【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.