Fleet Modernization

I typically start new airlines with extremely old planes. Does anyone have a good rule of thumb of when to pause/slow expansion in exchange for new(er) airframes?

Thanks in advance!

So, I haven’t used this way of doing things in a while. When I have it’s been to expand quickly, so I usually change things, when I’m getting prepared for long haul. I want the best ratings on my long haul flights. Once I have 1500 to 2000 daily out of my hub, I’ll start the process. Others may have other opinions, but that’s what has worked for me. Good luck.