What type of airspace needs to be designated for a flight route on aeronautical charts?

High-level air routes are specifically designed to guide aircraft flying at higher altitudes and are depicted on aeronautical charts. These routes are established to facilitate safe and efficient navigation for aircraft operating above the congested airspace typically found at lower altitudes. The designation of high-level air routes is crucial because it provides pilots with predefined paths that enhance their situational awareness and aid air traffic control in managing air traffic.

Contrarily, while Class D airspace typically surrounds airports with control towers, it is not universally applicable to all flight routes, particularly those at higher altitudes. Class G airspace also plays a role in some lower altitude operations, but it is not specifically designated for structured route use like high-level air routes. Pass-through airspace isn't a standard classification, so it lacks the relevance needed in this context. Understanding the functional purpose of high-level air routes elucidates their necessity for clear navigation at cruising altitudes in the airspace system, making them essential on aeronautical charts.