fly-by vs flypast

fly-by and flypast both are nouns.

Word	Noun	Adjective	Verb	Adverb
fly-by	Yes	No	No	No
flypast	Yes	No	No	No

As nouns, fly-by and flypast are synonyms defined as:

fly-by and flypast: a flight at a low altitude (usually of military aircraft) over spectators on the ground

Other synonyms of fly-by include flyover.

fly-by (noun)	flypast (noun)
a flight at a low altitude (usually of military aircraft) over spectators on the ground	a flight at a low altitude (usually of military aircraft) over spectators on the ground