Idling below 1000 RPM

If the carb heat is on AND the idle is set too low AND the idle mixture is set a little too rich the engine is likely to quit when you pull the throttle back to idle. That's why you were taught that way.
 
Richard Stefanick said:
Higher RPM is required on the ground to maintain the aircraft battery and prevent discharge. This rpm is called the critical rpm. My light for the alternator will come on below 1300rpm which for most aircraft I would assume to be about 1,000 rpm minimum. It is not all about engine as the above comments suggest.
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You must have a low amp rated alternator.
 
woodchucker said:
Come to think of it, in carbed 172s (and the 150 I just got checked out in) the runup checks carb heat by a slight decrease in rpm, then check engine idle with no carb heat. Seems to be more prudent to leave the carb heat on then check idle. Ideas on that?
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Yeah. I had a C177. It idled at what I thought was to slow. Mechanic said it was set to specs. I told him to warm the engine up and set it with the carb heat on. It seems to me that should be normal procedure
 
woodchucker said:
Come to think of it, in carbed 172s (and the 150 I just got checked out in) the runup checks carb heat by a slight decrease in rpm, then check engine idle with no carb heat. Seems to be more prudent to leave the carb heat on then check idle. Ideas on that?
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I was taught to turn on carb heat after the mag check (1700rpm in the 172 I trained in), observe the drop, turn it off, observe the rise, turn it on again, observe the drop, then pull the throttle to idle. Verify idle with carb heat on, turn it off, verify idle with carb heat off, then return to 1000rpm. Thus spoketh my musty crusty cranky old CFI.
 
The 1000 rpm idle minimum is to protect the overhead camshaft in Lycoming crankcases. Calculations can show that there would be little oil thrown that high at lower rpms.
 
I remember initial engine start (after overhaul) instructions that said to avoid speeds lower than 1800 rpm to prevent excessive cam pressure. I never thought much about it. My typical idle speed is about 1200 rpm. The only time I go lower is to verify the engine won't quit when I pull the throttle all the way back.
 
flyingcheesehead said:
I was just reading through the Cold Weather Operations section of Lycoming's Key Operations manual, and came across this on Page 50:



Wait, what?

Okay, so in a big truck, you avoid idling below about 750 RPM due to low oil pressure. Is this why you shouldn't idle aircraft engines below 1000 RPM? Or is there a different reason?

I also have seen notations in a couple of different POH's that say not to idle at 1000 RPM or greater until a certain oil temperature is reached.

So, which is it? And why?
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Well for float plane ops it’s normally under 1000, or full power till you’re on step.
 
Bill Jennings said:
Verify idle with carb heat on, turn it off, verify idle with carb heat off, then return to 1000rpm. Thus spoketh my musty crusty cranky old CFI.
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Same way I was taught. I mean.. if the engine can't run at idle with carb heat on because it's too rich do you really want to take it flying that day? I've safetied for people who are afraid to check the idle after run up "I don't know man, what if it quits?" <- really buddy? Do you really want to take off if you don't trust the engine to stay running under 1K RPM?
 
In my experience the Lycoming engines I've flown behind sound terrible at anything under 1K RPM... like they're about to shake themselves to death. Oil pressure *may* be part of the limitation, but you should be vigilant of the pressure gauge anyway and keeping it in a reasonable regime. I would bet it has more to do with vibrations and torsional stresses
 
In all my years of flying and mechanicking I never encountered any engine that suffered for idling at 650 RPM as per the manufacturer's instructions. Cold idle maybe should be a bit higher, but sticking with 1000 all the time is just rote behavior. If you ever set your idle so that it's at 1000 minimum, throttle closed, you will have really long landing flares and rollouts. And you'll burn out lots of brake linings and discs. Tires, too. They scrub away if you have to ride the brakes all the time while taxiing.
 
nrpetersen said:
The 1000 rpm idle minimum is to protect the overhead camshaft in Lycoming crankcases. Calculations can show that there would be little oil thrown that high at lower rpms.
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I was Director of Maintenance for a fleet of Lyc-poowered trainers in a flight school. The idles were set as per Lyc (600-700) and the camshafts never, ever gave us any trouble. Those engines ran to TBO and beyond without a hiccup. Lyc had some cam problems in the early '90s but it was a metallurgy problem, not an RPM problem. And I used Aeroshell 15W50 with the Lyc-recommended additive in it. And I was always on the students and instructors to stop riding the brakes during taxi. Bad habit.

Now Lycs are being built with roller lifters. Been doing that for ten years or more. Don't need much oil. There's also an aftermarket camshaft available that has a gallery drilled in it and outlet holes in the cam heels to lube the cam/lifter interface. The oil from the cam bearings is carried through that gallery and outlets. Don't know why Lyc didn't adopt that a long time ago.

When warm, there's lots of oil being flung off the crank at low idle. The con rods barely clear the camshaft; it's not as if the oil has to be thrown a long way. The centrifugal force at 600 RPM is far stronger than gravity.
 
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