In aviation, what is primarily used to define vertical separation between different aircraft types?

Vertical separation between different aircraft types primarily relies on flight levels. Flight levels are standardized altitudes expressed in hundreds of feet, with the use of pressure settings to define these levels. This system is essential for maintaining safe distances between aircraft operating at different altitudes, ensuring that there is sufficient vertical space to avoid collisions, particularly in congested airspace.

Flight levels are determined based on atmospheric pressure and are standardized so that all aircraft can interpret them similarly. This consistency is crucial for air traffic control (ATC) and pilot navigation, allowing for efficient scheduling and safe operation of multiple aircraft in the same airspace.

The other options, while relevant in certain contexts, do not specifically pertain to the definition of vertical separation. Flight duration pertains to the time an aircraft is in the air, communication frequency relates to the radio channels used by pilots and air traffic controllers, and weather data is important for flight planning and safety but does not directly influence vertical separation protocols. Thus, flight levels are the key factor in defining and managing vertical separation among different aircraft types.