Instrument approach

thumb|300px|An "[[approach plate" depicting an instrument approach procedure for an ILS approach to Tacoma Narrows Airport in the United States]]

In aviation, an instrument approach or instrument approach procedure (IAP) is a series of predetermined maneuvers for the orderly transfer of an aircraft operating under instrument flight rules from the beginning of the initial approach to a landing, or to a point from which a landing may be made visually. These approaches are approved in the European Union by EASA and the respective country authorities, and in the United States by the FAA or the United States Department of Defense for the military. The ICAO defines an instrument approach as "a series of predetermined maneuvers by reference to flight instruments with specific protection from obstacles from the initial approach fix, or where applicable, from the beginning of a defined arrival route to a point from which a landing can be completed and thereafter, if landing is not completed, to a position at which holding or en route obstacle clearance criteria apply."

There are three categories of instrument approach procedures: precision approach (PA), approach with vertical guidance (APV), and non-precision approach (NPA). A precision approach uses a navigation system that provides course and glidepath guidance. Examples include precision approach radar (PAR), instrument landing system (ILS), and GBAS landing system (GLS). An approach with vertical guidance also uses a navigation system for course and glidepath deviation, just not to the same standards as a PA. Examples include baro-VNAV, localizer type directional aid (LDA) with glidepath, LNAV/VNAV and LPV. A non-precision approach uses a navigation system for course deviation but does not provide glidepath information. These approaches include VOR, NDB, LP (Localizer Performance), and LNAV. PAs and APVs are flown to a decision height/altitude (DH/DA), while non-precision approaches are flown to a minimum descent altitude (MDA).

IAP charts are aeronautical charts that portray the aeronautical data that is required to execute an instrument approach to an airport. Besides depicting topographic features, hazards and obstructions, they depict the procedures and airport diagram. Each procedure chart uses a specific type of electronic navigation system such as an NDB, TACAN, VOR, ILS/MLS and RNAV.

Before satellite navigation (GNSS) was available for civilian aviation, the requirement for large land-based navigation aid (NAVAID) facilities generally limited the use of instrument approaches to land-based (i.e. asphalt, gravel, turf, ice) runways (and those on aircraft carriers). GNSS technology allows, at least theoretically, to create instrument approaches to any point on the Earth's surface (whether on land or water); consequently, there are nowadays examples of water aerodromes (such as Rangeley Lake Seaplane Base in Maine, United States) that have GNSS-based approaches.

Instrument approach segments

An instrument approach procedure may contain up to five separate segments, which depict course, distance, and minimum altitude. These segments are

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. The ceiling must be reported or expected to be at least 1000 feet AGL (above ground level) and the visibility is at least 3 SM (statute miles).

When a pilot accepts a visual approach, the pilot accepts responsibility for establishing a safe landing interval behind the preceding aircraft, as well as responsibility for wake-turbulence avoidance, and to remain clear of clouds.

ILS approach

These are the most precise and accurate approaches. A runway with an ILS can accommodate 29 arrivals per hour. This type of approach takes advantage of the runway or more commonly, the oil platform, standing out from its surrounding environment when viewed on a radar. For additional visibility on a radar, radar reflectors may be installed alongside the runway.

Localizer approach

These approaches include a localizer approach, localizer/DME approach, localizer back course approach, and a localizer-type directional aid (LDA). In cases where an ILS is installed, a back course may be available in conjunction with the localizer. Reverse sensing occurs on the back course using standard VOR equipment. With a horizontal situation indicator (HSI) system, reverse sensing is eliminated if it is set appropriately to the front course.

Precision approaches and systems

Precision approach systems provide both lateral (heading) and vertical (glidepath) guidance.

Ground controlled approach (GCA)
GBAS landing system (GLS)
Instrument landing system (ILS)
Joint Precision Approach and Landing System (JPALS)
Microwave landing system (MLS)
Precision approach radar (PAR)
Transponder landing system (TLS)

Basic concepts

===Decision height or altitude===

thumb|Illustration of DA and DH

In a precision approach, the decision height (DH) or decision altitude (DA) is a specified lowest height or altitude in the approach descent at which, if the required visual reference to continue the approach (such as the runway markings or runway environment) is not visible to the pilot, the pilot must initiate a missed approach. The pilot may descend to the MDA, and may maintain it, but must not descend below it until visual reference is obtained, and must initiate a missed approach if visual reference has not been obtained upon reaching the missed approach point (MAP).

DH/DA, the corresponding parameter for precision approach, differs from MDA in that the missed approach procedure must be initiated immediately on reaching DH/DA, if visual reference has not yet been obtained: but some overshoot below it is permitted while doing so because of the vertical momentum involved in following a precision approach glide-path.

If a runway has both non-precision and precision approaches defined, the MDA of the non-precision approach is almost always greater than the DH/DA of the precision approach, because of the lack of vertical guidance on the non-precision approach. The extra height depends on the accuracy of the navaid the approach is based on, with ADF approaches and SRAs tending to have the highest MDAs.

All published minimums assume full operation of all components and visual aids. When any component is malfunctioned, the minimums increase. If more than one component is inoperative, the minimums are raised to the highest minimum required by any single inoperative component.

Rate-of-descent formula

A useful formula pilots use to calculate descent rates (for the standard 3° glide slope):

: Rate of descent = (ground speed ⁄ 2) × 10

: Rate of descent = ground speed × 5

For other glideslope angles:

: Rate of descent = glide slope angle × ground speed × 100 / 60,

where rate of descent is in feet per minute, and ground speed is in knots.

The latter replaces tan α (see below) with α/60, which has an error of about 5% up to 10°.

Example:

120 kn × 5

120 kn / 2 × 10

= 600 ft/min

The simplified formulas above are based on a trigonometric calculation:

: Rate of descent = ground speed × 101.27 × tan α

where:

α is the descent or glideslope angle from the horizontal (3° being the standard)
101.27 (ft/min⁄kn) is the conversion factor from knots to feet per minute (1 knot = 1 NM⁄h ≈ 6076 ft⁄h ≈ 101.27 ft/min)

Example:

Ground speed = 120 kn

α = 3°

120 kn × 101.27ft/min/kn × tan 3°

≈ 640 ft/min

Airport requirements

Special considerations for low visibility operations include improved lighting for the approach area, runways, and taxiways, and the location of emergency equipment. There must be redundant electrical systems so that in the event of a power failure, the back-up takes over operation of the required airport instrumentation (e.g., the ILS and lighting). ILS critical areas must be free from other aircraft and vehicles to avoid multipathing.

In the United States, the requirements and the standards for establishing instrument approaches at an airport are contained in the FAA Order 8260.3 "United States Standard for Terminal Instrument Procedures (TERPS)". ICAO publishes requirements in the ICAO Doc 8168 "Procedures for Air Navigation Services – Aircraft Operations (PANS-OPS), Volume II: Construction of Visual and Instrument Flight Procedures".

Mountain airports such as Reno–Tahoe International Airport (KRNO) offer significantly different instrument approaches for aircraft landing on the same runway, but from opposite directions. Aircraft approaching from the north must make visual contact with the airport at a higher altitude than a flight approaching from the south, because of rapidly rising terrain south of the airport. This higher altitude allows a flight crew to clear the obstacle if a landing is not feasible. In general, each specific instrument approach specifies the minimum weather conditions that must be present in order for the landing to be made.

Cold temperature airports in colder climates and high altitudes are subject to barometric altimeter errors and require cold temperature error corrections.

Under the performance-based navigation (PBN) framework, many instrument approaches are published as RNAV (GNSS), RNP, or LPV procedures rather than traditional ground-based navaid approaches. These designs use GNSS, SBAS, and in some cases baro-VNAV to provide lateral and vertical guidance with obstacle protection comparable to conventional precision systems. RNP AR approaches, which include authorization-required curved paths and radius-to-fix (RF) legs, are used at airports with challenging terrain or airspace constraints and require specific aircraft capabilities and crew training.

References

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)

External links

Flight Crew Guide – Precision approach – Category I operations
Flight Crew Guide – Precision approach – Category II operations
Flight Crew Guide – Precision approach – Category III operations
Flight Crew Guide – Non-precision approach