Instrument flight rules (IFR) are one of two sets of regulations governing all aspects of civil aviation aircraft operations; the other are visual flight rules (VFR).
Federal Aviation Regulations (FAR) 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.[1]” It is also referred to as, “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.[2]
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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, 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.[3] 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.[4] Flight conditions reported as equal to or greater than these VFR minimums are referred to as visual meteorological conditions (VMC).
Visual flight rules are much 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.[5] 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 [6] (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).[7]
Instrument flight rules permit an aircraft to operate in instrument meteorological conditions (IMC), which have much lower weather minimums than VFR. 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.[8]
As compared to VFR flight, instrument pilots must meticulously evaluate weather, create a very detailed flight plan based around specific instrument departure, en route, and arrival procedures, and dispatch the flight.[7][9]
It is essential to differentiate between flight plan type (IFR or VFR) and weather conditions (VMC or IMC). While current and forecasted 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.
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.
The main purpose of IFR is the safe operation of aircraft in Instrument Meteorological Conditions (IMC). The weather is considered to be IMC when it does not meet the minimum requirements for Visual Meteorological Conditions (VMC). To operate safely in IMC, a pilot controls the aircraft relying on flight instruments, and ATC provides separation.[10]
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 common scenario is a VFR pilot taking off in VMC conditions, but encountering deteriorating visibility while en route. "Continued VFR flight into IMC", as it is known, is responsible for a significant number of light-airplane crashes.
During flight under IFR, there are no visibility requirements, so flying through clouds is permitted. There are still minimum conditions that must be present in order for the aircraft to take off and land; these will 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. For example, landing at mountain airports such as Reno (KRNO) offer significantly different instrument approaches for aircraft landing on the same runway, but from opposite directions. Aircraft approaching from the north must make visual contact with the airport at a higher altitude than a flight approaching from the south, because of rapidly rising terrain south of the airport.{FAA approach plate AL 346} This higher altitude allows a flight crew to clear the obstacle if a landing is not feasible.
Although large airliners and, increasingly, smaller aircraft now carry their own terrain awareness and warning system TAWS, these are primarily backup systems providing a last layer of defense if a sequence of errors or omissions causes a dangerous situation.
Under IFR, the primary means of navigation are either via radio beacons on the ground, such as VORs and NDBs, or GPS. In areas of radar coverage, ATC may also assign headings to IFR aircraft, also known as radar vectors. Radar vectors are one of several methods which ATC uses to provide separation between aircraft for landing, especially in busy traffic environments; in less congested airspace, aircraft are increasingly responsible for their own traffic awareness and collision avoidance.
Modern Flight Management Systems have evolved sufficiently to allow a crew to plan a flight not only as to route and altitude, but 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 at large respectively.
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.
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 3,000 expect 5,000 five minutes after departure. Departure frequency is 121.3, squawk 4263." |
Detailed explanation:
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.
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. Instrument approaches are categorized as precision and non-precision. Despite the names, a precision approach simply indicates that vertical guidance (as well as lateral guidance) is being used. Non-precision indicates only lateral guidance.
In either case, an instrument approach will terminate either in the pilot(s) acquiring sufficient visual reference to carry out a normal landing, or in a missed approach if the required visual reference is not encountered in time. The point at which the crew of an aircraft has to make a decision to either proceed visually, or "miss" the approach is called either the Decision Altitude (DA) or Decision Height (DH) for precision approaches, and missed approach point (MAP) for non-precision approaches. During precision approaches the altitude of the aircraft is determined by the navigational instruments. For non-precision approaches the crew will descend to specific altitudes during the approach procedure, with the final altitude termed the Minimum Descent Altitude (MDA).
Some commercial aircraft are equipped with automatic landing systems that allow the aircraft to land without transitioning from instruments to visual conditions for a normal landing. Such autoland operations require specialized equipment, procedures and training, and involve the aircraft, airport, and the crew. Autoland is the only way some major airports such as Paris-Charles de Gaulle Airport remain operational every day of the year. Some modern aircraft are equipped with enhanced vision systems based on infrared sensors, that provide a day-like visual environment and allow operations in conditions and at airports that would otherwise not be suitable for a landing. Commercial aircraft also frequently use such equipment for takeoffs when takeoff minima are not met.[11]
Documents describing the approach procedure are frequently called approach plates in reference to the plate-like appearance of single-page sheet that it is printed on.
An instrument approach that terminates in a missed approach will continue using missed approach procedure information shown on the approach plate. Typically it describes a transition to a nearby navigational fix, from which the instrument approach can be attempted again. In practice an approach that terminates in a missed approach rarely flies the missed approach procedure as published. Instead, ATC will assign headings and altitudes that will assimilate the aircraft into the arriving traffic for a repeated approach attempt. The crew may also request an alternate destination, either a previously stated alternate airport, or other suitable airport considering the prevailing weather conditions.
To fly under IFR, a pilot must have an instrument rating, and must meet currency of experience requirements. In the United States, these currency of experience requirements include six instrument approaches, NAVAID intercepting and tracking, and holding procedures in the past six calendar months. The aircraft must also be equipped and type-certified for instrument flight, and the related navigational equipment must have been inspected within a specific period of time prior to the instrument flight.
As a partial alternative to the instrument rating, the UK used to issue an "IMC rating", the privileges of which include flight under IFR in airspace classes B to G in instrument meteorological conditions. With this rating, non-instrument-rated pilots 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.
In the United States, instruments required for IFR flight are those that are required for VFR flight in addition to: 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.[12]