id say the route's of that plane will not be availabel for the duration of the maintenance. lets say 3 maintenance would be avail, small, medium, big.
so if a plane has to get a small maint no problem routes wil be out for lets say 2 days
a big maint would last for a week if you have a plane of the same kind you could put this on the route.
if no plane is avail you could charter a plane from a other company and pay for it.
something like this.