I claim the ornamental design and appearance of an airplane seat back, substantially as shown and described.
FIG. 1 is a perspective view of a seat back taken from the front, top and left, showing the rod length indicator fully closed;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a right side elevation view thereof;
FIG. 4 is a left side elevation view thereof; and,
FIG. 5 is a top plan view thereof.
The broken line showing of an airplane seat back is included for the purpose of illustrating environmental structure and forms no part of the claimed design.