Airbases can (and do) draw resources during any player turn on demand from any city, they are not restricted to one (like it happens in the supply phase for resupply). The same happens when moving units need to draw fuel (infantry units can still get full MP even with fuel shortages; they later draw that fuel on demand, so at least there is no movement "for free").
Super, thank you, also good to know about infantry.
For railheads and supply depots, I know. It's just that we're used to think about distance to railhead when calculating supply to an HQ and we used to think it better to position airbases on railheads as much as possible. But now my understanding is that it doesn't really matter anymore if an airbase is on a railhead or not, since the distance to the city depot is what really matters.
"The closer you get to the meaning, the sooner you'll know that you're dreamin'" -Dio