TopCat wrote:This is quite correct. 3000 feet is in fact a magic altitude, above which engines work differently.
And as everyone knows, it is impossible to recover from a stall in less than 3000 feet.
Since this is the student forum, can I just clarify that this is sarcarm.
Flyin'Dutch' wrote:The 3000ft must have a different origin as you correctly state that 65% can easily be exceeded with WOT at that altitude, also by other jalopies than the mighty Cheetah.
Perceived wisdom is that 75% power can be achieved up to around 8000ft density altitude, though it depends on the aeroplane. 75% air density is at about 9200-9500ft but an engine will produce 75% power on full throttle somewhere between 8000 and 8500ft generally.
In some POHs I believe Cessna state that you have to lean above 3000ft, so it's probably got something to do with the engine running too rich if you try and run it full rich above that. Some people have taken this to mean "never lean below 3000ft" which is incorrect logic.
I just saw something on a US forum where someone states, "Lycoming say never lean above 75% power." This is also wrong, it's just that above 75% power you have to be careful to do it properly, running sufficiently rich (or lean!) to cool the engine. 100 degrees F rich of peak EGT is where the perceived wisdom lies. This is also where LOP (Lean Of Peak EGT) wisdom also lies but that's another discussion.