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

Instrument Flight Rules (IFR) are mandated when visibility is compromised by weather conditions. This requirement exists because IFR allows pilots to fly by relying on their instruments instead of visual references. Situations such as poor visibility caused by clouds, fog, rain, or low ceilings require adherence to IFR to ensure safety and compliance with aviation regulations.

In contrast, flying over mountainous terrain can be done under Visual Flight Rules (VFR) as long as the weather allows for visual navigation; thus, not all flights in such areas necessitate IFR. Night flying typically does not mandate IFR by itself unless visibility is also impaired. Similarly, IFR is not required when visual flight conditions are safe, as pilots can operate under VFR in such circumstances.