Updated from 8:09 a.m. ESTDelta Air Lines' ( DAL ) has a new president at the helm of its Comair unit, following the regional airline carrier's Christmas travel debacle. Fred Buttrell has replaced Randy Rademacher, who resigned suddenly late Monday after five years in the post, according to media reports. Comair canceled hundreds of flights around the Christmas holiday after severe weather stressed its infrastructure and its computer system for flight-crew scheduling crashed, triggering an investigation by the U.S. Transportation Department. The Cincinnati-based regional airline normally operates 1,160 flights a day, serving 119 cities. Delta shares were recently up 24 cents, or 3.9%, to $6.37.