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Instrument approach
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Terminal procedures for an ILS approach.
An instrument approach or instrument approach procedure (IAP) is a type of air navigation that allows pilots to land an aircraft in weather restricting visibility (known as instrument meteorological conditions or IMC), or to reach visual conditions permitting a normal landing.
Approaches are classified as either precision or nonprecision, depending on the accuracy and capabilities of the navigational aids (navaids) used. Precision approaches utilize both lateral (course) and vertical (glideslope) information. Nonprecision approaches provide course information only.
Pilots refer to charts known as terminal procedures or approach plates during an approach, both precision and nonprecision. These depict the navaid's approach path and radio frequencies used in addition to landmarks, airspace, and other relevant data.
Contents[hide]
1 Basic principles
2 Precision approaches
3 Nonprecision approaches
4 Terminology
4.1 Decision Altitude
4.2 Minimum Descent Altitude
4.3 Straight-in Approach
4.4 Circling Approach
5 References
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Basic principles
Instrument approaches are generally designed such that a pilot of an aircraft in instrument meteorological conditions (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. The whole of the 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.
Instrument approaches generally involve five phases of flight:
Arrival: where the pilot navigates to the Initial Approach Fix (IAF: a navaid or reporting point), and where holding can take place.
Initial Approach: the phase of flight after the IAF, where the pilot commences the navigation of the aircraft to the Final Approach Fix (FAF), a position aligned with the runway, from where a safe controlled descent back towards the airport can be initiated.
Intermediate Approach: an additional phase in more complex approaches that may be required to navigate to the FAF.
Final Approach: between 4 and 12 nms of straight flight descending at a set rate (usually an angle of between 2.5 and 6 degrees).
Missed Approach: an optional phase; should the required visual reference for landing not have been obtained at the end of the final approach, this allows the pilot to climb the aircraft to a safe level and navigate to a position to hold for weather improvement or from where another approach can be commenced.
When aircraft are under radar control, air traffic controllers 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 the final approach, to allow traffic levels to be increased over those of which a fully procedural approach is capable. It is very common for air traffic controllers to vector aircraft to the final approach aid, e.g. the ILS, which is then used for the final approach.
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Precision approaches
ILS - Instrument Landing System
MLS - Microwave Landing System
PAR - Precision Approach Radar (Military)
GPS (with vertical navigation via WAAS or EGNOS) - Global Positioning System
LAAS - Ground Based Augmentation System (GBAS) for Global Satellite Navigation Systems (GNSS)
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Nonprecision approaches
Localizer
VOR
NDB
GPS
TACAN
SRA - Surveillance Radar Approach
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Terminology
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Decision Altitude
A decision altitude (DA) or decision height (DH) is a specified altitude or height in the precision approach at which a missed approach must be initiated if the required visual reference to continue the approach has not been established. DA is referenced to mean sea level [MSL], while DH is referenced to the runway threshold elevation. Conducting an approach, the pilot must decide whether either to land or to go around. The latest the decision can be safely made is at the specified DH or DA. The altitude specified allows the pilot enough time to safely re-configure the aircraft to climb and to climb to a safe altitude avoiding obstacles.
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Minimum Descent Altitude
A minimum descent altitude (MDA) or minimum descent height (MDH) is the equivalent of the DA/DH for non-precision approaches, however there are some significant differences. It is the level below which a pilot making such an approach must not allow his or her aircraft to descend unless the required visual reference to continue the approach has been established. The significant difference compared to a DA is that a missed approach need not be initiated once the aircraft has descended to this level: in non-precision approaches the point at which a missed approach must be initiated is defined as a separate point known as the missed approach point (MAPt). Thus, in non-precision approaches, a pilot may descend to the minimum descent altitude and, having not gained visual reference, fly level at the MDA attempting to gain visual reference until the MAPt is reached, at which point a missed approach must be initiated if the required visual reference to continue the approach has not been obtained.
If a runway has both precision and non-precision approaches defined, the MDA of the non-precision approach is almost always greater than the DA of the precision approach, due to the lack of vertical guidance of the non-precision approach: the actual difference will also depend on the accuracy of the navaid upon which the approach is based, with NDB approaches tending to have the highest MDAs.
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Straight-in Approach
An approach where the track of the instrument approach procedure is aligned to within 15 degrees of the runway heading, therefore allowing aircraft to land easily after making the approach.
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Circling Approach
A circling approach is an instrument approach to a runway which is not aligned to within 15 degrees of the track of the instrument approach procedure, and therefore requires some visual maneuvering of the aircraft in the vicinity of the airport after the instrument portion of the approach is completed for the aircraft to become aligned with the runway to land.
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References