eg. in SYD we have a stand that support cat A~C and a C jet, why cant we just buy the stand with cat C but have to buy from A through C? that’s costly, especially for beginners 
That’s how the game mechanism works…
Try to sell some planes at hangar and you will have more money to go with.
Because an A or B aircraft is smaller than a C aircraft. So when you own a C stand it physically allows smaller planes to park there too.
You cannot build a C stand that cannot hold an A or B aircraft.
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