To lean or NOT to lean?

[Chimbu chuckles]

Yes they are WRONG...because they tell you to operate the engine in such a way that, if you're above about 65% power, you WILL heat damage the cylinders. Over time the cylinders oval because of repeated high temps, the pistons don't so you get increased cylinder wear and low compression leading to top overhaul.

Interestingly enough years ago, way before the current technology was available, we used to get to TBO too....mostly because at the extremely high DAs we were operating at we were operating them correctly by fluke...combined with flying them hard all day every day....by pure luck we were at or below 65% pwr more often than not because of operating at high DAs...at 65% pwr and below you can do whatever you want with the mixture and you will never see high enough temps to do damage.

The original fit triplex gauge on my Bonanza is actually reading one of the lower CHTs so if that was all I had and I operated as per the POH 3 of my cylinders could be continously over 400F and if I was operating with the one I have info on around 380-400F, which is in the green band therefore 'OK', 3 of my cylinders would be, unknown to me, up around 450F...which is still in the green band.

You might say so what...it's in the green band....under 460F which is the limitation as per the POH...but if you look at Deakins article on Mixture you will see a graph from Pratt & Whitney that shows quite clearly that at temps much over 400F the cylinder metal is permanently weakened...and there we are, right back to top overhauls again.

There is a reason why, over many years, owners and 'experienced' pilots have grown wary of the red knob...because they followed recommended procedures, for the most part, and engines typically needed top overhauls around 500-800hrs before TBO....more often than not.

Typically the compressions of two or three cylinders drop and owners make the mistake of changing ALL the cylinders...reasoning that if those 2 or 3 are stuffed the others can't be far behind...WRONG! They most likely are the cooler running cylinders and have not been exposed to temps over 400F....I would hate to think how many cylinders have been changed for no good reason.

I cannot think of a single Ops manual/POH in ANY of the piston engine aircraft I have flown over 3500 hrs sitting between or behind 300hp/6 cylinder Continentals and Lycomings that does NOT recommend leaning at 75% power and below to peak EGT for best economy or 50 ROP for best power...that ENTIRE recommended mixture range is deadly for cylinders at power settings much over 65%.

That list includes every Cessna and Piper SE aircraft in production post 1970, Helio Couriers, Pitts, Bonanza, Baron,Bn2,Aerostar, Queenairs, C310, 402, 404, Twin Commanche, Partenavia...and a few more I can't remember.