Marc CYBW wrote:
http://www.washingtonpost.com/wp-dyn...121001562.html


SWA and ATA had FAA waivers concerning minimum visibilities:
http://www.chicagotribune.com/news/l...,4692055.story

The pilots of Southwest Airlines Flight 1248 were permitted to attempt
a landing in extremely bad weather at Midway Airport because the
low-cost carrier holds a special FAA waiver to operate below regular
minimum-visibility safety regulations, federal officials said
Wednesday.
....
Among the 10 airlines serving Midway, Southwest and ATA Airlines have
been granted waivers from the Federal Aviation Administration allowing
planes to land when visibility is below three-fourths of a mile--the
established standard on Runway 31 Center, where the Southwest accident
occurred.

Pilots must be specially trained and log a required number of landings
with a certified pilot before receiving their own certification
allowing them to land at Midway with only one-half mile visibility,
called a "31Z approach."