Pilots should not include the effect of thrust reversers when computing safe landing distances for jet aircraft, the NTSB said last week. Calculations tend to assume the thrust reversers will deploy immediately, when in practice there can be a lag of several seconds. The difference can be critical, especially when runway conditions are poor, the NTSB said. The thrust-reverser credit was used when calculating the safe landing distance required for a Southwest 737-7H4 that landed at Chicago Midway Airport on Dec. 8, during a snow storm. The airplane ran off the runway, crashed through a fence, and hit two cars, killing a 6-year-old boy. If the thrust-reverser credit had not been employed, the calculations would have shown that a safe landing was not possible, the NTSB said. "We believe this recommendation needs the immediate attention of the FAA since we will be experiencing winter weather conditions in many areas of our nation for several more months to come," NTSB Acting Chairman Mark Rosenker said. The board asked the FAA to immediately prohibit all Part 121 operators from using the reverse thrust credit in landing performance calculations. Although the recommendation would prohibit the thrust-reverser credit on all runways, its practical effect would be felt on planned landings only on contaminated runways, which is when the credit is included in stopping distance calculations.