Yes, Instrument Flight Rules (IFR) flight plans are absolutely required for any flight operating in "Instrument Meteorological Conditions" (IMC) or within specific classes of controlled airspace (such as Class A airspace in the U.S., which starts at 18,000 feet). Even in clear weather, commercial airliners and many private jets file IFR flight plans to ensure they receive positive air traffic control (ATC) separation and guidance throughout their journey. Filing an IFR flight plan involves submitting specific details—including the aircraft's ID, type, planned route, altitude, and estimated time en route—to an FAA or international ATC facility. Once filed, the pilot must receive an official "IFR Clearance" before entering controlled airspace. This system is the backbone of modern aviation safety, as it allows controllers to track and sequence aircraft precisely, preventing mid-air collisions. For general aviation pilots, IFR flight is only permitted if the pilot is IFR-rated and the aircraft is properly equipped with certified navigation instruments.