It's a bird. It's a plane. It's even a pterodactyl this Saturday at the Andrews Air Force Base Joint Services Open House, where all branches of the military put on their annual air show. New to this year's program is a $700,000 replica of a pterodactyl which the Air and Space Museum will send into the air for a five-minute flight beginning at 9. The mechanical creature has an 18-foot wingspan and will retire to the museum after the event. 0fficial entertainment begins at 10, when you can see a parade, a helicopter demonstration and a fly-over by F-4s. You can also see the Blue Angels, an F-16 and other fighter planes in the air.Paratrooping exhibitions cap off the show with the 82nd Airborne Division and the Golden Knights performing free-fall and mass paradrops. On the ground, there'll be military working dogs, bands, and planes you can wander past, including the B-1 and the SR-71. Gates are open from 8:30 to 3:30; admission is free. To get there, take Beltway Exit 9 and follow signs to base or take the Suitland Parkway from Washington. 568-5995 or 981-4511.

If you want to get primed for the air show, you can hook up with "Threshold: The Blue Angel Experience," the first movie in the American Film Institute's seven-week series, A 21-Film Salute to the 75th Year of Naval Aviation. The film is Friday at 6:30. Among the other films in the series are "Task Force" with Gary Cooper and "Hell Divers," starring Clark Gable. Admission is $4. 828-4040.