The ornamental design for an airplane, as shown and described.
FIG. 1 is a top, front and left side perspective view of an airplane in a closed configuration according to the present invention, the top, front and right side perspective view being a mirror image of the perspective view shown;
FIG. 2 is a top, front and left side perspective view thereof in an open configuration, the top, rear and right side perspective view being a mirror image of the perspective view shown; and,
FIG. 3 is a top, rear and left side perspective view thereof in an open configuration, the top, rear and right side perspective view being a mirror image of the perspective view shown.