What indicates poor longitudinal stability in an airplane?

The correct choice indicates that a tendency to gradually increase a pitching moment is a sign of poor longitudinal stability in an airplane. Longitudinal stability refers to an aircraft's ability to maintain a steady pitch attitude. When an airplane shows a tendency to continuously increase its pitching moment, it means that the forces acting on the aircraft (like aerodynamic lift and weight) are not balanced effectively, leading to an unstable flight situation. This condition can result in undesired and uncontrollable pitching motions, making it difficult for pilots to maintain level flight.

In stable aircraft, any perturbation should result in a restoring moment that returns the airplane to its original attitude. If the pitching moment continually increases, it suggests that the aircraft's design may not provide adequate stability or control, posing a risk during flight operations. Understanding this concept is essential for evaluating an aircraft's stability characteristics and ensuring safe flight conditions.