What is required for an aircraft to maintain a steady trajectory?

For an aircraft to maintain a steady trajectory, it is essential that the forces acting on it are balanced. Specifically, this means that the lift generated by the wings must equal the weight of the aircraft, allowing it to stay at a constant altitude. Simultaneously, the thrust produced by the engines must be equivalent to the drag the aircraft encounters from the air. When these forces are in balance, the aircraft can fly straight and level at a constant speed.

This concept is grounded in Newton's laws of motion, which describe how objects behave under the influence of various forces. In the case of steady flight, the equilibrium of these forces is crucial to preventing any acceleration in any direction—either vertically or horizontally.

A steady trajectory does not rely solely on constant thrust or unchanging weight; rather, it depends on the dynamic relationship between all four forces acting on the aircraft—lift, weight, thrust, and drag. This interplay ensures that the aircraft maintains a uniform path in the sky. Therefore, balanced lift and weight, along with balanced thrust and drag, are necessary for steady flight.