LOS ANGELES (AP) -- Federal aviation regulators, reacting to a December 2005 accident in which a Southwest Airlines Co. jet skidded off the end of a snowy Chicago runway, have issued new landing policies aimed at preventing such accidents.

Two weeks before a scheduled public hearing about the accident, the Federal Aviation Administration has called for more stringent and uniform pilot calculations to ensure an adequate safety margin when landing jetliners on snowy or slippery runways. The FAA wants at least a 15 percent safety margin when computing landing distances in such adverse weather conditions.

The document released earlier this week says an agency investigation prompted by the Boeing Co. 737 accident found that about half of U.S.

carriers lacked clear policies "for assessing whether sufficient landing distance" is available when operating on wet or snowy runways. The FAA says it also found that airlines use "inconsistent" safety margins when computing landing distances in such circumstances, including some calculations conflicting with data provided by aircraft manufacturers.

The upcoming hearing by the National Transportation Safety Board is expected to be critical of both the FAA and Southwest. Issues expected to be raised at the hearing include inadequate runway-condition reports often transmitted to pilots, lack of uniform standards in determining safe runway-braking distances and confusion about pilot procedures to account for slippery strips. The 2005 accident killed a six-year-old boy in a car that was struck as the Southwest plane skidded off the end of a runway at Chicago's Midway Airport in the middle of a snow storm.

The policies have broad impact on airlines because they call for using "the most adverse expected conditions" when assessing required stopping distances, and then requiring a 15 percent safety margin before a landing is attempted. By October, according to the FAA policy document, the agency will put out specific operating and training requirements.

Under the new procedures, dispatchers will be required to have specific instruction in landing distances required in adverse weather conditions. And pilots will be required to become familiar with such factors as part of their routine flight training.