Final answer:
Protected circling airspace for Category D aircraft is an area where the aircraft is safeguarded during instrument approach maneuvers. The size of this airspace takes into account the aircraft's speed and maneuvering capabilities. In the context of air defense, a 30 nautical mile effective perimeter around SAM sites is maintained for aircraft protection.
Step-by-step explanation:
Protected circling airspace for Category D aircraft refers to the designated airspace surrounding an airport within which a Category D aircraft is protected when performing a circling maneuver during an instrument approach. Category D aircraft are typically larger and faster, including most commercial jetliners, with approach speeds between 141 and 165 knots. The circling airspace dimensions are defined by the FAA and take into account various factors including aircraft speed and maneuverability.
The objective of protecting airspace around a confirmed SAM site or a possible SAM site is to ensure aircraft safety from ground-based air defense systems. An effective air defense perimeter approximates a 30 nautical miles radius, where air traffic is carefully managed to mitigate the risks posed by these sites.