Instrument flight rules (IFR) allow an aircraft to be flown in weather conditions that do not meet the minimum requirements for visual flight rules (VFR). These are referred to as instrument meteorological conditions (IMC). In such conditions the pilot will control the attitude, altitude, and course of the aircraft by watching the flight instruments. The pilot must have an instrument rating and meet recency of experience requirements pertaining to instrument flight. The aircraft must be equipped and type-certified for instrument flight. In airspace where air traffic control (ATC) services are available, an IFR flight plan must be filed, and the pilot must maintain voice radio communication with ATC. The pilot will usually navigate by using electronic navigation equipment, compass headings assigned by ATC, or in some cases compass bearings corrected for forecast winds. While weather conditions can be much worse than allowed for VFR flight, there are still minimum conditions that must be present in order for the aircraft to take off or land. These will vary according to the type of electronic navigation aids available, the location and height of terrain and obstructions in the vicinity of the airport, and in some cases according to qualifications of the crew and aircraft.
Contact with ATC is normally maintained using VHF (very high-frequency) radios. On trans-oceanic flights, HF (high-frequency) is used because of its ability to communicate over long distances. In areas where ground-based radar service is available, IFR flights are provided with radar separation from other aircraft by ATC. Terrain separation is provided either by published minimum altitudes, or by altitudes assigned by ATC. Where radar service is not available, non-radar separation techniques are used in which ATC keeps track of aircraft locations via position reports provided by pilots. In airspace where ATC service is available but not radar service, the course flown must be along published routes called airways, or along courses specified in published instrument approach or departure procedures.
IFR flights can also be conducted when conditions are good enough for VFR. Most airline flights are conducted under IFR. When weather conditions permit, the pilot is responsible for seeing and avoiding other aircraft even though ATC may be providing aircraft separation service.
See also: VOR