Airline Club

Instead of upgrading to level 10, try to improve your quality a bit, either by nudging up service quality or by increasing the autorenew %.

For bases, the slot formula is basically (50 + 50 per level) * (average flight quality of all routes from that base, passenger-weighted). So anything that improves quality will give you slots, potentially a LOT of slots for a high-level base.

The three ways to increase quality: star rating of the specific route, service funding, and airplane condition.

Star Rating

Always have this at 5 stars. Just do it, I promise you, this is by far the cheapest way to increase quality. At 5 stars you are getting 30 route quality base.

Airplane Condition

This is a little complicated. Airplane condition's contribution to quality runs from 0 (at 0% condition) to 20 (at 100% condition). Your average condition is (100 - autorenew) / 2 %, i.e. 70% at a 40% autorenew. So every 10% you increase autorenew adds 5% to your average condition and adds 1 to your average quality. Note: when I say "average" I mean it, your quality will actually continuously decline and then jump up on any given route when the plane is renewed, and this means slots will appear and vanish over time.

The cost of a 10% increase in autorenew % is based on how much extra "adjusted" depreciation you will pay. The idea of "adjusted" depreciation is that if you are renewing at, say, 40%, then you are taking a 20% * 40% = 8% loss on the purchase price of the plane when you renew. If we re-amortize that 8% over the owned lifetime of the plane (i.e from condition = 100 to condition = 40), then that represents a 13.33% addition to the straight-line depreciation.

If you increase that autorenew to 50%, the addition is 20% of the straight-line deprecation. So the cost of increasing autorenew from 40% to 50% is equal to 6.67% of the straight-line depreciation listed on the income statement.

At 60% autorenew the adjustment is 30% of straight-line depreciation, which means the cost of increasing from 50% to 60% is 10% of the income statement's depreciation line item. i.e. the cost goes up as the autorenew goes higher. (At 100%, obviously, the cost would be infinite as you'd be constantly renewing every plane.)

Increasing airplane condition also provides some rewards in the form of reduced delays, although I won't really go into the formula for evaluating that here.

Service Funding

Every point of global service quality adds 0.5 points of quality to your individual flights. The cost of service funding is based purely on total passenger volume, not adjusted for mileage or anything. The cost is also quadratic. At service funding of $50 per passenger you will have a service quality of 50; at a funding of $200 per passenger you will have a service quality of 100. Note that this means that at a service quality of 100, you should be mentally adjusting your profits on any given route down by $200 per passenger which is almost never worth it unless the only routes you run are big moneymaking international routes. (And at today's margins, probably not even then.)