Instrument flight rules
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Instrument Flight Rules (IFR) is a set of regulations and procedures for flying aircraft without the assumption that pilots will be able to see and avoid obstacles, terrain, and other air traffic; it is an alternative to Visual flight rules (VFR), where the pilot is primarily or exclusively responsible for see-and-avoid.
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Separation
The most important concept of IFR flying is separation: in controlled airspace, air traffic control (ATC) separates IFR aircraft from terrain, obstacles, and other IFR aircraft by time, distance, and altitude, relying either on radar or on pilot position reports over the radio -- IFR aircraft require an ATC clearance for each part of the flight, typically providing a heading or route, altitude, and clearance limit (the farthest the aircraft can fly without a new clearance). In very busy areas, typically near major airports, clearances may also be required for VFR aircraft, and ATC may also provide separation between IFR and VFR aircraft or even between VFR aircraft. In uncontrolled airspace, IFR aircraft do not require clearances, and they separate themselves from each other by using charted minimum altitudes to avoid terrain and obstacles, standard cruising altitudes to avoid aircraft flying in different directions, and radio reports over mandatory locations. In much of the world, all airspace from 18,000 to 60,000 feet (5,586 to 18,288 meters) is designated as Class A, requiring all aircraft to operate under IFR.
Weather
One advantage of IFR is the ability to fly an aircraft in instrument meteorological conditions (IMC), weather conditions that do not meet the minimum visibility requirements for VFR. In such conditions the pilot will control the attitude of the aircraft by watching the flight instruments, and will rely entirely on ATC for separation (though large airliners and, increasingly, smaller aircraft now carry their own terrain- and collision-avoidance systems such as TCAS). It is important, however, not to confuse IFR with IMC: the vast majority of IFR flying is done under visual meteorological conditions, and in many cases, the pilot will be controlling the aircraft primarily by outside visual references, as with VFR. Under VMC in some types of airspace, ATC will not provide separation between IFR and VFR aircraft (in fact, some VFR aircraft might not even show up on ATC's radar), so pilots are responsible for seeing and avoiding other traffic just as they would be under VFR.
The pilot will usually navigate by using electronic navigation equipment, vectors (compass headings and altitudes 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.
Procedures
There are three stages to an IFR flight: departure, enroute, and approach. For each stage, there are standard, published procedures to allow IFR aircraft to move in a safe, orderly way, from the moment the wheels leave the runway to the moment they touch down again. These procedures also allow an IFR aircraft to complete a flight predictably in the case of lost communications with ATC (lost-comms), since they contain default altitudes and headings for every stage.
Departures are described by simple departure procedures (DP), normally providing an initial heading and altitude, or (for busier airports) by standard instrument departures (SID) providing more detailed instructions, often accompanied by diagrams or charts. Enroute flight is described by IFR charts showing navigation aids, fixes, and standard routes called airways with minimum safe altitudes for each segment. Approaches are described by approach plates (now often called terminal procedures), describing a series of steps and segments to make the transition from enroute flight to a position where the aircraft can complete a landing visually (often from a low altitude and close to the airport). All instrument approaches have minimum altitudes: if it is not possible to complete a landing visually from the specified altitude and location, the pilot must commence a missed approach and return to enroute flight. Busy airports may also have standard terminal arrivals (STARS) providing an additional connection between enroute flight and the final approach.
Qualifications
To fly under IFR, a pilot must have an additional instrument rating added to his or her license, and must meet additional recency of experience requirements. In the United States, these recency of experience requirements include six instrument approaches, navaid intercepting and tracking, and holding procedures. The aircraft must be equipped and type-certified for instrument flight.
See also
de:Instrumentenflug fr:Vol aux instruments nl:Instrument flight rules