Editing Instrument landing system (section)

===Special CAT II and CAT III operations===
[[File:ILS CAT II III.JPG|thumb|right|[[Taxiway signs]] indicating the ILS category of a runway as CAT II/III]]
In contrast to other operations, CAT III weather minima do not provide sufficient visual references to allow a manual landing to be made. CAT IIIb minima depend on roll-out control and redundancy of the autopilot,{{citation needed|date=June 2011}} because they give only enough time for the pilot to decide whether the aircraft will land in the touchdown zone (basically CAT IIIa) and to ensure safety during rollout (basically CAT IIIb). Therefore, an [[Autoland|automatic landing system]] is mandatory to perform Category III operations. Its reliability must be sufficient to control the aircraft to touchdown in CAT IIIa operations and through rollout to a safe taxi speed in CAT IIIb (and CAT IIIc when authorized).<ref name="AMC and GM to Part-SPA">{{cite web |url= https://www.easa.europa.eu/sites/default/files/dfu/Annex%20to%20ED%20Decision%202012-019-R.pdf |title= Acceptable Means of Compliance (AMC) and Guidance Material (GM) to Part-SPA |work= Annex to ED Decision 2012-019-R |date= 25 October 2012 |publisher= EASA |access-date= 21 July 2018 |archive-date= 21 July 2018 |archive-url= https://web.archive.org/web/20180721221607/https://www.easa.europa.eu/sites/default/files/dfu/Annex%20to%20ED%20Decision%202012-019-R.pdf |url-status= dead }}</ref> However, special approval has been granted to some operators for hand-flown CAT III approaches using a [[head-up display]] (HUD) guidance that 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.

In the United States, airports with CAT III approaches have listings for CAT IIIa and IIIb or just CAT III on the instrument approach plate (U.S. Terminal Procedures). CAT IIIb RVR minimums are limited by the runway/taxiway lighting and support facilities, and are consistent with the airport [[surface movement radar|surface movement guidance control system]] (SMGCS) plan. Operations below 600&nbsp;ft RVR require taxiway centerline lights and taxiway red stop bar lights. If the CAT IIIb RVR minimums on a runway end are {{convert|600|ft|m}}, which is a common figure in the U.S., ILS approaches to that runway end with RVR below {{convert|600|ft|m}} 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&nbsp;ft RVR or better. Order 8400.13D (2009) 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 HUD that allows the pilot to perform aircraft maneuvers rather than an automatic system is considered as fail-operational. A HUD allows the flight crew to fly the aircraft using the guidance cues from the ILS sensors such that if a safe landing is in doubt, the crew can respond in an appropriate and timely manner. HUD is becoming increasingly popular with "feeder" airlines and most manufacturers of regional jets are now offering HUDs as either standard or optional equipment.{{citation needed|date=June 2011}} A HUD can provide capability to take off in low visibility.

[[File:CAT IIIA Landing.jpg|thumb|[[Cockpit]] view of a CAT IIIa landing ([[autoland]])]]
Some commercial aircraft are equipped with automatic landing systems that allow the aircraft to land without transitioning from instruments to visual conditions for a normal landing. Such [[autoland]] operations require specialized equipment, procedures and training, and involve the aircraft, airport, and the crew. Autoland is the only way some major airports such as [[Charles de Gaulle Airport]] remain operational every day of the year. Some modern aircraft are equipped with [[Head-up display#Enhanced flight vision systems|enhanced flight vision systems]] based on infrared sensors, that provide a day-like visual environment and allow operations in conditions and at airports that would otherwise not be suitable for a landing. Commercial aircraft also frequently use such equipment for takeoffs when ''takeoff minima'' are not met.<ref>For example, Southwest Airlines flies HUD equipped Boeing 737 aircraft to fog-prone airports such as [[Sacramento International Airport|Sacramento International]] (KSMF), allowing flights to take off when they would otherwise be unable to do so.</ref>

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 additional safety requirements for operating an aircraft close to the ground and the ability of the flight crew to react to a system anomaly. The equipment also has additional maintenance requirements to ensure that it is capable of supporting reduced visibility operations.

Nearly all of this pilot training and qualification work is done in simulators with various degrees of fidelity.