WASHINGTON, July 15 (Reuters) - The Federal Aviation Administration (FAA) on Thursday issued a directive to operators of all Boeing (BA.N) 737 series airplanes to conduct inspections to address possible failures of the cabin altitude pressure switches. The directive requires operators to conduct repetitive tests of the switches and replace them if needed. The directive covers 2,502 U.S.-registered airplanes and 9,315 airplanes worldwide. It was prompted after an operator reported that on three different 737 models both pressure switches failed the on-wing functional test.