Hi Folks
Ill give you the lowdown on Autoland.
The Categories refer to the operational requirements not directly to the aircraft autopilot.
CAT 1 = This is an IFR approach with a Min Decent Altitude of no less than 200ft. and a Runway Visual range of 1800ft. This is a manual Landing.
CAT 2 = This is a precision approach with a DH=100ft and a RVR=1200ft.
CAT 3 is split into 3
3a is a Precision Approach with no DH minima and RVR not less than 700ft. The aircraft must have Fail passive or Fail operational Autopilot.
3b is a Precision Approach with RVR less than 200ft but not less than 150ft and a DH of 50ft or less. Aircraft must have a Fail operational autopilot system.
3c Is designed for Zero-Zero approaches no RVR limit and no DH limit. 3c operations are currently not authorized!
Now as for the aircraft,
Fail Passive, this means if one Autopilot channel or one or more sensors to the A/P (EG Radio Alt) fails the A/P will continue the approach safely with out massive deviation, but the landing would have to be carried out manually.
Fail operational means where one channel or input fails the aicraft can continue to Autoland, but it now becomes fail passive. A futher subsequent failure will mean a manual land.
The 747-400 i work on has a fail operational autopilot system, it has 3 autopilots and the inputs are all triplicate too. with everything ok all 3 autopilots engauged on the approach it is said to be fail operational, CAT 3b, if somthing were to fail it would downgrade to land 2 and become fail passive max CAT3a.
As for the 737 it has 2 autopilots this means its fail passive and will be CAT3a
BA 737-400 CAT 3 Placard on flight deck
http://www.!!!!!!!!!!!!!!/open.file/264226/M
Stuart.