Originally Posted by
alex90
At the risk of stating something obvious, and probably why a lot of organisations teach, and attempt to push people towards rich operations...
There was a historical "myth" that running LoP (lean of peak) would damage engines. This is probably why many POH of old planes often prohibit this. I have put myth in quotes, because, although LoP if done right and accurately, with accurate engine instrumentation has been proved to actually be better for the engine, than running ROP (or over-rich for that matter). Doing it wrong however, could cause both short and long term problems with the engine.
Most spam-cans (pa28/c172/152) that I flew during my training were often 40+ years old, with engine instrumentation somewhat basic and old. Temps being drawn from only one CHT/EGT probe. (whose accuracy I could only guess was roughly working as the needle was moving...) If anyone has ever flown with a digital engine management system with each cylinders displayed, they will know that there is a range of temperatures, especially in old(er) engines. So flying without each cylinders, would be leaving you with 3 unknown temps which may be causing damage without you having the faintest idea.
If you cannot accurately manage the engine temps, you do run the risk of making things worse for yourself. Running on the rich side, with the exception of fouling the plugs (rare above 1000rpm), you don't really have that much risk, and therefore a more cautious option for most of us flying crappy, old and tired planes.
If however, you do have a fancy digital engine management with proper calibration, and installation - you are better (so data tells me) running LoP, but this is often a continual adjustment to ensure proper operations. You can't get complacent about it - saying "oh I did that about 30 minutes ago in the cruise" because any slight difference (OAT, ALT, Pressure) will cause fluctuation and as you are running on the edge, you do need to manage it actively to ensure proper operation. (which is why I tend to prefer ROP operations - and enjoy the scenery a little more)
On a side note, a number of Mooney acclaim and ovations have been reported to have had shortened engine lifespan as a direct result of running over-lean, (so people tell each other) but I can only attribute that to poor engine management (despite their fancy kit).
Back to the main question however, leaning on the ground.... Do you ever remember your instructor telling you to maintain at least 1000rpm whilst taxying, and that you had to close the throttle, and use the brakes to slow down before returning to at least 1000rpm? Well that is so as to not foul the plugs at low rpm. An alternative to this, is to reduce the amount of excess fuel being fed to the engine, hence the leaning on the ground. The main reason why instructors that I know are not so keen on teaching to lean on the ground is because if you are a newly qualified PPL, with 3 friends in the plane, all excited and distracting you, the likelihood of you forgetting something is increased, and they believe (quite rightly in my mind) that forgetting the mixture would not be a great idea on takeoff!
With regards to being on final, I was always taught to go full rich as part of my downwind checks. I don't think it is a good idea to leave it till the go-around, or that moment when you suddenly need full power during final due to severe downdrafts / windshear, because chances are you'll need it full rich, and not have the power you need, when you need it, as a result.
Restrictions on running LoP were not so much a "myth" as a certain limitation to specific types, I might be wrong but I think this was specifically related to higher-powered engines in Ye Olde Worlde Days when lower-octane fuel was in common usage. Higher-powered engines are more sensitive to detonation and for correct operation higher-octane fuels are a necessity. Just why we've ended up with having to run 180hp 4-cylinder engines on 100-octane fuel that requires toxic additive is a mystery to me (like many things in aviation, "Cos that's the way we've always done it") however we have what we have. Running LoP on certain engines or general cack-handed operation of the mixture control was a sure-fire way of inducing detonation which causes catastrophic engine damage in the medium to long term no mattery what type. Hence we ended up with people who were terrified of attempting to run LoP or even trying to lean the mixture at all.
Most engines in common GA aircraft today will quite happily run LoP (if done correctly in accordance with the POH and manufacturer's data, as alluded to in the previous posts with) without any detriment to the engine operation or lifespan if done correctly.
Often in horizontally-opposed engines different cylinders will run slightly leaner or richer than the others (and at a different temp), so unless you have a fancy engine analyser leaning can often be a bit of a compromise as one or two cylinders may start rough running as they're running a bit leaner than the other cylinders.
P.S. I was always taught that closing the throttle completely on taxiing before applying brakes was to save wear on the brakes, as even at 1000rpm there's enough thrust there to work against the brakes - saves Mx costs!