What was found
AC RETURNED THIRTY MINUTES AFTER TAKEOFF, THE (RT OIL STRAINER CLOGGING) LIGHT CAME ON. CAPTAIN INFORMED THAT THE ENGINE WAS NOT TURNED OFF AND IT WAS PUT AT FLIGHT IDLE POWER. AT PLATFORM THE MAIN FILTER ASSY AND THE CHIP DETECTORS WERE INSPECTED, THE FILTER WAS FOUND CONTAMINATED WITH COAL. THE FILTER WAS CLEANED AND THE PROCEDURE OF FILTER PRESS SENSING WAS PERFORMED IAW MM. AT STATION THE MAIN OIL FILTER, DIFFERENTIAL PRESSURE ALARM, RESTRICTORS P5 AND P6 AND FLIGHT DATA RECORDER WERE REPLACED IAW MM, TESTS OK, ALL CORRECT.