When a plane is in for B,C or D check the impact on its routes is nil that you need to adjust for, sure you will lose the revenue earning capability but you can leave everything else in an 'unattended' mode.
In other words -
- route CI is not impacted
- slots are not affected (e,g, no unused slot violations)
Minor pedant: CI is not affected, you don't get a penalty for missed flights. RI may be affected, if your only plane flying that route, or all your planes flying that route, are in C-check, there can be 0 flights for the week, which means RI will drop slightly.
That said, just ignore C-checks. You can manually do them early to stagger them a bit better, not get a 20 C-check lump sum payment in a single week that you'd forgotten was coming. But trying to cover routes by moving schedules between planes, and trying to set up C-checks to the point you've constantly got 1 of your 40 737s in C-check, is a lot of tedium for almost no benefit.
Until maintenance staff lose the ability to put in their 40 hours for the week by doing 8 simultaneous A-checks on a Sunday and then taking the rest of the week off, you can simply set & forget maintenance.