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 visual 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 (localizer) 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 list 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:
- 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.
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 |
|---|---|---|
| I | greater than 200 ft | 550m or 1800 ft |
| II | less than 200 ft | 350m or 1200 ft |
| IIIa | less than 100 ft | 150m - 200m (see below) |
| IIIb | less than 50 ft | 75m - 150m (see below) |
| IIIc | No DH | No RVR |
Cat III mimima depend on Roll Out Control & Redundancy of the Autopilot.
Low visibility approaches are those in categories II and III.
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 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)
- 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)
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 - Global Positioning System
- TACAN
- SRA - Surveillance Radar Approach (known in some countries as an ASR approach)
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 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 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. An aircraft must not descend below the MDH until visual reference is obtained, which differs from a DH in that an aircraft may descend below DH without visual reference so long as the missed approach procedure was initiated at or prior to the DH. For example, with a DH of 500ft AMSL, it is legal for a pilot to allow the aircraft to descend to 450ft AMSL if the missed approach procedure was initiated at or prior to 500ft. This would not be legal during a non-precision approach with a MDH of 500ft. This difference is due to the presence of vertical guidance during a precision approach, and thus terrain clearance near DH being less of an issue than near MDH during a non-precision 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 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 instrument approach is one where the final approach is begun without first having executed a procedure turn, not necessarily completed with a straight-in landing or made to straight-in landing minimums.
Circling To Land
A circle to land maneuver is a maneuver used when a runway is not aligned to within 30 degrees of the track 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.
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 because 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
- FAAO 7110.65: Pilot/Controller Glossary
- FAA Instrument Procedures Handbook
- FSF ALAR Briefing Note 7.2 - Constant Angle Nonprecision Approach Flight Safety Foundation
Audio and Multimedia Resources
- Audio and commentary of a full-procedure RNAV (GPS) approach into Flint Bishop International Airport (KFNT)
- Audio of a US instrument rating checkride - Part 1 (including RNAV 18 at KFNT)
- Audio of a US instrument rating checkride - Part 2 (including VOR 9 at KFNT partial panel and the ILS 9R at KPTK)
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