What is one key factor of vertical separation in crowded airspace?

Maintaining a safe altitude between aircraft is a fundamental aspect of vertical separation in crowded airspace. This principle ensures that aircraft are separated by a minimum vertical distance, typically measured in feet, to prevent mid-air collisions and enhance overall safety. Vertical separation is crucial, especially in busy airspace, where multiple aircraft may operate at similar altitudes. By ensuring that aircraft maintain designated altitudes or altitude blocks, air traffic control can effectively manage traffic and reduce the risk of accidents. This practice is vital for both the safety of the aircraft and the efficiency of airspace use, allowing for more aircraft to operate safely within limited and busy airspace environments.

In contrast, the other options, while related to flight operations, do not directly address the cornerstone principle of vertical separation necessary for safe aircraft operation in crowded airspace.