Under the aviation regulations of 2026, you cannot legally operate an aircraft under Instrument Flight Rules (IFR) in controlled airspace without a filed and activated IFR flight plan. The core of the IFR system is "separation," which is managed by Air Traffic Control (ATC). For ATC to ensure you do not collide with other aircraft in clouds or low visibility, they must have your intended route, altitude, and destination in their system to provide a "clearance." Flying IFR without a plan would essentially make you "invisible" to the structured flow of traffic, creating a severe safety hazard. There is a minor technical exception: a pilot may "pick up" an IFR clearance while already airborne (often called a "Pop-up IFR") if they encounter unexpected weather, but this still requires the pilot to provide flight plan details to the controller and receive a formal clearance before entering IMC (Instrument Meteorological Conditions). In uncontrolled Class G airspace, the rules are slightly more relaxed in some countries, but for the vast majority of commercial and private flying, a filed flight plan is the mandatory "entry ticket" to the IFR system.