What is required before entering controlled airspace under IFR?

Before entering controlled airspace under Instrument Flight Rules (IFR), it is essential to file a flight plan. A flight plan serves multiple critical functions: it notifies air traffic control of the intended route of flight, helps ensure air traffic safety by informing controllers of potential conflicts, and provides a structured way to manage and monitor the aircraft's movements.

In controlled airspace, proper communication and coordination with air traffic control (ATC) are vital, and a filed flight plan is a fundamental requirement. This plan details the departure and destination airports, waypoints, altitudes, and estimated times, allowing ATC to provide necessary services and instructions to pilots, enhancing safety and efficiency in airspace management.

While other options like obtaining a weather briefing can be important for flight safety, and having a passenger list might be necessary for certain operators, they do not specifically pertain to the formal requirements for entering controlled airspace under IFR. The focus on a filed flight plan underscores the structured nature of IFR operations and the importance of communication with ATC.