Thanks for the replies, guys.
These are defined spawns in single mission mode. I became aware of the problem when I tried missions that were working fine last week (same spawn files, same mission files) and no air targets appeared. I have tried numerous spawns in numerous mission files, each one multiple times, and cannot get air spawns to appear. Ground vehicles appear normally. I have not knowingly changed any settings. I have tried a computer re-start. Air spawns DO continue to appear normally in PTO.