When are ATC clearances and instructions mandatory for pilots to follow?

ATC clearances and instructions are mandatory for pilots to follow while operating in controlled airspace. This is because controlled airspace is designated for the purpose of providing air traffic control services, ensuring the safe and orderly flow of air traffic. Within controlled airspace, air traffic controllers have the responsibility to separate aircraft, manage traffic, and provide necessary clearances and instructions to maintain safety margins.

While pilots must adhere to procedural requirements in uncontrolled airspace, the nature of operations there allows for pilots to exercise more freedom without needing direct clearance from ATC. Additionally, the obligation to follow ATC instructions does not specifically extend to all phases of flight or are only relevant to specific approach segments; it is primarily focused on the complexities present when operating within controlled airspace, where ATC guidance is essential to mitigate risk and avoid collisions.