Topic Author
Posts: 57
Joined: Mon May 31, 1999 1:40 am

Catagory I, II,and III Landings

Thu Nov 04, 1999 7:21 am

Could somebody please explain the difference between Catagory I, II, and III landings. I understand a lot about ILS and how it works, just trying to get the terminology down.

Posts: 1066
Joined: Fri Mar 16, 2007 1:29 pm

RE: Catagory I, II,and III Landings

Thu Nov 04, 1999 8:48 am

The difference isn't so much in "the landings" as in the onboard and surface based equipment, procedures followed and level of training of the flight crew.

Cat II surface based equipment requires a more precisely powered and monitored ILS (LOC/GS), more stringent sterile areas in the vicinity of the localizer beam (hence the Cat II/III) hold lines vs the normal hold lines), a speciallized approach lighting system, amongst other things, and special training for the flight crew.

Cat III requires enhanced onboard equipment including multiple autopilots, autothrottle, and automatically switchable power systems.

Visibility restrictions are variable from 1/2 mile on most Cat I approaches to zero RVR in some countries (USA I believe is min 300' RVR).

That's only the "nutshell" version fom a pilot's point of view - hope it gives you the idea.

Best Regards,


RE: Catagory I, II,and III Landings

Thu Nov 04, 1999 9:14 am

These are the different classifications of ILS landings depending on the airport you're landing and its surroundings. A category I allows a decision height of 200 ft minimum with a visual range of around 600 meters. A cat. II has a decision height of not less than 100 ft and a visual range of 400 meters. A category III has a reduced visual range of 200 meters. In other words, the higher the cat. is, the lesser visual range of the rwy is needed, and more dependant on the instruments you become, which permits landings in difficult weather. Category III is located at larger airports or where fog is constantly present. Also, the higher it gets, the greater need of both ground and airborne equipment such as the markers, transmitters, lights etc.
Posts: 448
Joined: Thu Jul 29, 1999 8:12 am

RE: Catagory I, II,and III Landings

Thu Nov 04, 1999 1:39 pm

To add to what's already posted: CAT III is sub-divided into IIIa, IIIb, and IIIc. These refer to the decision height / visibility at the effected airport, as well as the aircraft. For example: Delta's MD11s are CATIIIc, because they have enough redundant systems and power sources to perform an auto-land in Zero / Zero weather (0 visibility at 0 altitude). If, say, one of the three autopilots were to fail, the aircraft would be restricted to CATIIIb landings untill the problem was repaired and the aircraft sucessfully completed a full auto-land (in good weather). Any modern autopilot is capable of performing at least CATIIIa, but the aircraft may be a limiting factor. When I first hired on at Delta, we had two 737-200s leased from Guiness Pete (EI-BON and EI-BOM, if anyone cares). They were limited to CAT III a due to a lack of autopilot feed back input instrumentation, among other things.

CAT IIIc also refers to airport equipped with runway, taxiway, and gate 'guidance systems'. So far as I know, only Heathrow is CATIIIc active. This system consists of a wire buried in the runways, taxiways, and ramps, which act as an antenna for the signal telling the aircraft where on the airport it is. Technically, the aircraft rolls out, turns off, and taxis right to the gate in 0/0 conditions under autopilot commands. All the pilot has to do is set the brakes and shut down the engines!