when pigs fly

fly blind

To do by guesswork; to feel one's way.

fly by

To pass or go past quickly, often without much interaction

fly by the seat of one's pants

To pilot an aircraft without the aid of instruments and without a flight plan, using only instinct, visual observation, and practical judgment.

fly in the face of

To act in a manner highly contrary to; to counteract or contradict.

fly in the ointment

Something (especially a minor thing) which ruins or spoils everything else, or makes it less pleasant; a nuisance or problem; a disagreeable or unpleasant detail.

fly low

To have one's fly (zipper) undone.

fly off

To flee rapidly; to run away.

fly off at a tangent

To digress from a topic.