From Wiki :

In some missions, extra AI aircraft are included but aren’t available to the player in Free Flight. These aircraft include:
TBM Avenger ghost planes of Flight 19 – from Lost in the Triangle
Boeing 787-9 – from Paris Airshow Demonstration Flight, Loopy Larry & Jet Truck Drag Race
Ekranoplan – from Aleutian Cargo Run & Tokyo Executive Transport
Airbus A380-800 – from Paris Airshow Demonstration Flight, Loopy Larry & Jet Truck Drag Race
Boeing 747-8 – from Loopy Larry & Jet Truck Drag Race
Antonov AN-225 – from Paris Airshow Demonstration Flight, Loopy Larry & Jet Truck Drag Race
Aerospatiale-BAC Concorde – from Loopy Larry & Jet Truck Drag Race (in British Airways livery)
Two variations of UFOs – a flying saucer is seen while flying and nearly crashes into you and two black triangular UFOs; one is seen landing at Area 51 just before the player lands and launches again while the player is taxiing, and a larger, similar one parked on skids – all from Secret Shuttle
Two UFOs – a small UFO and a much larger UFO, that can be seen in the background – High Altitude Intercept (Acceleration)
Variations of military jets and helicopters – different types from Tutorial 1: First Take-off, Tutorial 11: Helicopter Maneuvers 1, Flour Power, Loopy Larry, Jet Truck Drag Race, Civil Air Patrol Search, Africa Relief and Catalina Day Spa
Lockheed SR-71 Blackbird – from Tutorial 1: First Take-off and Secret Shuttle.
Boeing B-52H Stratofortress – from Tutorial 1: First Take-off, Executive Tour and Loopy Larry
V-22 Osprey from Tutorial 1: First Take-off and Executive Tour (Acceleration)
F-117 Nighthawk from Tutorial 1: First Take-off and Executive Tour
F-22 Raptor at Paris Airshow Demonstration Flight, Tutorial 1: First Takeoff and Secret Shuttle
Su-37 Flanker at Paris Airshow Demonstration Flight. There are 2 on the flight line next to an F-22.
A fictional NASA orbiter named “Delta V” in Rocket Launch Cover.