Which airspace classification typically requires pilots to obtain a clearance before entering?

Class B airspace is characterized by its high level of control and traffic density, typically surrounding major airports in highly populated areas. Pilots must obtain an air traffic control clearance before entering this airspace to ensure the safe and efficient movement of both commercial and general aviation flights. The requirement for clearance helps manage the complexity of operations in this busy environment, which is essential for maintaining safety and minimizing the risk of mid-air collisions.

In contrast, Class G airspace, which is uncontrolled, does not require a clearance, allowing pilots to operate freely. Class E airspace can be either uncontrolled or controlled, depending on its designated purposes, and generally does not require a clearance unless specified. Class D airspace, while requiring communication with air traffic control, does not necessitate a formal clearance for entry, only a two-way radio communication with the control tower. Thus, the requirement for a clearance before entry is distinctly applicable to Class B airspace due to its operational complexity and traffic management needs.