Instrument flight rules

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"IFR" redirects here. For other uses, see IFR (disambiguation).

Instrument flight rules (IFR) are a set of regulations and procedures for flying aircraft whereby navigation and obstacle clearance is maintained with reference to aircraft instruments only, while separation from other aircraft is provided by Air Traffic Control. In layman's terms, a pilot who is rated for IFR can keep a plane in controlled flight solely on the data provided by his instruments, even if that pilot cannot see anything (useful) out the cockpit windows; indeed, one of the benefits of these regulations is the ability to fly through clouds, which is otherwise not allowed.

IFR is an alternative to visual flight rules (VFR), where the pilot is ultimately responsible for navigation, obstacle clearance and traffic separation using the see-and-avoid concept. The vast majority of commercial traffic (any flight for hire) and all scheduled air carriers operate exclusively under IFR. Commercial aircraft providing sight seeing flights, aerial photography, or lift services for parachute jumping usually operate under VFR.

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[edit] Separation

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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 meteorological visibility conditions. In controlled airspace, Air Traffic Control (ATC) separates IFR aircraft from obstacles and other IFR and known VFR aircraft by applying a flight clearance based on route, time, distance, speed, and altitude differences between aircraft. ATC monitors IFR flights by relying either on radar or aircraft position reports. Aircraft position reports are traditionally sent as voice radio transmissions, but increasingly also as electronic data exchanges. Aircraft position reports are not necessary if ATC has an aircraft in radar contact. In the United States a flight operating under IFR is required to fall back to position reports if advised radar contact lost.

IFR flights 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. An aircraft operating VFR must also obtain a clearance to enter class B and class C airspace, and is required to maintain an assigned heading or altitude restriction as long as it does not conflict with the safe operation of the 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 the Southern Domestic Airspace of Canada (SDA), airspace from 18,000 to 60,000 feet (5,586 to 18,288 meters) is designated as class A, requiring an IFR clearance for all aircraft. In other countries class A airspace begins higher or lower. For example, in France class A airspace begins at 19,500 feet (5,850 meters).

In the United States even when on a filed IFR flight plan, if conditions permit the pilot is responsible to maintain a watch for, and avoid other air traffic and obstructions. Separation may also be referred to as 'protection'.

[edit] Weather

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. To operate safely in IMC , a pilot controls the aircraft relying on flight instruments, and ATC provides separation.

VMC Flying under IFR Rules
VMC Flying under IFR Rules

It is important to not to confuse IFR with IMC. The vast majority 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 other traffic, however, since the flight is conducted under Instrument Flight Rules, ATC still provides separation services.

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 and land; these will vary according to 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 according to 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. This higher alltitude allows a flight crew to start a climb earlier in case landing is not feasible.

Although large airliners and, increasingly, smaller aircraft now carry their own terrain- and collision-avoidance systems such as TCAS, these are primarily backup systems providing a last layer of defense if a sequence of errors or omissions causes a dangerous situation.

[edit] Navigation

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 the primary method for ATC to provide separation between aircraft for landing.

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.

[edit] 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 case of communication failure (lost-comm) with ATC, with default altitudes and headings for every stage. , and any modifications to the route. Here is an example of an IFR clearance, for a Cessna aircraft traveling from Palo Alto airport (KPAO) to Stockton airport (KSCK).


"Cessna 6253G is cleared to Stockton Airport. After departure, turn right heading zero-six-zero within one mile of the airport. Radar Vectors San Jose, Victor-334, SUNOL, Victor-195, Manteca, direct. Climb and maintain 3,000 expect 5,000 five minutes after departure. Departure frequency is 121.3, squawk 4263." Note: Not to be used for Real World Navigation

Detailed explanation:

"Cessna 6253G"
Verifies that only this specific aircraft is cleared.
"is cleared to Stockton Airport."
Clearance Limit: the farthest destination the aircraft is allowed to go under IMC (in most cases it is the destination airport).
"After departure, turn right heading zero-six-zero within one mile of the airport."
The Pilot is expected to execute the turn without further ATC prompting.
"Radar Vectors San Jose"
The departure controller will provide directional guidance to the San Jose VOR.
"Victor-334, SUNOL, Victor-195, Manteca, direct."
After arriving at the San Jose VOR, the pilot will likely resume navigation without ATC prompts along the described airways and intersection to the Manteca VOR and then direct to the destination airport.
"Climb and maintain 3,000 ..."
After takeoff, climb to an altitude of 3000 feet above sea level.
"... expect 5,000 five minutes after departure."
Your final 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 121.3, ..."
Contact with NORCAL Departure on the specified communication frequency, after Palo Alto Tower tells you to switch.
"... squawk 4263."
Program your transponder to 4263 so that ATC can positively identify you on radar.

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 visual conditions sufficient for a normal landing of the airplane, or in a missed approach if such conditions are 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 CDG 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 minimums are not met.[1]

Documents describing the approach procedure are also 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 procedure. 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 weave 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.

[edit] Qualifications

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 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.

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. As a partial alternative to the instrument rating, the UK issues an "IMC rating", the privileges of which include flight under IFR in controlled (classes D and E) airspace and flight outside controlled airspace in instrument meteorological conditions.

[edit] References

  1. ^ For example, Southwest Airlines flies Head Up Display (HUD) equipped Boeing 737 aircraft to fog-prone airports such as Sacramento International (KSMF), allowing flights to take off when they would otherwise be unable to do so.


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