The ornamental design for an airplane, as shown and described.
FIG. 1 is a front view of my new invention;
FIG. 2 is a first perspective view thereof;
FIG. 3 is a second perspective view thereof;
FIG. 4 is a third perspective view thereof;
FIG. 5 is a fourth perspective view thereof;
FIG. 6 is a right side view thereof;
FIG. 7 is a reduced rear view thereof;
FIG. 8 is a first greatly enlarged partial perspective view showing details of a portion thereof;
FIG. 9 is a second slightly enlarged partial perspective view showing details of another portion thereof;
FIG. 10 is a slightly enlarged partial top view showing details of another portion thereof;
FIG. 11 is a slightly enlarged partial left side view showing details of another portion thereof;
FIG. 12 is an enlarged third partial perspective view showing details of another portion thereof; and,
FIG. 13 is an enlarged fourth partial perspective view showing details of another portion thereof.
Details of the external environment shown around the airplane and details of the interior of the airplane visible through the windows in the drawing figures are provided for illustrative purposes only and form no part of the claimed invention.