Setting high idle vs leaning
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Setting high idle vs leaning
Throughout the UK we are taught (everywhere it seems, and as seen on generic type checklists from pilot shops) to idle our conti/lyco engines at 1200rpm, the thinking behind this is to reduce the chance of spark plug fouling. These engines idle smoothly enough at 800-1000, and it's quieter, less wear and tear etc, so why don't we do that? I trained at a school in the U.S. on PA28s, and that's what they did. After start checks mix leaned, and set idle 900rpm. After landing, mix leaned.
So the question is, does leaning alone give the same results and stop fouling? And if so, why don't we teach leaning of the mixture across the board instead of this idling high tactic, and doing things like running at 1500rpm for 30 seconds before shutting down to clear the plugs?
Is the problem an excess of fuel, or a cool engine? Or a bit of both?! Is there also an argument that it's easier teaching people in the UK to leave the red lever alone as they don't often go above 3000' during the whole flight and that it's just an on/off lever?! (Not how I do it, but daft students, lowest common denominator and all that)
Would be interested to hear some sensible answers based on experience, and/or reference to POHs and engine operating manuals.
So the question is, does leaning alone give the same results and stop fouling? And if so, why don't we teach leaning of the mixture across the board instead of this idling high tactic, and doing things like running at 1500rpm for 30 seconds before shutting down to clear the plugs?
Is the problem an excess of fuel, or a cool engine? Or a bit of both?! Is there also an argument that it's easier teaching people in the UK to leave the red lever alone as they don't often go above 3000' during the whole flight and that it's just an on/off lever?! (Not how I do it, but daft students, lowest common denominator and all that)
Would be interested to hear some sensible answers based on experience, and/or reference to POHs and engine operating manuals.
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I think a lot of instruction time you are varying power a lot so many find it easier to just leave the mixture in rich, personally I teach leaning as part of exercise 4, it then largely gets forgotten about until moving onto nav when it again comes in, though I do try to include it in earlier lessons when it fits in.
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The problem is, does lean for taxi going to cook the cylinders?
Does 1200 provide enough cooling when stationary?
I think it's the Piper PA28 manual says something like 'don't idle for more than 2 minutes summer or 4 minutes winter'.
Why, will the engine overheat.
Also no requirement to see oil temp rise into green.
If throttle responds without hesitation, take off.
Unlike some checklists that say t & p's green, well if the temp. isn't, going to sit there for 10 minutes warming engine.
Plug fouling more likely due all the rubbish recon engines burning oil.
Does 1200 provide enough cooling when stationary?
I think it's the Piper PA28 manual says something like 'don't idle for more than 2 minutes summer or 4 minutes winter'.
Why, will the engine overheat.
Also no requirement to see oil temp rise into green.
If throttle responds without hesitation, take off.
Unlike some checklists that say t & p's green, well if the temp. isn't, going to sit there for 10 minutes warming engine.
Plug fouling more likely due all the rubbish recon engines burning oil.
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Rather than copy and paste the paragraph it might be more useful to have a look at the entire Lycoming Key Reprints - Operations. There's loads of good stuff in there:
http://www.lycoming.com/Portals/0/te...Operations.pdf
There's a bit on page 66 which explains why 
ifitaint...
http://www.lycoming.com/Portals/0/te...Operations.pdf
Throughout the UK we are taught (everywhere it seems, and as seen on generic type checklists from pilot shops) to idle our conti/lyco engines at 1200rpm, the thinking behind this is to reduce the chance of spark plug fouling. These engines idle smoothly enough at 800-1000, and it's quieter, less wear and tear etc, so why don't we do that?
ifitaint...
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On the tommies, i was told to follow the checklist which is leave the engine running at 1200rpm, check the mags, close the throttle and then mixture ico in shutdown. But, my fic instructor did it his own way by adjusting the mixture and throttle levers! my guess it would takes some experience to understand sparkplug fouling and if not done correctly then fouling can occur. So best bet is follow the checklist and is simple to understand.
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leaning
We operated a fleet of single engine (lycoming) aircraft for many years. We started operations in 1988 and initially left the choice of leaning to the pilots. After a few years, we found several cracked cylinder heads, probably due to leaning and forgetting to readjust during the descent. I decided then that the extra fuel burn was less expensive than replacement heads so instructed pilots to operate in full rich throughout flights. We had no more cylinder head cracks. We did not suffer from plug fouling - plugs were cleaned and checked at 50 hours. I must admit that I did lean on longer flights where fuel economy became important.
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Strange there seemed to be a lot of broken cylinder heads and dropped valves in the late eighties and early nineties.
Could have been a bad batch out there.
Is it less common these days?
Now seeing use of multigrade oils and seeing some high oil temp's and low oil pressures during normal ops.
Not sure if these oils have the capacity to take away the heat efficiently or perhaps flowing through the cooler too quick to be cooled?
When hot these oils have the consistency of water.
Could have been a bad batch out there.
Is it less common these days?
Now seeing use of multigrade oils and seeing some high oil temp's and low oil pressures during normal ops.
Not sure if these oils have the capacity to take away the heat efficiently or perhaps flowing through the cooler too quick to be cooled?
When hot these oils have the consistency of water.
Last edited by BigEndBob; 8th Jun 2015 at 07:17.
