First Paradox need to give us nice and working tools and basement for game, no bugs, no dissapearing units in naval transports, no paratroops that are starving in air transports because they cannot leave them until organisation of airplanes reach maximum, no minister's skills that works even after minister is off (air to ground and others...) :rofl: , and many many many others that are still in this game after beeing hundreds of tousends time pointed out!
If basement rules/tools will work than they can think about AI, ok.
So according to the topic, please do it like this:
one airtransport with strenght 100 should have possibility to supply easly two/three divisions (or even more) making one flight per day (we all know that it's possible to make more than one flight per 24hours - player can try to supplie more units but it's gamble and risky). It should looks as an option, "supply unit" added to existing naval/tactical... bombing, and supply stock should appear in sourrunded area increasing if there will be more supplies that needed. Such units musn't reinforce as it's obious, only extra paratroops could be added (by paradrop) to support sourrunded units.
If enemy will have airsuperiority, it means that airbridge will be impossible to be done - you will lose all your transoprts

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Another example of supplying units by airbridge: North Africa in 1943 almost all supplies for German units was made by airplanes Ju52 and Me 232 Gigant. So please don't write me that is impossible!
Cheers,
Orthank