Under what condition does the FAA FIO document allow active air defense missions?

The correct answer highlights that active air defense missions are permitted when those missions cannot be conducted within the Air Traffic Control (ATC) system. This condition reflects the importance of airspace management and safety protocols that the FAA prioritizes.

In situations where an active air defense mission would interfere with normal ATC operations or pose risks to civilian aircraft, it is crucial that such a mission is executed outside of the ATC framework. This is done to ensure that air traffic remains safe and that military operations do not compromise the overall airspace organization.

The focus of this condition signifies the balance between military operational needs and the safety of civilian air traffic, underscoring why the FAA has established this guideline. This answer also implicitly acknowledges that circumstances where missions are to be executed outside the constraints of the regular commercial airspace structure warrant a different approach to ensure operational security and efficacy.