Under what circumstances must Instrument Flight Rules (IFR) be used?

The correct circumstance for the requirement to use Instrument Flight Rules (IFR) is when visual cues are obscured by weather or darkness. IFR is designed for conditions where pilots cannot rely on visual references outside the aircraft, such as in low visibility due to fog, heavy rain, or at night when no external visual cues are available. Under such scenarios, the pilot must navigate and control the aircraft using instruments instead of visual sight, ensuring safety and compliance with aviation regulations.

Maintaining situational awareness and safely navigating in adverse conditions is crucial; thus, IFR provides standardized procedures and guidelines for pilots to follow. In contrast, flying at night or operating in controlled airspace does not automatically necessitate IFR; these situations may still allow for Visual Flight Rules (VFR) operations if visibility and conditions are favorable.