Obtain a fighter jet from Ford Zancudo to unlock the ability to complete all 15 Knife Flight aerial challenges with it. You can also use a stunt airplane (they occasionally land at the airport in the desert). Visit the Flight School at Los Santos Airport to learn how Knife Flights work, and maxxing out the "Flying" attribute for one of your characters to have more control over the aircraft is recommended. If your aircraft gets badly damaged, quick save the game, and immediately reload the game to repair it. It is also recommended you quick save the game before attempting the more challenging Knife Flight stunts.
View the following video to see the location of every Knife Flight challenge: