Instrument flight rules
Instrument flight rules permit an aircraft to operate in instrument meteorological conditions (IMC), which have much lower weather minimums than 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.
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.
Confusing flight rules with weather conditions
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.
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 onradar, 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.
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 due to spatial disorientation. 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 United States.
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 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, 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 altitude allows a flight crew to clear the obstacle if a landing is not feasible. In general, each specific instrument approach specifies the minimum weather conditions that must be present in order for the landing to be made.
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.
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 navigation systems are available to pilots, including ground-based systems such as DME/VORs and NDBs as well as the satellite-based GPS system. In addition, air traffic control may assist in navigation by assigning pilots specific headings to fly, known as “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, amongst other things.
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.
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 3,000 expect 5,000 five minutes after departure. Departure frequency is 121.3, squawk 4263.”|
- “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° 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 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.
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 andnon-precision. A precision approach is one in which both vertical and lateral guidance are given, such as in an ILS or WAAS (LPV)approach. A non-precision approach is one in which only lateral guidance is given, such as a VOR, NDB, or Localizer approach. Due to the fact that they provide vertical guidance, the weather criteria required for precision approaches is less restrictive than it is for non-precision approaches.
In either case, an instrument approach will terminate either in a) the pilot acquiring sufficient visual reference to carry out a normal landing, or in b) 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 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). Documents describing the approach procedure are frequently called approach plates in reference to single-page sheet that it is printed on.
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 Airportremain 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.
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 pilot can elect to either attempt the approach again or proceed to an alternate airport. In practice (esp. when flying in radar environment) the missed portion of an approach is rarely flown as published. Instead, ATC will usually assign headings and altitudes that will assimilate the aircraft into the arriving traffic for a repeated approach attempt.
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 has 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 suitableavionics 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.