Aircraft moving through the air develop lift, or a force upward that overcomes weight, by air moving over the wings. One way an aircraft moves is when the nose or front of the airplane moves up or down, often referred to as pitch. Pitching moment is a measurement of the movement up and down for different angles of air across the wings, known as angle of attack.
Most fixed-wing aircraft have two or four wings about halfway along the fuselage, which is the main body of the plane. Wings have movable ailerons that move the wings up or down, known as rolling the aircraft. There is a horizontal stabilizer with a movable elevator panel at the tail or rear of the fuselage to control pitch up or down. The horizontal stabilizer often looks like a smaller wing on each side of the tail in a flat or horizontal position.
A vertical stabilizer with a movable rudder panel is placed vertically up from the horizontal stabilizer to move the nose back and forth, which is yaw control. All movable surfaces are connected to a pilot control wheel or stick and to rudder pedals controlled by the pilot's feet. The pilot can bank or roll, turn left and right, and yaw or move the nose back and forth with the controls.
If the aircraft moves up or down from movement in the elevator, power from the engine, or weather turbulence, the angle of attack changes for air flowing both across the wings and horizontal stabilizer. The horizontal stabilizer is designed as an upside-down wing, and it creates a pitching moment upward to force the nose down. Other parts of the aircraft are trying to push the nose upward due to aerodynamic forces, which are effects of air moving across the different surfaces.
Forces created by the horizontal stabilizer are often referred to as torque, which is a measurement of the force times the distance from a rotation point. The point of rotation on aircraft is normally the center of gravity, which is an imaginary point where the airplane could be lifted and be in perfect balance. Passenger weight, baggage and fuel will change the center of gravity or CG, and calculations are made by pilots to determine that their aircraft flies within an acceptable range of CG.
The pitching moment created by the horizontal stabilizer occurs from a wing much smaller than the main wings. This is possible because of the torque calculation. For a desired amount of force, the wing can be smaller because it is further away from the center of gravity. Almost all aircraft have a long tail with horizontal and vertical stabilizers at the far end for this reason.
When the angle of attack becomes too great, air will no longer flow smoothly across the top and bottom of the wing. Turbulence occurs, the air no longer flows along the wing, and the wing does not create lift. This is known as an aerodynamic stall, and the plane can no longer maintain level flight. The CG range is carefully designed and tested by manufacturers so a aircraft's nose will drop when a stall occurs. This allows the plane to gain speed and restore airflow across the wings and tail, and is caused by the designed pitching moment of the aircraft.
If a pilot mistakenly adds too much weight toward the rear before flight, an aircraft may not recover from a stall. The horizontal stabilizer cannot develop enough thrust to overcome excess weight and lower the nose. This is known as an aft or rear CG condition, and is very dangerous if not corrected by the pilot.
The pitching moment can also change from aerodynamic effects that occur close to the ground, called ground effect. Ground effect is caused by changes in the way air moves over and under the wings, and affects lift and pitching moment. This can cause the nose to pitch down just before landing and contribute to accidents if not understood by the pilot.