For 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 flight conditions from the beginning of the initial approach to a landing, or to a point from which a landing may be made visually.[1]
There are two main classifications for IAPs: precision and non-precision. 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. Non-precision 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.
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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:
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.
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 | 550 m or 2400 ft (1200 ft is approved at some airports)[3] | 800 m |
| II | 100 ft | 350 m or 1200 ft | N/A |
| IIIa | 50 ft < DH < 100 ft | 200 m | N/A |
| IIIb | 0 < DH < 50 ft | less than 200 m but not less than 75 m | N/A |
| IIIc | No DH | No RVR | N/A |
CAT III minima depend on roll-out control and redundancy of the autopilot. 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 (HUD) 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 area, runways, and taxiways as well as the location of emergency equipment.
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.
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 non-precision 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 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 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..
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.
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.
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 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.
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.[5]
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 II or CAT III) 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.
In the United States, to maintain IFR currency, a pilot must have flown 6 instrument approaches and holds within the last 6 months. After this period there is a grace period of another 6 months, that allows flight with a safety pilot; after that, examination by an instructor is required.
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.
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.
A visual approach is a nonprecision 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.
In the United States, the requirements for an airport to offer instrument approaches are contained in FAA Order 8200.97 AIRMAN AND AIRCRAFT APPROVAL FOR REDUCED VISIBILITY FLIGHT OPERATIONS, INCLUDING CATEGORY II/III OPERATIONS.
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