After cash reaches $5 million,
1. Lease used aircraft until $15-20 million
2. Lease new aircraft until $25-30 million
3. Buy used aircraft until $150-200 million
4. Buy new aircraft
I belive i have readed this in the AE tutorial in the wiki?
I do believe I was the one who wrote that, years ago (back when I was AeroKid, or possibly Aeroplaneboy. I am bad at usernames). Haha. Well, now that I have started playing again, it's proved useful (although temporary inactivity lead to two airlines going bust even when they seemed to be doing so well financially).
Tip: Never leave your airline too long without checking on it; first task should normally be to check to see if the competition has lowered their prices, decreasing your loadfactor. This can get longer and more tedious the more your airline grows, but also slightly less relevant. Never operate a route at a loss (although I know that that has been said a lot)