Build more than two airports per city.
Configuration file: moreairports
Command line: -J
Normally, you can only build two airports within a city's area of influence. Lobbying by powerful Transport Tycoons has led the politicians to allow this option, though. However, the population is still upset by all the noise so you cannot build an unlimited number of airports, even with this option.
So how many can you build then? The basic idea is the concept of "noise units". In total, you are allowed to have up to nine noise units per city. And here's how many noise units the different types of airports use up:
|Type||Noise units||Maximum number per city|
Of course you can also combine the types, and just add up the noise units. For example, you can now build one large airport, two small ones, and still two heliports, because that adds up to exactly nine noise units.