Expert: D. Norkus - 12/19/2006

Question
why is it that airports (particularly commercial ones) are often closed due to fog when modern planes are so capable of landing "blind" at night due to the spophisticated computers in use nowadays.

Answer
Not all airplanes or airports are "so capable". Landing "blind" is not really totally blind for most aircraft in most instances. To execute a totally automated touchdown in zero forward visibility not only must the airports approach be a Catagory 3/"CATIIIc" certified "ILS/Instrument Landing System", the aircraft must be equipped to use it (CATIIIc certified aircraft/autopilot) and the crew must also be trained and have a recency of experience to do the approach.

CATEGORY I/II/III approaches refer to different levels of low visibility ILS approaches. CAT I is the least severe at 1/2 mile or 1800 RVR/runway visual range (a measuremt of runway visibility via transmissomitters near the runway.) CATIIIc is a zero/zero ILS requiring and autoland capable aircraft.

Catagory - Ceiling - Visibility:
CAT I 200 feet 1/2 mile
CATII 100 feet 1/4 mile
CATIIIa 50 feet Runway Visual Range - 1200 feet
CATIIIb 50 feet Runway Visual Range - 700 feet
CATIIIc 0 0 (zero/zero)


Airplanes with CATIIIc capability have at least two, and usually three autopilots that operate together during an autoland so if one fails there is a backup to get the airplane down. Each of the autopilots has its own dedicated inputs (i.e. power sources, control positions sensors, attitude and directional data, etc) to reduce the possibility of a single failure in the system that would result in bad things. If any one factor is missing it cannot be done. Additionally, most aircraft are not CATIIIc certified for zero/zero landings. Many are CAT II or even CATI certified, so in foggy conditions below their landing criteria, they will not be able to even initiate an approach to the airfield per Federal Aviation Regulations. In instances where this is the case, that is why the airport is "closed" to those flights, even if the airport does have a CATIIIc ILS.

Read all about an ILS and how it works here- http://en.wikipedia.org/wiki/Instrument_Landing_System

Hope that helps
Dottie