When aircraft are assigned to a city, are they "garrisoned" there? If so, are they covered by the "garrisoned units have no upkeep" bonus from the Tradition culture tree?
My (apparently-dashed) hope was that Triplanes/Fighters/Jet Fighters could be set to intercept and would be upkeepless, but that apparently isn't the case.
The short answer is no. You can't.
Source: look at Thresian's answer. To question number 2.
Aircraft (and Missiles) are dissimilar to land and naval units, and they don't count as units for purposes of either garrisons or the "One Unit per Tile" rule.
As far as I know, only Land units can Garrison cities. If you are looking for a cheap garrison unit to benefit from Tradition, I'd suggest scouts. Beyond that, every garrisoned unit will provide identical benefits.
