in flight—
1. while flying:
- Gliders are now fitted with a means of cleaning off the leading edges of the wings in flight.
2. running away; fleeing:
- The two men are hiding in a hotel apparently in flight from the police.
on the fly—
1. = in flight 1:
- Peregrine falcons are predators and their prey consists mainly of birds, caught on the fly.
2. in motion; moving fast:
- She is constantly on the fly as she directs, plays piano or walks through the rows of children.
3. do smth. quickly or in passing (while busy with other things):
- Changes must be made on the fly, as the program progresses.
4. (sl.) having a lively, merry time; on the spree:
- My father had been on the fly in that town for nine or ten days.