Which of the following is NOT one of the four forces acting on an airplane?

In flight dynamics, the four primary forces acting on an airplane are lift, gravity, thrust, and drag. Lift is the force that enables an aircraft to rise and overcome gravity, which pulls the aircraft down towards the Earth. Thrust is the forward force generated by the airplane's engines, propelling it through the air. Drag, the fourth force, resists the aircraft's motion through the atmosphere.

Pressure itself is not classified as a force acting directly on the airplane within the context of these four forces. While pressure plays a role in the generation of lift (due to differences in air pressure over and under the wings), it is not one of the primary forces that are consistently accounted for in the basic flight dynamics framework. Thus, identifying pressure as one of the forces acting on an airplane is inaccurate, making this choice the correct answer in the context of the question.