In what airspace is it required to file an IFR flight plan?

Filing an IFR (Instrument Flight Rules) flight plan is mandatory in Class A airspace, as this airspace is specifically designated for instrument flight operations and is located generally from 18,000 feet up to and including 60,000 feet MSL (Mean Sea Level). In this airspace, all flights must operate under IFR, regardless of whether they are conducted by aircraft equipped for visual flight.

The primary purpose of requiring IFR plans in Class A airspace is to ensure safe separation of aircraft flying at high altitudes, which often involves greater traffic density and complex air traffic control procedures. By mandating IFR flight plans, air traffic controllers can effectively manage and coordinate the flow of air traffic, ensuring that pilots adhere to prescribed altitudes, routes, and any changes due to weather or traffic conditions.

In contrast, while Class B, Class C, and Class D airspace have their own regulations, they do allow for visual flight operations under specified weather conditions, meaning that pilots can opt not to file an IFR flight plan if they are flying under visual flight rules (VFR).