There are two main classifications for IAPs: precision and nonprecision. A third type of approach, available only to IFR flights but not considered a true instrument approach, is discussed below. Precision approaches utilize both lateral (localizer) and vertical (glideslope) information. Nonprecision approaches provide lateral course information only.[2]
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 list missed approach procedures and commonly-used radio frequencies.
In the past, the requirement for large land-based navigation aid (NAVAID) facilities has generally limited the use of instrument approaches to land-based (i.e. asphalt, gravel, turf, ice) runways (including those aboard aircraft carriers). However, recent advances in GPS approach technology have permitted the creation of instrument approaches at water aerodromes such as Rangeley Lake Seaplane Base in Maine, USA.
Contents
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.An instrument approach procedure may have as many as four separate segments depending on how the approach procedure is structured.[1] These segments include:
- Initial approach: The segment between the initial approach fix (IAF) and the intermediate fix (IF), or the point where the aircraft is established on the intermediate course or final approach course.
- Intermediate Approach: The segment between the IF or point, and the final approach fix (FAF).
- Final approach: The segment between the FAF or point, and the runway, airport, or missed approach point (MAP).
- Missed approach: The segment between the MAP or the point of arrival at decision height and the missed approach fix at the prescribed altitude.
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 | Visibility |
|---|---|---|---|
| I | 200 ft | 550m or 2400 ft (1200 ft is approved at some airports)[3] | 800m |
| II | 100 ft | 350m or 1200 ft | N/A |
| IIIa | DH < 100 >50 ft | 200m | N/A |
| IIIb | DH <50Ft to No DH | 75m | N/A |
| IIIc | No DH | No RVR | N/A |
Smaller aircraft will generally only be equipped to fly a cat I ILS.
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.
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 manufacturers 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 close 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.
For all cat II or III approaches, 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 IIIc approaches are rare, but category IIIb 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
- ILS - Instrument landing system
- MLS - Microwave Landing System
- PAR - Precision Approach Radar (Military)
- 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)
- GLS - GNSS Landing System
Nonprecision approaches and systems
- Localizer
- VOR
- Non-Directional Beacon or NDB with complementary Automatic Direction Finder (ADF) installed on board
- Localizer Type Directional Aid or LDA
- Simplified Directional Facility or SDF
- GPS (with or without vertical navigation via WAAS, EGNOS, or other signal correction systems) - Global Positioning System
- TACAN
- SRA - Surveillance Radar Approach (known in some countries as an ASR approach)
- ASR - Airport Surveillance Radar (Military designation for SRA)
Concepts in detail
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, such as the runway or runway environment, 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 altitude
The minimum descent altitude (MDA) is the lowest altitude (in feet MSL) to which descent is authorized on final approach, or during circle-to-land maneuvering in execution of a nonprecision approach. [4] Unlike a DH, a missed approach need not be initiated immediately upon reaching the altitude; that decision can be made at any time before reaching the missed approach point (MAP). A pilot flying a non-precision approach may descend to the MDA and maintain it until reaching the 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 in that while the missed approach procedure must be initiated at or prior to the DH, because of its vertical momentum, during a precision approach an aircraft may end up descending slightly below the DH 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 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.
Reverse Sensing
Reverse sensing is an instrument error only associated with the localizer. It occurs when the aircraft flies a "localizer back course" approach. The CDI in the aircraft will cease to be function as a command instrument and instead will read the opposite of what the pilot is to fly; that is, the CDI will command you to fly left when the aircraft in fact needs to fly right to intercept the approach course, and vice versa. Reverse sensing is not a factor in aircraft equipped with a Horizontal Situation Indicator (HSI).Multipathing
Multipathing is the second error associated with the ILS. In this case, unlike reverse sensing, it is associated with both the glideslope and the localiser. This occurs when distortions of the glide slope or radio waves are received by the aircraft. Distorted signals can reach the aircraft when a large metal object moves into the radiation zone of the transmitter. For instance an aircraft ahead of you or a taxiing aircraft or truck on the taxi way will produce a distorted signal.Reversal procedure
If conducting approach, and the pilot is not lined up for a Straight-in Approach, then a course reversal may be necessary. This can be used on either a VOR, ADF or ILS approach. The idea of a reversal is to turn the aircraft around by 180 degrees therefore lining the aircraft up with the runway. This will line the aircraft up for final, and can be accomplished in three different ways: Procedure Turn, Holding Pattern, or Teardrop Course Reversal.- Procedure turn: Standardized way of reversing course to get lined up on final approach. The approach chart must indicate that a procedure turn is authorized for the approach, via a procedure turn barb. Note that when a procedure turn exists for an approach, the maximum speed of the aircraft should never exceed 200 knots IAS. This is typically entered by tracking the localizer outbound, and then turning 45 degrees off of the localizer. After that, the pilot extends the leg, and conducts a roughly 180-degree turn, depending on the wind, and then flying back toward the localizer path and reintercepting it, so the pilot can track the localizer inbound.
- Holding pattern: commonly referred to as the racetrack pattern. It is another method of course reversal, but it can also be used for losing altitude within protected airspace. Only one circuit within the holding pattern is approved. ATC must be advised if the pilot needs more circuits, and ATC must approve the request prior to doing so. A holding pattern used for this purpose will be depicted in U.S. Government publications as the "hold-in-lieu-of-PT" holding pattern symbol.
- Teardrop procedure turn: If the controlled airspace is extremely limited, a teardrop may be used to reverse the direction of the aircraft and permit the aircraft to lose altitude. This procedure is shaped like a teardrop, hence the name. The approach chart, usually the profile view section, will give limitations as to how far you can get from the VOR. So, this method of course reversal is all about perception. The pilot must use timing, given a known airspeed, or DME to remain within the limits described on the approach chart.
Direct Approach
A Direct instrument approach requires no procedure turn or other reversal course procedures for alignment (NoPT), as the arrival direction coincides with the final approach course. The direct approach can be finished with a straight-in landing or circle to land procedure, if necessary and published.Circling To Land
A circle to land maneuver is the opposite of 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 of descent (or more) 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 for the aircraft to become aligned with the runway to land.It's very common for a circle to land maneuver 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 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 to land is considered more difficult and less safe than a straight-in landing, especially under Instrument meteorological conditions, due to the fact that the aircraft is at a relatively low altitude and must remain within a small distance from the airport in order to be assured of obstacle clearance (often only a couple of miles, even for larger, faster aircraft). In any case, the pilot must maintain visual contact with the runway at all times - loss of visual contact must result in an immediate climb to the published safe altitude.
Instrument Currency
In some countries [1] 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 will be required if the pilot practicing instrument approach wears 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. Logging instrument approaches toward license currency has long been an area of much confusion. The regulations have been revised and rewritten many times to meet the needs of pilots.[2]Back Course Approach
A back course approach is a type of approach in which a pilot flies the localizer on the opposite (back) side from the original direction it was primarily designed to be flown. Usually, when one flies a front course approach, the shaded side of the localizer will be on the right on an approach plate. However, if flying a back course approach, the shaded side of the localizer would be on the left, due to the back course heading. By flying the back course, the Course Deviation Indicator (CDI) needle will deflect to the opposite side, depending on what type of equipment exists in the aircraft. If the needle starts to move away from center, the aircraft would be flown away from the needle in order to re-intercept the correct inbound track; turning toward the needle, such as is required on a front course, would cause the aircraft to deviate further from the correct inbound track.The localizer transmits on both sides, making this approach possible. Because the glide slope is not transmitted on the back side of the localizer, a back course approach is classed as a non-precision approach as it has no vertical guidance. Any (false) movements of the glideslope needle 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 the older localizer antennas are less directional. 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.
Simultaneous close parallel approaches
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 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.
Visual Approach
A visual approach is a precision approach carried out using visual references to the runway, when weather conditions permit. 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. 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 3 statute miles) before issuing the clearance. 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.
Visual approaches are very commonly used for IFR flights at some airports that routinely experience good visual meteorological conditions.
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