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For the aircraft operating under instrument flight rules (IFR), an instrument approach or instrument approach procedure (IAP) is a series of predetermined maneuvers for the orderly transfer of an aircraft under instrument meteorological conditions (IMC) from the beginning of the initial approach to a landing, or to a point from which a landing may be made visually. The concept was also commonly known as a blind landing or blind approach when first introduced, although these terms are no longer common.
There are two main classifications for IAPs: precision and non-precision. Precision approaches utilize both lateral (localizer) and vertical (glideslope) information. Non-precision approaches provide lateral course information only.
Publications depicting instrument approach procedures are called Terminal Procedures, but are commonly referred to by pilots as approach plates. These documents depict the specific procedure to be followed by a pilot for a particular type of approach to an airport. They depict prescribed altitudes and headings to be flown, as well as obstacles, terrain, and potentially conflicting airspace. They list missed approach procedures and commonly used radio frequencies.
Before GNSS was available for civilian aviation, the requirement for large land-based navigation aid ("navaid") facilities generally limited the use of instrument approaches to land-based (i.e. asphalt, gravel, turf, ice) runways (and those on aircraft carriers). GNSS technology allows, at least theoretically, to create instrument approaches to any point on the Earth's surface (whether on land or water); consequently, there are nowadays examples of water aerodromes (such as Rangeley Lake Seaplane Base in Maine, USA) that have both land navaid-based as well as GNSS-based approaches.
Instrument approaches are generally designed such that a pilot of an aircraft in IMC, by the means of radio, GPS, or INS navigation with no assistance from air traffic control, can navigate to the airport, hold in the vicinity of the airport if required, then fly to a position from where he or she can obtain sufficient visual reference of the runway for a safe landing to be made, or execute a missed approach if the visibility is below the minimums required to execute a safe landing. The whole approach is defined and published in this way so that aircraft can land if they suffer from radio failure; it also allows instrument approaches to be made procedurally at airports where air traffic control does not use radar or in the case of radar failure.
An instrument approach procedure may contain up to five separate segments (some of which are mandatory). These segments are:
When an aircraft is under radar control, air traffic control (ATC) may replace some or all of these phases of the approach with radar vectors (the provision of headings on which the controller expects the pilot to navigate his aircraft) to allow traffic levels to be increased over that which is possible when following the full procedures. It is very common for ATC to vector aircraft to intercept the final approach navaid's course, e.g., the ILS, which is then used for the final approach. In case of the rarely used ground-controlled approach (GCA), the instrumentation (normally Precision Approach Radar) is on the ground and monitored by a controller, who then relays precise instructions for adjustment of heading and altitude to the pilot in the approaching aircraft.
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Most instrument approaches allow for landing in conditions of low visibility. See "Airport requirements" section. There are air traffic control considerations with low visibility approaches. There must be longer gaps between aircraft on final approach both to protect the ILS signal and to cope with slower runway vacating times.
A decision height (DH) or decision altitude (DA) is a specified lowest height or altitude in the approach descent at which a missed approach must be initiated if the required visual reference to continue the approach, such as the runway markings or runway environment, has not been acquired. This allows the pilot sufficient time to safely re-configure the aircraft to climb and execute the missed approach procedures while avoiding terrain and obstacles.
The minimum descent altitude (MDA) is the lowest altitude (relative to MSL) to which descent is authorized on final approach, or during circle-to-land maneuvering in execution of a non-precision approach. Unlike with DH or DA, a missed approach need not be initiated immediately upon reaching the altitude. A pilot flying a non-precision approach may descend to the MDA and maintain it until reaching the missed approach point (MAP), then initiate a missed approach if the required visual reference was not obtained. An aircraft must not descend below the MDA until visual reference is obtained, which differs slightly from a DH/DA in that while the missed approach procedure must be initiated at or prior to the DH/DA, because of its vertical momentum, during a precision approach an aircraft may end up descending slightly below the DH/DA during the course of the missed approach.
If a runway has both precision and non-precision approaches defined, the MDA of the non-precision approach is almost always greater than the DH/DA of the precision approach, because of the lack of vertical guidance on the non-precision approach: the actual difference depends on the accuracy of the navaid upon which the approach is based, with ADF approaches and SRAs tending to have the highest MDAs.
A direct instrument approach requires no procedure turn or any other course reversal procedures for alignment (usually indicated by "NoPT" on approach plates), as the arrival direction and the final approach course are not too different from each other. The direct approach can be finished with a straight-in landing or circle-to-land procedure.
When conducting any type of approach, if the aircraft is not lined up for a straight-in approach, then a course reversal might be necessary. The idea of a course reversal is to change course by 71° or more to line the aircraft up with the final approach course. This is accomplished in one of three ways: a procedure turn, a holding pattern, or a teardrop course reversal.
A back course approach is a type of approach in which a pilot flies the localizer's course guidance on the opposite (back) side from the original direction it was primarily designed to be flown. A front course approach is usually depicted by the right half of the localizer symbol being shaded on the approach plate. A back course approach is depicted by the left half of symbol being shaded, as well as by clear indication (typically, in capitalized words) that the approach uses localizer for the non-standard use (namely for the back course approach). When flying a back course, the course deviation indicator (CDI) needle deflects to the opposite side with certain types of equipment. This reverse sensing means that the CDI in the aircraft indicates the opposite of what the pilot is expecting on a standard localizer approach (or during course tracking using VORs); that is, the CDI indicates to fly left when the aircraft in fact needs to fly right to intercept the approach course, and vice versa. Using CDI, if the needle moves away from center, the aircraft should be flown from the needle toward the center in order to re-intercept the correct inbound track. (Turning toward the needle, such as done on a front course, causes the aircraft to deviate further from the correct inbound track.) Reverse sensing does not occur on a horizontal situation indicator (HSI), which gives correct course guidance during both front-course and back-course approaches.
The unshielded localizer transmits in both directions to give course guidance. Because the glide slope is not transmitted on the back side of the localizer, a back course approach is classified as a non-precision approach as it has no vertical guidance. The glideslope indications during a back course approach must always be ignored.
This type of approach typically is found at smaller airports that do not have ILS approaches on both ends of the runway, where often citation needed] These transmit a signal from the back that is sufficient enough to be used in a back course approach. Newer localizer antennas are highly directional, and often cannot be used for a back course approach.[
At some airports, multiple parallel runways are available for operations, but are so closely spaced (less than 4300 feet between centerlines) that they present a hazard for simultaneous use under ordinary conditions. Simultaneous operations on such runways can be carried out using ILS and special precision runway monitor (PRM) radars and three controllers, with special procedures known as simultaneous close parallel approaches.
In this type of approach, two aircraft approach and land simultaneously on closely spaced parallel runways, with extra air traffic controllers assigned to monitor each approach path on special PRM radar. A zone between the runways is designated as the no-transgression zone (NTZ), and if either of the aircraft nears or strays into this zone, the other approaching aircraft is told to break off by the PRM controller, at which point that aircraft must veer away from the approach path (without the use of autopilot). The aircraft must have two radios, one tuned to the tower controller in the usual way, and another tuned (for monitoring only, no transmission) to the PRM controller.
If runways are less than 3000 feet apart but at least 750 feet apart, simultaneous offset instrument approaches (SOIAs) may be used. The procedure is similar to that described above, except that one aircraft flies the ILS/PRM approach, and the other flies an offset LDA/PRM approach at an angle to the runway centerline. The aircraft flying the LDA/PRM approach with glide path is positioned to be behind the ILS/PRM aircraft, and must have the ILS/PRM aircraft in sight before beginning a visual segment to the approach at or before the missed approach point. During the visual segment, the LDA/PRM aircraft must keep the ILS/PRM aircraft in sight as it aligns with the centerline of the runway.
A visual approach is an approach carried out using visual references to the runway, when weather conditions permit. It is officially not considered a non-precision approach. While it is not an instrument approach in the strict sense, visual approach clearances are issued only to IFR flights (because VFR flights must always approach and land visually).
A visual approach may be requested by the pilot or offered by ATC. Visual approaches are possible when weather conditions permit continuous visual contact with the destination airport. They are issued in such weather conditions in order to expedite handling of IFR traffic.
A pilot may accept a visual approach clearance as soon as he has the destination airport in sight. According to ICAO Doc.4444 it is enough for a pilot to see the terrain to accept a visual approach. The point is that if a pilot is familiar with the terrain in the vicinity of the airflield he/she may easily find the way to the airport having the surface in sight. ATC must ensure that weather conditions at the airport are above certain minima (in the U.S., a ceiling of 1000 feet AGL or greater and visibility of at least 3 statute miles) before issuing the clearance. According to ICAO Doc.4444 it is enough if the pilot reports that in his/her opinion the weather conditions allow a visual approach to be made. In general the ATC gives the information about the weather but it's the pilot who makes a decision if this weather is suitable for landing. Once the pilot has accepted the clearance, he assumes responsibility for separation and wake turbulence avoidance and may navigate as necessary to complete the approach visually. According to ICAO Doc.4444 ATC continues to provide separation between the aircraft making a visual approach and other arriving and departing aircraft. The pilot may get responsible for the separation with preceding aircraft in case he/she has the prededing in sight and instructed so by ATC.
Visual approaches are very commonly used for IFR flights at some airports that routinely experience good visual meteorological conditions. However, at some airports visual approaches are not allowed for environment protection reasons as low altitude flights over populated areas produce high level of noise.
A circle-to-land maneuver is an alternative to a straight-in landing. It is a maneuver used when a runway is not aligned within 30 degrees of the final approach course of the instrument approach procedure or the final approach requires 400 feet (or more) of descent per nautical mile, and therefore requires some visual maneuvering of the aircraft in the vicinity of the airport after the instrument portion of the approach is completed to align the aircraft with the runway for landing.
It is very common for a circle-to-land maneuver to be executed during a straight-in approach to a different runway, e.g., an ILS approach to one runway, followed by a low-altitude pattern flying, ending in a landing on another runway. This way, approach procedures to one runway can be used to land on any runway at the airport, as the other runways might lack instrument procedures or their approaches cannot be used for other reasons (traffic considerations, navigation aids being out of service, etc.).
Circling to land is considered more difficult and less safe than a straight-in landing, especially under instrument meteorological conditions because the aircraft is at a low altitude and must remain within a short distance from the airport in order to be assured of obstacle clearance (often within a couple of miles, even for faster aircraft). The pilot must maintain visual contact with the airport at all times; loss of visual contact requires execution of a missed approach procedure.
Pilots should be aware that there are significant differences in obstacle clearance criteria between procedures designed in accordance with ICAO PANS-OPS and US TERPS. This is especially true in respect of circling approaches where the assumed radius of turn and minimum obstacle clearance are markedly different.
A useful formula pilots use to calculate descent rates (for the standard 3° glide slope):
For other glideslope angles:
where rate of descent is in feet per minute, and ground speed is in knots.
The latter replaces tan α (see below) with α/60, which has an error of about 5% up to 10°.
120 kt × 5 or 120 kt / 2 × 10 = 600 fpm
The above simplified formulas are based on a trigonometric calculation:
Ground speed = 250 kt α = 4.5° 250 kt × 101.27fpm/kt × tan 4.5° = 1993 fpm
Special considerations for low visibility operations include improved lighting for the approach area, runways, and taxiways, and the location of emergency equipment. There must be redundant electrical systems so that in the event of a power failure, the back-up takes over operation of the required airport instrumentation (e.g., the ILS and lighting). ILS critical areas must be free from other aircraft and vehicles to avoid multipathing.
In the United States, the requirements and the standards for establishing instrument approaches at an airport are contained in the FAA Order 8260.3 "United States Standard for Terminal Instrument Procedures (TERPS)". ICAO publishes requirements in the ICAO Doc 8168 "Procedures for Air Navigation Services – Aircraft Operations (PANS-OPS), Volume II: Construction of Visual and Instrument Flight Procedures".
Mountain airports such as Reno-Tahoe International Airport (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.