Expert: D. Norkus - 12/21/2006

Question
Why can't planes take off and land in heavy fog?

Answer
Damon

They can, with certain criteria met. With certain minimum visual referances, planes can take off in fog. Landing is another matter. Of course, certain criteria must also be met for landing as well.

The Federal Aviation Regulations, Part 121.651 set out the rules for 'takeoff and landing weather minimums' for airlines. You can read FAR 121.651 here if you like-
http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&sid=fa4f9ae84013129b8aaff12...

There are several different levels of bad weather approach catagories. Depending on how an aircraft is equipped, they can only execute the highest level for which they are equipped.

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)

There are some jets that can do totally automated landings in very dense fog, but they are the exception more than the norm. 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.
Due to the time and expense needed versus the actual times this can be used many airlines chose not to install it.

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