
 
Having flown a friend's C-172 A model that had the carb misadjusted and we did NOT do a proper idle check... And having the engine quit in the flare...

I think I'll keep checking.

Do as you like. I've had skin in that game... Once. Long ago. Not gonna invite it back. :)
 
denverpilot said:
Having flown a friend's C-172 A model that had the carb misadjusted and we did NOT do a proper idle check... And having the engine quit in the flare...

I think I'll keep checking.

Do as you like. I've had skin in that game... Once. Long ago. Not gonna invite it back. :)
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AMEN.

For Fuhrer Fuchs sake, there's a Donner Dam good reason for every step on the checklist and it is proven every year to be unwise to omit an item or trust your memory over the list.

While it is great to inquire why a check is on the list or how to perform a check, it's foolish to use that information to justify ignoring the item.

Edit: Just because you can't imagine any more modes of failure doesn't mean there aren't any more. If an item is on the list then its operation has been deemed critical for safe flight.
 
Very informative thread. A lot of Cessnas don't have an idle check on the checklist, although the Skycatchers I currently fly do. I will certainly make sure I check it.
 
Palmpilot said:
Very informative thread. A lot of Cessnas don't have an idle check on the checklist, although the Skycatchers I currently fly do. I will certainly make sure I check it.
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I also check it with carb heat on as a just-in-case.
 
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