What would be a technical or slang term for 'in the air'?
She kept the Squadron Leader updated as much as she needed to, but she
preferred to keep comms chatter to a minimum when she was...

Military-related terms:

...on patrol
...on station
...on a sortie
...on a shout [British English, specifically used for search and rescue]
...on recce/on recon [British and American English, respectively]

General terms:

...in the air
...airborne
...at the controls
...flying
...in flight
...on a mission
...doing (her) rounds
...aloft
...wheels up

There might be other, more technical terms in use in the military for specific operations or missions but I have no experience there.