The ornamental design for an airplane, as shown and described.
FIG. 1 is a top plan view showing the new ornamental design;
FIG. 2 is a perspective view, the portions shown in broken lines forming no part of the design sought to be patented;
FIG. 3 is a front elevation view, the portions shown in broken lines forming no part of the design sought to be patented;
FIG. 4 is a side elevation view, the left and right views being a mirror image, the portions shown in broken lines forming no part of the design sought to be patented;
FIG. 5 is a rear elevation view, the portions shown in broken lines forming no part of the design sought to be patented;
FIG. 6 is a side perspective view, the portions shown in broken lines forming no part of the design sought to be patented; and,
FIG. 7 is a bottom plan view thereof, the portions shown in broken lines forming no part of the design sought to be patented.