The idea behind the "airbase disallowed" rule was that one got way more benefits from an airbase than Sid intended. I.e., an airbase gives roads/railroads, farmland as well as an actually airbase, and, as the airbase magic table shows, one gets food/shields from some terreign types that are more than intended. The rule is not intended to ban airbases, completely, just to stop players from using them to: 1) do stuff not normally available, like irrigating a mountain, and 2) building an airbase instead of RR, irrigation or mining a hill. Its much quicker to just build an airbase rather than spending the engineer time to do the usual work. You can build any airbases you want, as long as they are used as a legit airbase to project airpower. Putting airbases on top of every mtn just to get an extra food (as well as a free RR), is not allowed. Likewise, mining a hill, than putting in an airbase just to get the extra food is not allowed. IIRC, the purpose of the rule was to "disallow" the extras that were available according to the "airbase magic table". That said, unless one is trying to get a high score landing game ending in 2020, the airbase trick is not of much use in regular game play.