Going back to an employer you chose to leave is seldom wise - they rarely change so much as to be worth a second chance, and if you were unhappy once, you'll be unhappier still second time around. The only appropriate time to go back, cap in hand, is if in desperation as a short-term solution to keep a roof over your head, but airlines recognise this is the case and that is why they only welcome pilots back when they too are desperate.