Instrument flight rules

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. Since navigation and control of the aircraft under IFR is done by instruments, flying through clouds is allowed; under VFR it is not.
Commercial traffic (a flight carrying paying passengers or cargo) operates under IFR almost exclusively.

Separation

The most important concept of IFR flying is that it allows continued flight operations in reduced visibility, during which time the ability of a pilot to physically see and avoid collision with other aircraft or obstacles is severely reduced, or even impossible. The distance that is achieved when avoiding obstacles or other aircraft is termed separation. In controlled airspace, air traffic control (ATC) separates IFR aircraft from obstacles and other IFR aircraft by applying separations based on time, distance, and altitude differences between aircraft, by relying either on radar or reports of aircraft positions traditionally sent as voice radio transmissions, but increasingly as electronic data exchanges. Generally, in most controlled airspaces, 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 the United States and Canada, 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. It should be noted however that in the United States even when on a filed IFR flight plan, if conditions permit the pilot is ultimately responsible for see and avoid.

See also: Airspace classes

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. Although large airliners and, increasingly, smaller aircraft now carry their own terrain- and collision-avoidance systems such as TCAS, these are primarily backup systems.

It is important, however, not to confuse IFR with IMC: the vast majority of IFR flying is done under visual meteorological conditions (VMC), 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, in areas with limited radar coverage, ATC may not know the location of VFR aircraft), so pilots are responsible for seeing and avoiding other traffic just as they would be under VFR.

During flight under IFR, there are no visibility requirements, and as such flying through clouds is permitted. 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 according to qualifications of the crew and aircraft. See Instrument Landing System

Under IFR, the primary means of navigation is via radio beacons on the ground such as VORs and NDBs, although GPS is rapidly taking over. In areas of radar coverage, navigation may be done by receiving radar vectors (compass headings assigned by ATC). This is one of the primary ways for ATC to provide separation between aircraft, and to sequence them for landing. Because of the importance of reliable navigation, aircraft are required to carry several receivers (of varying types), and an aircraft may not be operated under IFR if one is defective. Likewise, the flight instruments must work correctly and will be checked before each flight.

Procedures

There are three stages to an IFR flight: departure, en route, 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), with 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. En route 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 terminal procedures (often called approach plates), describing a series of steps and segments to make the transition from en route 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 (decision height) and location, the pilot must commence a missed approach and return to en route flight. The most commonly used approach system is the Instrument Landing System, other systems are also used but these generally have higher weather minima so ILS is usually preferred.

Busy airports may also have standard terminal arrivals (STARS) providing an additional connection between en route flight and the final approach. These usually start at an airway and end at the holding stack.

Qualifications

To fly under IFR, a pilot must have an instrument rating, and must meet recency of experience requirements. In the United States, these recency of experience requirements include six instrument approaches, NAVAID intercepting and tracking, and holding procedures in the past six months. The aircraft must also be equipped and type-certified for instrument flight.

The UK differs from pilot licensing practice in the U.S. In the UK any pilot can decide to which flight rules he adheres given that the meteorological conditions for those rules are met. The pilot does need an instrument rating to fly in instrument meteorological conditions, and under IFR in controlled airspace. The upshot of this is that non-instrument qualified pilots can elect to fly under IFR in visual meteorological conditions outside controlled airspace. Compared to the rest of the world the UK's flight crew licensing regime is somewhat unusual in this respect by licensing for meteorological conditions and airspace, rather than flight rules.

Separation

Weather

Navigation

Procedures

Qualifications

See also