Instrument flight rules
||The examples and perspective in this article deal primarily with USA and do not represent a worldwide view of the subject. (July 2012)|
FAA's 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.
Basic Information 
Visual flight rules 
To put instrument flight rules into context, a brief overview of VFR is necessary. Flights operating under VFR are flown solely by reference to outside visual cues (horizon, buildings, flora, etc.) which permit navigation, orientation, and separation from terrain and other traffic. 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 cloud distance of 500' below, 1,000' 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).
Visual flight rules can be simpler than IFR, 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. Pilots are not required to file a flight plan, do not have to communicate with ATC (unless flying in certain types of "busier" airspace), and are not limited to following predefined published routes or flight procedures.
VFR pilots may use cockpit instruments as secondary aids to navigation and orientation, but are not required to. However, any aircraft operating under VFR must have the required equipment on board, as described in FAR Part 91.205  (which includes instruments necessary for IFR flight); but the view outside of the aircraft is the primary source for keeping the aircraft straight and level (orientation), flying where you intended to fly (navigation), and for not hitting anything (separation).
Instrument flight rules 
Instrument flight rules permit an aircraft to operate in instrument meteorological conditions (IMC) in contrast to VFR. They are also an integral part of flying in class A airspace. "Class A" airspace exists over and near the 48 contiguous U.S. states and Alaska from 18,000 feet above mean sea level to flight level 600 (approximately 60,000 feet in altitude depending on variables such as atmospheric pressure). Flight in "class A" airspace requires pilots and aircraft to be instrument equipped and rated and to be operating under Instrument Flight Rules (IFR). Most jet aircraft operate in "class A" airspace for the cruise portion of their flight and are therefore required to utilize IFR procedures. Procedures and training are significantly more complex as a pilot must demonstrate competency in conducting an entire cross-country flight in IMC conditions, while controlling the aircraft solely by reference to instruments.
Separation and clearance 
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. Aircraft position reports are not necessary if ATC reports that the aircraft is in radar contact. 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.
IFR flights in controlled airspace require an ATC clearance for each part of the flight. A clearance always specifies a clearance limit, which is the farthest the aircraft can fly without a new clearance. In addition, a clearance typically provides a heading or route to follow, altitude, and communication parameters, such as frequencies and transponder codes.
In uncontrolled airspace, ATC clearances are unavailable. In some states a form of separation is provided to certain aircraft in uncontrolled airspace as far as is practical (often known under ICAO as an advisory service in class G airspace), but separation is not mandated nor widely provided.
Despite the protection offered by flight in controlled airspace under IFR, the ultimate responsibility for the safety of the aircraft rests with the pilot in command, who can refuse clearances.
It is essential to differentiate between flight plan type (VFR or IFR) and weather conditions (VMC or IMC). While current and forecast weather may be a factor in deciding which type of flight plan to file, weather conditions themselves do not affect one's filed flight plan. For example, an IFR flight that encounters VMC en route does not automatically change to a VFR flight, and the flight must still follow all IFR procedures regardless of weather conditions. In the US, weather conditions are forecast broadly as VFR, MVFR, or IFR.
The main purpose of IFR is the safe operation of aircraft in instrument meteorological conditions (IMC). The weather is considered to be MVFR or IMC when it does not meet the minimum requirements for visual meteorological conditions (VMC). To operate safely in IMC ("actual instrument conditions"), a pilot controls the aircraft relying on flight instruments and ATC provides separation.
It is important not to confuse IFR with IMC. A significant amount of IFR flying is conducted in Visual Meteorological Conditions (VMC). Any time a flight is operating in VMC, the crew is responsible for seeing and avoiding VFR traffic; however, since the flight is conducted under Instrument Flight Rules, ATC still provides separation services from other IFR traffic.
Although dangerous and illegal, a certain amount of VFR flying is conducted in Instrument Meteorological Conditions (IMC). A scenario is a VFR pilot taking off in VMC conditions, but encountering deteriorating visibility while en route. "Continued VFR flight into IMC" may lead to spatial disorientation of the pilot, the cause of a significant number of light-airplane crashes. VFR flight into IMC is distinct from VFR-on-top, an IFR procedure in which the aircraft operates above IMC but remains in contact with ATC, and VFR over the top, a VFR procedure in which the aircraft takes off and lands in VMC but flies above an intervening area of IMC, both of which are legal in the US.
During flight under IFR, there are no visibility requirements, so flying through clouds (or other conditions where there is zero visibility outside the aircraft) is legal and safe. However, there are still minimum weather conditions that must be present in order for the aircraft to take off or to land; these vary according to the kind of operation, the type of navigation aids available, the location and height of terrain and obstructions in the vicinity of the airport, equipment on the aircraft, and the qualifications of the crew.
Although large airliners, and increasingly, smaller aircraft, carry their own terrain awareness and warning system, these are primarily backup systems providing a last layer of defense if a sequence of errors or omissions causes a dangerous situation.
Because IFR flights often take place without visual reference to the ground, a means of navigation other than looking outside the window is required. A number of navigational aids are available to pilots, including ground-based systems such as DME/VORs and NDBs as well as the satellite-based GPS/GNSS system. Air traffic control may assist in navigation by assigning pilots specific headings ("radar vectors"). The majority of IFR navigation is given by ground- and satellite-based systems, while radar vectors are usually reserved by ATC for sequencing aircraft for a busy approach or transitioning aircraft from takeoff to cruise, among other things.
Modern flight management systems have evolved to allow a crew to plan a flight as to route and altitude and to specific time of arrival at specific locations. This capability is used in several trial projects experimenting with four dimensional approach clearances for commercial aircraft, with time as the fourth dimension. These clearances allow ATC to optimize the arrival of aircraft at major airports, which increases airport capacity and uses less fuel providing monetary and environmental benefits to airlines and the public.
Specific procedures allow IFR aircraft to transition safely through every stage of flight. These procedures specify how an IFR pilot should respond, even in the event of a complete radio failure, and loss of communications with ATC, including the expected aircraft course and altitude.
Departures are described in an IFR clearance issued by ATC prior to takeoff. The departure clearance may contain an assigned heading, one or more waypoints, and an initial altitude to fly. The clearance can also specify a departure procedure (DP) or standard instrument departure (SID) that should be followed unless "NO DP" is specified in the notes section of the filed flight plan.
Here is an example of an IFR clearance for a Cessna aircraft traveling from Palo Alto airport (KPAO) to Stockton airport (KSCK).
|"Cessna 21756, cleared to Stockton Airport via turn right heading zero-six-zero within one mile of the airport. Radar vectors San Jose, then as filed. Maintain three thousand expect five thousand five minutes after departure. Departure frequency is one two one decimal three. Squawk four two six three."|
- "Cessna 21756"
- Verifies that only this specific aircraft is cleared.
- "cleared to Stockton Airport"
- Clearance Limit: the farthest destination the aircraft is allowed to go under IFR (in most cases it is the destination airport).
- "via turn right heading zero-six-zero within one mile of the airport."
- The pilot is expected to execute the right turn to 060° magnetic heading without further ATC prompting within one mile of the departure airport.
- "Radar vectors San Jose"
- The departure controller will provide directional guidance to the San Jose VOR.
- "Then as filed."
- After arriving at the San Jose VOR, the pilot will likely resume navigation without ATC prompts along the airways and intersections that were filed in their flight plan.
- "Maintain three thousand ..."
- After takeoff, climb to an indicated altitude of 3000 feet above sea level.
- "... expect five thousand five minutes after departure."
- Your next altitude assignment is probably going to be 5000 feet above sea level. However, you must follow actual ATC altitude assignments throughout the flight. This portion of the clearance provides a backup if communications are lost, allowing you to proceed to climb and maintain 5000 feet.
- "Departure frequency is one two one decimal three."
- After you are airborne and the tower controller tells you to "contact departure", you are to contact the departure controller on this communication frequency.
- "Squawk four two six three."
- Program your transponder with beacon code 4263 so that ATC can positively identify you on radar.
En route flight is described by IFR charts showing navigation aids, fixes, and standard routes called airways. Aircraft with appropriate navigational equipment such as GPS, are also often cleared for a direct-to routing, where only the destination, or a few navigational waypoints are used to describe the route that the flight will follow. ATC will assign altitudes in its initial clearance or amendments thereto, and navigational charts indicate minimum safe altitudes for airways.
The approach portion of an IFR flight may begin with a standard terminal arrival route (STAR), describing common routes to fly to arrive at an initial approach fix (IAF) from which an instrument approach commences. An instrument approach terminates either by the pilot acquiring sufficient visual reference to proceed to the runway, or with a missed approach because the required visual reference is not seen in time.
To fly under IFR, a pilot must have an instrument rating and must be current (meet recency of experience requirements). In the United States, to file and fly under IFR, a pilot must be instrument-rated and, within the preceding six months, have flown six instrument approaches, as well as holding procedures and course interception and tracking with navaids. Flight under IFR beyond six months after meeting these requirements is not permitted; however, currency may be reestablished within the next six months by completing the requirements above. Beyond the twelfth month, examination ("instrument proficiency check") by an instructor is required.
Practicing instrument approaches can be done either in the instrument meteorological conditions or in visual meteorological conditions – in the latter case, a safety pilot is required so that the pilot practicing instrument approaches can wear a view-limiting device which restricts his field of view to the instrument panel. A safety pilot's primary duty is to observe and avoid other traffic.
For all ILS Cat II or Cat III approaches, additional crew training is required and a certain number of low visibility approaches must either be performed or simulated within a fixed time for pilots to be 'current' in performing them.
In the UK, an "IMC rating" which permits flight under IFR in airspace classes B to G in instrument meteorological conditions, a non-instrument-rated pilot can also 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 its licensing for meteorological conditions and airspace, rather than flight rules.
The aircraft must be equipped and type-certified for instrument flight, and the related navigational equipment must have been inspected or tested within a specific period of time prior to the instrument flight.
In the United States, instruments required for IFR flight in addition to those that are required for VFR flight are: heading indicator, sensitive altimeter adjustable for barometric pressure, clock with a sweep-second pointer or digital equivalent, attitude indicator, radios and suitable avionics for the route to be flown, alternator or generator, gyroscopic rate-of-turn indicator that is either a turn coordinator or the turn and bank indicator.
- "Instrument Flying Handbook", Instrument Flight Rules (defined), Oklahoma City, OK: Federal Aviation Administration, 2008, pp. G–9, retrieved 2010-11-27
- "Aeronautical Information Manual", Instrument Flight Rules (defined), Oklahoma City, OK: Federal Aviation Administration, 2010-02-11, pp. PCG I−4, retrieved 2010-11-27
- Keel, Byron; Stancil, Charles; Eckert, Clifford; Brown, Susan (June 2000). Aviation Weather Information Requirements Recommendations. "Aviation Weather Information Requirements Study". Aviation Safety Program (Hampton, Virginia: NASA Langley Research Center). pp. 40–41. CiteSeerX: 10.1.1.32.1842.
- "14 CFR 91.155 [ Basic VFR weather minimums ]". Federal Aviation Regulations: 721–722. 2010-02-11. Retrieved 2010-11-27.
- Wallace, Lane (2010-05-03). "The Basics: VFR Flight Planning". Flying Magazine 137 (4): 61. Retrieved 2010-11-27.
- Collins, Richard (2006-11-11). "Getting an Instrument Rating". Flying Magazine 133 (11): 68. Retrieved 2010-11-27.
- Federal Aviation Regulations Title 14
- Goyer, Robert (2010-04-30). "IFR Flight Prep: A Whole New Game". Flying Magazine 137 (4): 56. Retrieved 2010-11-27.
- McCloy, John. "Safety Pilot in IMC". IFR magazine. Retrieved 2009-05-20.
- US Code of Federal Regulations, 14 CFR 61.57(c) and (d)
- 14 CFR 91.205
- FAA website
- Instrument Flying Handbook (FAA-H-8083-15A)
- Instrument Procedures Handbook (FAA-H-8261-1A)
- Hear audio of a US instrument rating checkride - Part 1
- "Blind Flying, January 1933, Popular Mechanics details on blind flying for that era
- ATC Communication, A complete pilot-to-ATC communication reference