Editing Autopilot (section)

== Autopilot for ILS landings ==

[[Autoland|Instrument-aided landings]] are defined in categories by the [[International Civil Aviation Organization]], or ICAO. These are dependent upon the required visibility level and the degree to which the landing can be conducted automatically without input by the pilot.

* '''CAT I''' – This category permits pilots to land with a [[Instrument approach#Decision height or altitude|decision height]] of {{convert|200|ft}} and a forward visibility or Runway Visual Range (RVR) of {{convert|550|m}}. Autopilots are not required.<ref>{{cite web|title=Aeronautical Information manual|url=http://www.faa.gov/air_traffic/publications/atpubs/AIM/Index.htm|website=faa.gov|publisher=FAA|access-date=16 June 2014}}</ref>
* '''CAT II''' – This category permits pilots to land with a decision height between {{convert|200|ft}} and {{convert|100|ft}} and a RVR of {{convert|300|m}}. Autopilots have a fail passive requirement.
* '''CAT IIIa''' – This category permits pilots to land with a decision height as low as {{convert|50|ft}} and a RVR of {{convert|200|m}}. It needs a fail-passive autopilot. There must be only a 10<sup>−6</sup> probability of landing outside the prescribed area.
* '''CAT IIIb''' – As IIIa but with the addition of automatic roll out after touchdown incorporated with the pilot taking control some distance along the runway. This category permits pilots to land with a decision height less than 50 feet or no decision height and a forward visibility of {{convert|250|ft}} in Europe (76 metres, compare this to aircraft size, some of which are now over {{convert|70|m}} long) or {{convert|300|ft}} in the United States. For a landing-without-decision aid, a fail-operational autopilot is needed. For this category some form of runway guidance system is needed: at least fail-passive but it needs to be fail-operational for landing without decision height or for RVR below {{convert|100|m}}.
* '''CAT IIIc''' – As IIIb but without decision height or visibility minimums, also known as "zero-zero". Not yet implemented as it would require the pilots to taxi in zero-zero visibility. An aircraft that is capable of landing in a CAT IIIb that is equipped with [[autobrake]] would be able to fully stop on the runway but would have no ability to taxi.

Fail-passive autopilot: in case of failure, the aircraft stays in a controllable position and the pilot can take control of it to go around or finish landing. It is usually a dual-channel system.

Fail-operational autopilot: in case of a failure below alert height, the approach, flare and landing can still be completed automatically. It is usually a triple-channel system or dual-dual system.