Instrument flight rules

In aviation, instrument flight rules (IFR) is one of two sets of regulations governing all aspects of civil aviation aircraft operations; the other is visual flight rules (VFR).

The U.S. Federal Aviation Administration's (FAA) Instrument Flying Handbook defines IFR as: "Rules and regulations established by the FAA to govern flight under conditions in which flight by outside visual reference is not safe. IFR flight depends upon flying by reference to instruments in the flight deck, and navigation is accomplished by reference to electronic signals." It is also a term used by pilots and controllers to indicate the type of flight plan an aircraft is flying, such as an IFR or VFR flight plan.

It is possible and fairly straightforward, in relatively clear weather conditions, to fly an aircraft solely by reference to outside visual cues, such as the horizon to maintain orientation, nearby buildings and terrain features for navigation, and other aircraft to maintain separation. This is known as operating the aircraft under visual flight rules (VFR), and is the most common mode of operation for small aircraft. However, it is safe to fly VFR only when these outside references can be clearly seen from a sufficient distance. When flying through or above clouds, or in fog, rain, dust or similar low-level weather conditions, these references can be obscured. Thus, cloud ceiling and flight visibility are the most important variables for safe operations during all phases of flight. The minimum weather conditions for ceiling and visibility for VFR flights are defined in FAR Part 91.155, and vary depending on the type of airspace in which the aircraft is operating, and on whether the flight is conducted during daytime or nighttime. However, typical daytime VFR minimums for most airspace is 3 statute miles of flight visibility and a distance from clouds of 500 feet below, 1,000 feet above, and 2,000 feet horizontally. Flight conditions reported as equal to or greater than these VFR minimums are referred to as visual meteorological conditions (VMC).

Any aircraft operating under VFR must have the required equipment on board, as described in FAR Part 91.205 (which includes some instruments necessary for IFR flight). VFR pilots may use cockpit instruments as secondary aids to navigation and orientation, but are not required to; the view outside of the aircraft is the primary source for keeping the aircraft straight and level (orientation), flying to the intended destination (navigation), and avoiding obstacles and hazards (separation).

Visual flight rules are generally simpler than instrument flight rules, and require significantly less training and practice. VFR provides a great degree of freedom, allowing pilots to go where they want, when they want, and allows them a much wider latitude in determining how they get there.

When operation of an aircraft under VFR is not safe, because the visual cues outside the aircraft are obscured by weather, instrument flight rules must be used instead. IFR permits an aircraft to operate in instrument meteorological conditions (IMC), which is essentially any weather condition less than VMC but in which aircraft can still operate safely. Use of instrument flight rules is also required when flying in "Class A" airspace regardless of weather conditions. Class A airspace extends from 18,000 feet above mean sea level to flight level 600 (60,000 feet pressure altitude) above the contiguous 48 United States and overlying the waters within 12 miles thereof. Flight in Class A airspace requires pilots and aircraft to be instrument equipped and rated and to be operating under instrument flight rules (IFR). In many countries commercial airliners and their pilots must operate under IFR as the majority of flights enter Class A airspace. Procedures and training are significantly more complex compared to VFR instruction, as a pilot must demonstrate competency in conducting an entire cross-country flight solely by reference to instruments.

Instrument pilots must carefully evaluate weather, create a detailed flight plan based around specific instrument departure, en route, and arrival procedures, and dispatch the flight.

The distance by which an aircraft avoids obstacles or other aircraft is termed separation. The most important concept of IFR flying is that separation is maintained regardless of weather conditions. In controlled airspace, air traffic control (ATC) separates IFR aircraft from obstacles and other aircraft using a flight clearance based on route, time, distance, speed, and altitude. ATC monitors IFR flights on radar, or through aircraft position reports in areas where radar coverage is not available. Aircraft position reports are sent as voice radio transmissions. In the United States, a flight operating under IFR is required to provide position reports unless ATC advises a pilot that the plane is in radar contact. The pilot must resume position reports after ATC advises that radar contact has been lost, or that radar services are terminated.