At the weekend, I was practising circuits with my instructor. I noticed that at some point after take-off, he'd leaned the mixture without me even noticing. I asked why, but the response was 'I'll tell you later when you're not quite so busy.' Fair enough, but naturally by the time we were back on the ground, I'd forgotten all about it. This thread has reminded me about it, but no doubt by the time I go flying again I'll have forgotten. So, what do people think? Speculation and reasons why you'd lean the mixture in the circuit...