Instrument approach
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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 aids (navaids) used. Precision approaches utilize both lateral (course) and vertical (glideslope) information. Nonprecision approaches provide 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.
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[edit] 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:
- 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 altitude 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. 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.
[edit] 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:
| Category | Decision Height (above threshold) | RVR limit | Manual / Automatic |
|---|---|---|---|
| 1 | 200ft or greater | 550m | Manual or Automatic |
| 2 | between 100ft and 200ft | 350m | Normally Automatic |
| 3a | less than 100ft | 200m | Automatic |
| 3b | less than 100ft | 50m | Automatic |
| 3c | less than 100ft | 0m | Automatic |
Low visibility approaches are those in categories 2 and 3. In most cases all phases of approach and landing are automated and controlled by computers on board the aircraft. 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.
[edit] Precision approaches and systems
- 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)
- JPALS - Joint Precision Approach and Landing System
- GCA - Ground-Controlled Approach (mostly military)
[edit] Nonprecision approaches and systems
- Localizer
- VOR
- ADF or NDB
- GPS - Global Positioning System
- TACAN
- SRA - Surveillance Radar Approach (known in some countries as an ASR approach)
[edit] Terminology
[edit] 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.
[edit] 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.
[edit] 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.
[edit] 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.
[edit] 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 flying a single pilot aircraft, to practice Instrument Approaches in VMC, many pilots enlist a fellow pilot who rides in the right hand seat acting as a Safety Pilot.
