This is a recurring problem that needs to be sorted.
Slots need to either be increased or player numbers need to be decreased. If you want to fly to Heathrow, CDG, Schipol, Narita, JFK, Frankfurt and those type of airports after the first year its impossible. It may not be realistic to increase the slots available but it is also not realistic for 500 airlines to appear on the same day and all form by snapping up a huge variety of small aircraft and fly them from Heathrow. I'm pretty sure no airline took up a massive proportion the slots at one airport with over 50 30 seat aircraft flying them god knows how many times a day on short high demand routes.
I feel this game needs to find the happy medium between realism and playability. At the moment AWS seems too close to realism at the detriment of playability. It's worse in situations like this happen where a realistic part crashes head on into a game factor and all hell breaks loose.