In the Dam level, the opening sequence shows the airplane and someone looking like Agent Blastowitz ready to parachute out. Enable the /noclip code and go behind the dam wall (not the side where you can fall and die). Once there, look throughout the sky. You may notice an airplane. Float in and disable the /noclip code. The soldier will attempt to attack you, but does not have a gun. When you kill him, he stands frozen. There are also guns on the plane that are usable, but there is not anything to shoot.