Urban games didn't included air freights, probably for a reason.
1. Make sure you buy freight planes, not passenger planes.
2. Make sure the catchment area can reach the freight airport's terminals (the best is to place a station as close as possible to the airport, deliver from/to that station).
3. Make sure there is no competition of the cargo delivery at the destination. The game is calculating the fastest and cheapest route.

Start small, and build up slowly.

Okay thanks for help. I will check for ad.3. I have a loop here Air+train+Land so this is probably the reason