What was found
AT FL 390-410 ACRID BURNING TYPE SMELL COMMING FROM THE PACKS. UNABLE TO DETERMINE WHICH ONE. BACK TO NORMAL ON DESCENT. ACCOMPLISHED TROUBLSHOOTING IAW FIM REF 21-00-01/301. INSPECTED CABIN RECIRCULATION SYSTEM & ACCOMPLISHED ADJ /TEST OF CABIN RECIRCULATION FAN IAW 21-25-00/501. NO DEFECTS NOTED. INSPECTED LT & RT PACKS FOR OIL CONTAMINATION IAW AMM 21-00-01/201 NO CONTAMINATION FOUND IN EITHER PACK SYSTEMS. OPS LT & RT PACK SYSTEMS FOR 1 HR EACH IAW MM 21-00-00-862-001. NO ACRID ODOR DETECTED.