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An instrument approach or '''instrument approach procedure''' (IAP) is a type of Air Navigation that allows Pilots to Land an Aircraft in reduced 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 Aid s (''navaids'') used. Precision approaches utilize both lateral (course) and vertical ( Glideslope ) information. Nonprecision approaches provide lateral course information only. The publications depicting instrument approach procedures are called Terminal Procedures, but are commonly referred to by pilots as " Approach Plates ". These documents graphically depict the specific procedure to be followed by a pilot for a particular type of approach to a given runway. They depict prescribed altitudes and headings to be flown, as well as obstacles, terrain, and potentially conflicting airspace. In addition, they also listed missed approach procedures and commonly-used radio frequencies. 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, or execute a missed approach if the visibility is below the minimums required to execute a safe landing. 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:
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. In the 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. LOW VISIBILITY APPROACHES Many instrument approaches allow for landing in conditions of low visibility. ICAO classifies ILS approaches as being in one of the following categories: Low visibility approaches are those in categories 2 and 3. For larger aircraft it is typical that these approaches are under the control of the flight control system with the flight crew providing a supervisory role. Traditionally smaller aircraft which lacked redundancy in the flight control systems could not fly these approaches. (Imagine a radio getting a glitch at the moment of flare which causes the airplane to "think" that a large correction is required. The result would, most likely, be a sudden turn which at low altitude would be catastrophic.) A Head-Up Display allows the flight crew to fly the aircraft using the guidance cues from the ILS sensors so that if such a large deviation were seen, the pilot would be able to respond in an appropriate and safe manner. This is becoming increasingly popular with "feeder" airlines and most manufactures of regional jets are now offering HUDs as either standard or optional equipment. In addition a HUD can provide a low visibility take off capability. For both automatic and HUD landing systems, the equipment requires special approval for its design and also for each individual installation. The design takes into consideration all of the additional safety requirements for operating an aircraft in close proximity to the ground and takes into consideration the ability of the flight crew to react to a "system anomaly." Once installed, the equipment also has additional maintenance requirements to ensure that it is fully capable of supporting reduced visibility operations. In all cases, additional crew training is required for such approaches, and a certain number of low visibility approaches must either be performed or simulated in a set period of time for pilots to stay 'current' in performing them. For practical reasons Category 3c approaches are rare, but category 3b approaches are relatively common at major airports. There are also air traffic control considerations with low visibility approaches: when using ILS, the integrity of the signal must be protected, which requires that certain areas of the airport close to the installations being free of other aircraft and vehicles. Also there must be bigger gaps between aircraft on final approach to both protect the ILS signal and to cope with slower runway vacation times. In addition, the airport itself has special considerations for low visibility operations including different lighting for approach, runways, and taxiways as well as the location of emergency equipment. PRECISION APPROACHES AND SYSTEMS
NONPRECISION APPROACHES AND SYSTEMS
TERMINOLOGY Decision Height or Altitude A decision height (DH) or decision altitude (DA) is a specified Height or Altitude in the Precision Approach at which a Missed Approach must be initiated if the required visual reference to continue the approach 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. Minimum Descent Height or Altitude A minimum descent height (MDH) or minimum descent altitude (MDA) is the equivalent of the DH/DA 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. Unlike a DH/DA, a missed approach need not be initiated once the aircraft has descended to the MDH, that decision can be deferred to the Missed Approach Point (MAP). So a pilot flying a non-precision approach may descend to the minimum descent altitude and maintain it until reaching the MAP, then initiate a missed approach if the required visual reference was not 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 ADF approaches and SRAs tending to have the highest MDAs. Straight-in Approach A straight-in approach is an approach where the track of the instrument approach procedure is aligned to within 30 degrees of the runway heading (15 degrees for GPS approaches), therefore allowing aircraft to land easily after making the approach. Circling Approach A circling approach is an instrument approach to a runway which is not aligned to within 30 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. It's very common for circling approaches to use 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 a different runway. This way, approach procedures to one runway can be used to land on any runway at the airport, as the other runways may lack instrument procedures or their approaches cannot be used for other reasons (traffic considerations, navigation aids being out of service, etc). Circling approach is considered more difficult and less safe than straight-in approach, especially under Instrument Meteorological Conditions . Instrument Currency In some countries Instrument Rated Pilots are required to perform a minimum number of instrument approaches in a set period to remain current. Pilots may also have to fly a certain number of low visibility approaches (Cat 2 or Cat 3) to remain current at performing these. When practicing instrument approaches in Visual Meteorological Conditions , a Safety Pilot is required. This is due to the fact that the pilot practicing instrument approach must wear a view limiting device, which restricts his field of view to the instrument panel. A safety pilot's basic role is to observe and help to avoid traffic. AIRPORT REQUIREMENTS The requirements for an airport to offer instrument approaches is contained in FAA Order 8200.97 AIRMAN AND AIRCRAFT APPROVAL FOR REDUCED VISIBILITY FLIGHT OPERATIONS, INCLUDING CATEGORY II/III OPERATIONS. REFERENCES
AUDIO AND MULTIMEDIA RESOURCES Audio and commentary of a full-procedure RNAV approach into Flint Bishop International Airport (KFNT) Notes and approach plate for the audio above }}}}}} |
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