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This question is more about the indication of engine roughness. The procedure for leaning without an EGT is clear in the POH and Lycoming documentation which basically states as follows:

Best power:

  1. lean until "engine roughness is noted."
  2. Move mixture forward approx. 1/2 inch toward full rich.

Best Economy (75% power or less):

  1. lean until "engine roughness is noted."
  2. Enrich mixture until engine runs smoothly.

My question is what is engine roughness as defined by Lycoming in this situation?

I like to lean it until engine is clearly sputtering (no question its running rough), before I enrich, but most CFIs I've flown with lately seem to "detect" roughness with some magical senses, I think they are just going with their gut as opposed to empirical indications such as drop in MP or objectively detectable drop in engine performance.

My understanding of "engine roughness noted" would be a clear audible roughness of the engine being leaned, along with a slight, yet detectable drop in manifold pressure of said engine.

Engine Roughness is not a defined term in the POH or Lycoming documents I've read. So in my opinion it should be referring to a clearly objective condition that any person can easily identify.

I'm hoping someone can confirm or provide some objective guidance on how to identify "engine roughness" when leaning an engine on a twin with constant speed props.

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  • $\begingroup$ I would not lean to "sputtering", you are asking for in-flight failure (as in engine stops, maybe not damage) there. You should be able to hear/feel the engine getting rough. When I do this, I can notice the increased vibrations and differing engine tone. $\endgroup$ – Ron Beyer Aug 29 '18 at 19:53
  • $\begingroup$ CFIs do it (and much more full cut-off) all the time in multi-engine aircraft to simulate engine failure and to actually shut down the engine. Although I can appreciate what you're saying. $\endgroup$ – Devil07 Aug 29 '18 at 20:49
  • $\begingroup$ I've never had a CFI pull the mixture knob out to simulate engine failure, I'd probably have a long talk with the CFI and not in a good way. Yes, you pull the mixture knob to stop the engine, but you don't want to get so close to shut down in flight that your engine is sputtering. $\endgroup$ – Ron Beyer Aug 29 '18 at 20:54
  • $\begingroup$ In a twin I find it difficult to know for sure what I'm "hearing/feeling" is coming from the engine that it is supposed to be coming from, plus the twin I've been flying recently has a bit of a wandering RPM that continuously goes in and out of sync. At cruise altitudes (<75% power), there isn't much damage you can do by leaning momentarily until engine runs rough and cough a couple times. $\endgroup$ – Devil07 Aug 29 '18 at 20:55
  • $\begingroup$ @RonBeyer interesting, I'm going to have to go back and review that. Thanks for the info. I'm thinking back and I think they have always pulled mixture to simulate engine failure on me. How do CFI's simulate engine failures where you are? $\endgroup$ – Devil07 Aug 29 '18 at 20:57
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There is a big difference between "by feel" leaning of a carburated vs fuel injected engine. Carburated engines have less even fuel distribution and when leaning, one cylinder, the already leanest one, will start to misfire quite a bit before the others. So with a carb it's easy to detect one cylinder starting to misfire (the cylinder usually doesn't totally quit right away - you will pick up random misses in the vibration rhythm).

With a fuel injected engine the mixture distribution is more even and misfire of one cylinder will be accompanied by misfire of its sisters almost immediately. So on an injected engine you will lean and suddenly the engine will really start to stumble, whereas with a carburated engine there will just be these random dropouts in the engine's "beat", getting worse as you continue leaning.

So if you are leaning on an aircraft with fuel injection it is really fairly straightforward. The engine will go from smooth to pretty rough right away, whereas with a carb it takes careful attention to detect the early onset of misfire. You are at a disadvantage to some degree because the airplane is less familiar than to the instructors. It's a skill you will pick up over time as you develop at ear for it.

Don't worry too much about it. Err on the side of smoothness, knowing that the difference in fuel consumption between perfect and good enough is negligible (and if you're paying hourly on a hobbs meter, there is no benefit to you anyway), and it's wise to err on the side that is beneficial to the engine (too rich).

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  • $\begingroup$ Good point about carburated engines, they'll complain a lot before shutting down. $\endgroup$ – Devil07 Aug 29 '18 at 21:20

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