Four civilian crewmen were missing and presumed killed when two Learjets chartered by the Navy for radar training exercises collided about 27 miles southeast of San Clemente Island. A Navy spokesman said two crew members were aboard each of the planes, which collided in clear weather at an altitude of 2,000 feet. Petty Officer Rex Kramer said the two small jets were being tracked by personnel on the Gridley, a San Diego-based guided-missile cruiser, as part of a routine training mission. Navy ships and helicopters found debris at the crash site but found no survivors. A spokesman for Western Flight International Inc., owner of the two jets, said all four victims were residents of San Diego County. Their names were not immediately released.