How do you increase your credit rating in this game?
Age of airline, revenue and company value... increase all of them and your rating increases. Negative vs. positive cash can also have an impact, but it's more of a demotion that's available for being negative cash than it does to increase one's rating.
Have large profits, high CI and no loans.....in other words like in real life prove you don't need the money.
Loans do not appear to affect one's credit rating unless it puts your CV below the next threshold.
Time, CI, and don't miss a payment, I have a 341mill loan in a world and have an A rating.
Missing a payment does not impact your credit rating in the game. However, if you miss a payment, that means your cash is negative. Once your cash goes back to positive, then you'll bounce right back to where you were (eg AAA).