....and don't use more than three fleet types, that is number two.
This one is especially tricky because it's usually a very slow killer. Well, when you have 630 birds in operation like me, going to 4 fleet groups is an instant suicide. When you are at 200 birds, it's just annoying. So many players are tempted to say "it's not that big". And go up to 300, 400 birds. And begin to suffer more and more. And then comes an event that they should have survived(volcano, fule price heavy raise), and they are in great difficulty, despite a clever management on all other points.
4th fleet tupe can be used in specific moment, when you are still small, and need to grow fast. In the late 90s, it happened to me to take a few fleet types in extra to help my growth, and to take the place while it's free. But the right thing to do is to disengage quickly. Because it costs money. But for a few months, waiting for the "true" airplanes, in the beginning, it may be worth it.
It's dangerous. I tend to avoid as much as possible. But it's soooo tempting at the same time, your Ejets are not yet here, some BAC 1-11 are still available, and opposition is growing quickly where you are not yet.....