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45-493: The primary responsibility of Denver Center is sequencing and separation of over-flights, arrivals, and departures to provide safe, orderly, and expeditious flow of aircraft filed under instrument flight rules ( IFR ). Denver Center is the 14th busiest ARTCC in the United States. Between January 1, 2017 and December 31, 2017, Denver Center handled 1,828,575 aircraft. Denver Center covers approximately 285,000 square miles of

90-503: 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

135-566: 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;

180-569: 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 visual meteorological conditions (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 ( marginal visual flight rules ), IFR, or LIFR (low instrument flight rules). The main purpose of IFR

225-546: 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

270-448: 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

315-921: 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 . From 1999 single-engine helicopters could not be FAA-certified for IFR. Recently, however, Bell and Leonardo have certified the single engine helicopters for instrument flight rules. Instrument meteorological conditions In aviation , instrument meteorological conditions ( IMC ) are weather conditions that require pilots to fly primarily by reference to flight instruments , and therefore under instrument flight rules (IFR), as opposed to flying by outside visual references under visual flight rules (VFR). Typically, this means flying in cloud or poor weather, where little or nothing can be seen or recognised when looking out of

360-573: 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

405-478: Is an important factor in the VMC minima: as aircraft flying in clouds cannot be seen, a buffer zone from clouds established by the minimum separation requirements provides for time to react to an unseen/unknown aircraft exiting the clouds, especially when air traffic control may not be enforcing aircraft separation (as in airspace classes C-G). IMC should not be confused with IFR ( instrument flight rules ) – IMC describes

450-429: Is conducted in visual meteorological conditions (VMC). Anytime a flight is operating in VMC and in a volume of airspace in which VFR traffic can operate, the crew is responsible for seeing and avoiding VFR traffic; however, because the flight is conducted under instrument flight rules, ATC still provides separation services from other IFR traffic, and can in many cases also advise the crew of the location of VFR traffic near

495-456: 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. 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

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540-473: Is in the units of measurement as different regulatory authorities use different units of measurement in aviation. The VMC minima tend to be stricter in controlled airspace, where there is increased air traffic, therefore greater visibility and cloud clearance is desirable. The degree of separation provided by air traffic control is also a factor. For example, in strictly-controlled class A and B airspace , where all aircraft are provided with positive separation,

585-422: 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

630-447: 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. IFR flights in controlled airspace require an ATC clearance for each part of

675-484: 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

720-438: Is possible to be flying under VFR in conditions that are legally considered VMC, but still be forced to rely on flight instruments for attitude control because there is no distinct external horizon; for example, at night over water, which may create a so-called black hole effect if the sky and ground are equally dark, or when lights on the water cannot be distinguished from stars in the sky. If weather deteriorates during flight or

765-483: 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

810-501: 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

855-974: The Western United States , including all or parts of Colorado, Arizona, New Mexico, Utah, Kansas, Nebraska, South Dakota, Wyoming, and Montana. Denver Center is adjacent to five other Air Route Traffic Control Centers: Minneapolis Air Route Traffic Control Center , Kansas City Air Route Traffic Control Center , Albuquerque Air Route Traffic Control Center , Los Angeles Air Route Traffic Control Center , and Salt Lake City Air Route Traffic Control Center . The Denver Center overlies or abuts several approach control facilities (including Denver, Aspen, Colorado Springs, Pueblo, Grand Junction, Casper, Cheyenne, and Ellsworth AFB in Rapid City). 40°11′14″N 105°07′37″W  /  40.1873°N 105.1270°W  / 40.1873; -105.1270 IFR In aviation , instrument flight rules ( IFR )

900-532: 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

945-584: The Federal Aviation Administration indicate that spatial disorientation is a factor in approximately 15% of general aviation accidents; of those, approximately 90% are fatal. Other statistics indicate that 4% of general aviation accidents were attributable to weather; of those weather-related accidents, 50% resulted from VFR into IMC, and 72% of the VFR into IMC accidents were fatal. In the 180—Degree Turn Experiment conducted in 1954 by

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990-407: 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

1035-556: The University of Illinois, twenty student pilots flew from VFR into simulated IMC; after entry, all of them eventually reached a dangerous flight condition or attitude over a period ranging from 20 to 480 seconds. The average time to reach a dangerous condition was 178 seconds, echoed in the title of the "178 Seconds to Live" article distributed by the Federal Aviation Administration in 1993; however,

1080-496: The VMC minima feature visibility limits only, whereas in classes C–G airspace, where some or all aircraft are not separated from each other by air traffic control, the VMC minima also feature cloud separation criteria. With good visibility, pilots can determine the aircraft attitude by utilising visual cues from outside the aircraft, most significantly the horizon . Without such external visual cues, pilots may be subject to sensory illusions and must use an alternative reference for

1125-437: The actual weather conditions, while IFR describes the rules under which the aircraft is flying. Aircraft can (and often do) fly IFR in clear weather, for operational reasons or when flying in airspace where flight under VFR is not permitted; for example, in the United States, flight under VFR in class A airspaces is prohibited except in emergencies. Indeed by far the majority of commercial flights are operated solely under IFR. It

1170-432: The aircraft flies into clouds, a flight that started out under VFR may turn into a flight under IMC. This is known as inadvertent entry into instrument meteorological conditions (IIMC), or more briefly VFR into IMC . IIMC is a potentially dangerous situation that has resulted in many accidents, as pilots may succumb to spatial disorientation , leading to loss of control or controlled flight into terrain . Statistics from

1215-407: The aircraft operates in VMC using a hybrid of VFR and IFR rules, 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. Also possible in many countries is "Special VFR" flight, where an aircraft is explicitly granted permission to operate VFR within the controlled airspace of an airport in conditions technically less than VMC;

1260-424: 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. For example, Reno-Tahoe International Airport (KRNO) in a mountainous region has significantly different instrument approaches for aircraft landing on

1305-428: The attitude, which is usually provided by gyroscopically-driven instruments such as the attitude indicator ("artificial horizon"). The availability of a good horizon cue is controlled by meteorological visibility, hence minimum visibility limits feature in the VMC minima. Because the basic traffic avoidance principle of flying under visual flight rules (VFR) is to "see and avoid", it follows that distance from clouds

1350-409: 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

1395-571: 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

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1440-448: The flight path. Although dangerous and illegal, a certain amount of VFR flying is conducted in IMC. A scenario is a VFR pilot taking off in VMC conditions, but encountering deteriorating visibility while en route. Continued VFR flight into IMC can lead to spatial disorientation of the pilot which is the cause of a significant number of general aviation crashes. VFR flight into IMC is distinct from "VFR-on-top", an IFR procedure in which

1485-511: 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

1530-451: 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

1575-453: The instrument panel. A safety pilot's primary duty is to observe and avoid other traffic. In the UK, an IR (UK restricted) - formerly the "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

1620-428: 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

1665-422: The minima specified for visual meteorological conditions. Conditions that are above VMC minima but relatively close to one or more of them are sometimes referred to as marginal VMC , and flight in such conditions is referred to as marginal VFR . ICAO recommends the VMC minima internationally; they are defined and enforced by national regulations, which rarely significantly vary from ICAO. The typical variation

1710-435: 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

1755-416: The original 1954 study was noted for simulating an aircraft the subjects had little to no experience with, and only providing a partial instrument panel. In addition, the "178 seconds" average time was extracted from the preliminary evaluation; after training for a standardized procedure to exit IMC, each student pilot was tested three times, and 59 of the 60 resulting simulated flights successfully resulted in

1800-450: The pilot asserts they have the necessary visibility to fly despite the weather, must stay in contact with ATC, and cannot leave controlled airspace while still below VMC minimums. 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

1845-437: 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

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1890-615: The same runway surface, but from opposite directions. Aircraft approaching from the north must make visual contact with the airport at a higher altitude than when 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 aborted. In general, each specific instrument approach specifies the minimum weather conditions to permit landing. Although large airliners, and increasingly, smaller aircraft, carry their own terrain awareness and warning system (TAWS), these are primarily backup systems providing

1935-453: 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

1980-477: The window. Simulated IMC can be achieved for training purposes by wearing view-limiting devices , which restrict outside vision and force the trainee to rely on instrument indications only. The weather conditions required for flight under VFR are known as visual meteorological conditions (VMC). The boundary criteria between VMC and IMC are known as VMC minima . IMC and VMC are mutually exclusive. In fact, instrument meteorological conditions are defined as less than

2025-582: 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

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