What condition is required for an aircraft to be in equilibrium?

For an aircraft to be in equilibrium, it must maintain a steady flight path, which requires that there be no net external forces acting on it. This means there is no change in speed or direction. When these conditions are met, the forces acting on the aircraft, including lift, weight, thrust, and drag, are balanced.

When the aircraft experiences no change in speed or direction, it indicates a state where all forces are in equilibrium, allowing it to maintain a stable flight condition. This is a fundamental principle in aerodynamics, as it demonstrates that the aircraft's motion is uniform, which is essential for safe and efficient operations.

The remaining options, while related to aspects of flight, do not fully encapsulate the condition of equilibrium. For instance, constant altitude and speed suggest a stable flight but do not explicitly address the balance of forces. Similarly, minimal drag and thrust focuses on efficiency rather than equilibrium, and even distribution of weight pertains more to stability than to the equilibrium of forces acting on the aircraft.