Metal disks similar to the one used in the engine that blew up before a United Airlines DC-10 crashed in Sioux City, Iowa, are being recalled for examination from six unidentified commercial airliners, a government spokesman said. Investigators are seeking to determine the quality of the metal used in making the three-foot-diameter disks, which hold the fan blades of the General Electric engines, said National Transportation Safety Board spokesman Ted Lopatkiewicz. The investigators believe the fan section of the engine disintegrated in flight, possibly leading to hydraulic system failure, which is believed to have caused the jet to crash July 19, killing 111. The disk from Flight 232 has not been found, but it and the parts from the six airliners are believed to have come from a billet, or slab, of titanium used in 1970 at G.E.'s jet engine plant in Evendale, Ohio.