I’m playing on the Steam version on PC. So I was trying to go through and get some achievements I didn’t get on my first playthrough of Reach, and this is one of them. It’s only this achievement that won’t unlock. Other achievements in the same level worked fine, including the Club Errera-related ones, the pelican and phantom, and the par time/score achievements. It’s just “I Didn’t Train To Be A Pilot” that won’t unlock.
I did google the problem and found some other posts about this same achievement, and the generally-accepted solution seems to be to delete your saves and disable cloud saves, and then destroy the 3 turrets. However, I tried this, and it still doesn’t work. I’ve tried on normal and legendary with no success.