Instrument Landing System - ILS Categories

ILS Categories

There are three categories of ILS which support similarly named categories of operation. Information below is based on ICAO, FAA and JAA; certain states may have filed differences.

• Category I (CAT I) – A precision instrument approach and landing with a decision height not lower than 200 feet (61 m) above touchdown zone elevation and with either a visibility not less than 800 meters or 2400 ft or a runway visual range not less than 550 meters (1,800 ft) on a runway with touchdown zone and runway centerline lighting .
• Category II (CAT II) – A precision instrument approach and landing with a decision height lower than 200 feet (61 m) above touchdown zone elevation but not lower than 100 feet (30 m), and a runway visual range not less than 350 meters (1,150 ft) (ICAO and FAA) or 300 meters (980 ft) (JAA).
• Category III (CAT III) is subdivided into three sections:
  • Category III A – A precision instrument approach and landing with:
    • a) a decision height lower than 100 feet (30 m) above touchdown zone elevation, or no decision height (alert height); and
    • b) a runway visual range not less than 200 meters (660 ft).
  • Category III B – A precision instrument approach and landing with:
    • a) a decision height lower than 50 feet (15 m) above touchdown zone elevation, or no decision height (alert height); and
    • b) a runway visual range less than 200 meters (660 ft) but not less than 50 meters (160 ft) (ICAO and FAA) or 75 meters (246 ft) (JAA).
  • Category III C – A precision instrument approach and landing with no decision height and no runway visual range limitations. This category is not yet in operation anywhere in the world, as it requires guidance to taxi in zero visibility as well. "Category III C" is not mentioned in EU-OPS. Category III B is currently the best available system.

In contrast to other operations, CAT III weather minima do not provide sufficient visual references to allow a manual landing to be made. The minima only permit the pilot to decide if the aircraft will land in the touchdown zone (basically CAT III A) and to ensure safety during rollout (basically CAT III B). Therefore an automatic landing system is mandatory to perform Category III operations. Its reliability must be sufficient to control the aircraft to touchdown in CAT III A operations and through rollout to a safe taxi speed in CAT III B (and CAT III C when authorized). However, special approval has been granted to some operators for hand-flown Cat III approaches using "heads up display" (HUD) guidance which provides the pilot with an image viewed through the windshield with eyes focused at infinity, of necessary electronic guidance to land the airplane with no true outside visual references.

FAA Order 8400.13D allows for special authorization of CAT I ILS approaches to a decision height of 150 feet (46 m) above touchdown, and a runway visual range as low as 1,400 feet (430 m). The aircraft and crew must be approved for CAT II operations, and a heads-up display in CAT II or III mode must be used to the decision height. CAT II/III missed approach criteria applies.

In Canada, the required RVR for carrying out a Cat I approach is 1600 ft, except for certain operators meeting the requirements of Operations Specification 019, 303 or 503 in which case the required RVR may be reduced to 1200 ft.

In the United States, many but not all airports with CAT III approaches have listings for CAT IIIa, IIIb and IIIc on the instrument approach plate (U.S. Terminal Procedures). CAT IIIb runway visual range minimums are limited by the runway/taxiway lighting and support facilities, and would be consistent with the airport Surface Movement Guidance Control System (SMGCS) plan. Operations below 600 runway visual range require taxiway centerline lights and taxiway red stop bar lights. If the CAT IIIb runway visual range minimums on a runway end were 600 feet (180 m), which is a common figure in the U.S., ILS approaches to that runway end with runway visual range below 600 feet (180 m) would qualify as CAT IIIc and require special taxi procedures, lighting and approval conditions to permit the landings. FAA Order 8400.13D limits CAT III to 300 runway visual range or better. Order 8400.13D, which was released during 2009, also allows special authorization CAT II approaches to runways without ALSF-2 approach lights and/or touchdown zone/centerline lights, which has expanded the number of potential CAT II runways.

In each case, a suitably equipped aircraft and appropriately qualified crew are required. For example, CAT IIIb requires a fail-operational system, along with a crew who are qualified and current, while CAT I does not. A head-up display which allows the pilot to perform aircraft maneuvers rather than an automatic system is considered as fail-operational. CAT I relies only on altimeter indications for decision height, whereas CAT II and CAT III approaches use radar altimeter to determine decision height.

An ILS is required to shut down upon internal detection of a fault condition. With the increasing categories, ILS equipment is required to shut down faster, since higher categories require shorter response times. For example, a CAT I localizer must shutdown within 10 seconds of detecting a fault, but a CAT III localizer must shut down in less than 2 seconds.