Mixture & Pitch axis issues.

[bongodriver]

I don't know if this is accurate or not... But in il-2 sturmovik when you want to boom and zoom, read:surprise thine enemy from above, its wise to set your mix lean on high altitudes. otherwise the enemy could see you from miles away because you were spewing a nasty dark smoke trail. I am not talking about contrails at even higher altitudes.

It is true that mixture becomes richer at high altitudes (logically because the air becomes thinner) therefore a higher fuel to air ratio, but some aircraft had an automatic compensation system which meant the mixture was left in the rich setting.