I have just purchased 10 new aircraft and scheduling them takes ages, how do some airlines with 500 aircraft do it so quickly, is there a secret tool, or is it just something that takes a while and each one has to be done manually?
Once you schedule a ton you just get fast. There are tons of tiny little tricks that add up do very fast scheduling. In general, 7 long haul planes takes around 5-7mins for me, and 7 short haul planes takes around 15-20m
Airlines with 500 aircraft didn't schedule them all at once. It takes several game years to acquire that many planes. You schedule them as you receive them.
As already said, it takes time to gain the experience to do it efficiently.
The main thing is to plan ahead. At the start of the game or when I open a new base, I try to put two or three hours aside to make a spreadsheet of ALL possible routes and how long they take on the aircraft type(s) I might use. For this you really need a good desktop PC or laptop where you can have 20 or 30 tabs open at once. I have NEVER been able to schedule on my phone!
Once you have that, you can quite quickly pull together routes that would work together by sorting and searching the spreadsheet. I even wrote a couple of little Excel Macros that highlight groups of two or three routes that fill up exactly 24 hours and one that helps to choose a 7-day schedules quickly. You don't even have to worry too much about keeping the spreadsheet up-to-date though the game as demand, mostly, changes linearly. IT gets a bit messy to keep track of sometimes but works well in that important first year or so.
I create and update the spreadsheet manually as it's not allowed to run automatic scripts against the game itself (and I don't think I know how to anyway). Before anyone asks, no, you can't have a copy of my Macro. Hehe.