What was found
ATB - AIR TURN BACK DUE TO STRONG METALIC ODOR, FROM PRIMARALY #1 AND #2 DOOR. COULD ALSO SMELL AT #3 ORDER AND LESS AT # 4 DOOR. BOROSCOPE APU FIRST STAGE COMPRESSOR. CHECK OK. INSPECTED IP CHECK VALVE ON ENG #1 & #2, NO DEFECTS FOUND. PER AMM 36-11-41-200-804-A. FORWARD AND AFT AIR COND RECIRCULATION FITERS REPLACED IAW AMM 21-21-41. OPERATIONAL CHECK GOOD. PERFORMED PACK BURN OUT PER AMM 21-00-00. OK FOR SERVICE.