A new FAA policy will require Part 91, 121, 125 and 135 jet pilots never to land where available runway is not at least actual landing distance plus 15 percent. If conditions deteriorate en route, pilots will have to recalculate actual landing distance and the 15-percent safety margin. If the total is more than the available runway length, they will have to land elsewhere. The FAA will make the policy mandatory by issuing OpsSpec/MSpec C082 by June 30. Operators will have until October 1 to comply. This policy derives from the Southwest Airlines 737 runway overrun at Chicago Midway last December. (The FAA failed to mention in the policy notice that, according to the NTSB, the 737 pilots delayed for 17 seconds engaging the thrust reversers and landed with a 10-knot tailwind.) Among the problems the FAA found: about half of aircraft flight manuals (AFM) do not require checking if there is sufficient landing distance at time of arrival, even when conditions deteriorate; operators apply safety margins inconsistently and might not add a percentage to actual runway needed; AFM data developed with test pilots is difficult to duplicate in real life; and wet and contaminated runway data is often calculated from a dry-runway baseline, not using actual performance on wet runways.