Originally Posted by
englishal
Should be "above" 75% power.
Remember too that it may not be possible to achieve 75% rated power at altitude in a normally aspirated aeroplane so therefore you may be better off leaving the engine leaned while climbing at altitude. Some high altitude airports (not in the UK
) and you will need to lean on the ground before take off.
You are of course correct as I neglected the above 75% qualifier. Leaning at 75% power is permitted. You point is also well taken with respect to your comment about the engine my not being able to make 75% at altitude even at full throttle and thus could be leaned. This points to the requirement to know what power the engine is actually making at any time which will require consulting the POH and understanding what it is telling you.
There is one caveat to the above though. Overly rich mixtures will help reduce cylinder head temperatures therefor it is usually advisable to leave the mixture full rich on long climbs on hot day especially if the aircraft is not equiped with a cylinder head temperature guage.
Bottom line though is the mixture should always be leaned in cruise flight.