The Aviation Club is in the Level 5 Unique Buildings, which becomes available at the BIG TOWN (6) milestone.
The requirements to unlock are to have a population of 10,000 and "Construct an Airport".
The first airports available are the "Airport" and "Cargo Airport", which unlock at the "METROPOLIS" (12) milestone.
Followed later by "International Airport", "Cargo Airport Hub" and "Metropolitan Airport", which unlock at the "MEGALOPOLIS" (13) milestone.
You'd think a small airport like the Aviation Club could be available in a smaller area that doesn't even have an airport.
The requirements to unlock are to have a population of 10,000 and "Construct an Airport".
The first airports available are the "Airport" and "Cargo Airport", which unlock at the "METROPOLIS" (12) milestone.
Followed later by "International Airport", "Cargo Airport Hub" and "Metropolitan Airport", which unlock at the "MEGALOPOLIS" (13) milestone.
You'd think a small airport like the Aviation Club could be available in a smaller area that doesn't even have an airport.