Which description best fits straight and level flight?

Straight and level flight is characterized by maintaining a constant heading while also keeping a steady altitude. This means that the aircraft is neither climbing nor descending; it is flying horizontally at a consistent altitude above the ground. In this state, all forces acting on the aircraft – lift, weight, thrust, and drag – are balanced, allowing for stable flight without any elevation changes.

When considering the other choices, option A describes a scenario where altitude is changing, which does not align with the definition of straight and level flight. Option C involves turning, which introduces a change in heading and is therefore not straight flight. Finally, option D suggests simultaneous climbing and descending, which directly contradicts the concept of maintaining level flight. Therefore, the description that captures straight and level flight most accurately is the one that indicates a constant heading and maintained altitude.