Which mode of terrain flight is characterized by relatively constant airspeed and altitude?

The mode of terrain flight characterized by relatively constant airspeed and altitude is indeed Low-Level flight. In Low-Level operations, pilots fly at a sustained altitude that is below 1,000 feet above ground level and maintain a stable airspeed. This helps in avoiding detection and reduces the likelihood of being targeted by enemy defenses, allowing for a more stealthy approach while navigating through terrain.

Low-Level flight is crucial for tactical missions as it provides a balance between maneuverability and the ability to stay under radar coverage. By maintaining a constant airspeed and altitude, aircraft can effectively manage the effects of turbulence and other environmental factors, ensuring a smoother flight that can enhance operational effectiveness.

Other modes of terrain flight, such as NOE (Nap-of-the-Earth), involve flying extremely close to the surface, which necessitates varying speed and altitude to navigate obstacles, while Contour flying follows the shape of the terrain but may not maintain constant altitude and speed. High-Level flight, on the other hand, operates at much higher altitudes, which is not synonymous with the characteristics outlined in the question. Thus, Low-Level flight stands out for its emphasis on maintaining stable parameters in flight.