Negative cash is interest free currently. Not really a bug of any sort, just not modelled.
Not the highest priority in general, but I am seeing a lot of players (including me) go into red to obtain slots. The system lets them - I guess for playability reasons. Should the system let players buy up slots while their cash is negative? That's a separate subject.
As far as 2 scenarios interest:
1. negative cash, loan payment is due (Monday).
a) printipal: works fine. Prinicipal payment skipped, loan balance not decreased.
b) interest: interest and fees should be charged, cash balance should be updated (to go deeper into negative)
2. negative cash, end of each day
Interest should be charged. To make it simple, rate should be equal to D-credit rated airline. So interest expense field should be increased, cash balance should be decreased (to go deeper into negative)