Minor point, and fyi to noobs (and probably old news to any old time player I suspect)...
The message when an aircraft is in maintenance (C-check being a rather long one) reads as follows:
"Aircraft is currently in C check maintenance. It will be ready on (date here)"
This is confusing as "ready on", I read to mean "ready to fly on", as isn't that the purpose of being "ready" for? Turns out it is ready to fly on the next day.
This makes a small difference if you happen to have another aircraft you are temporarily assigning the routes to...looks like I lost a day's revenue on those routes after assigning them back.