What was found
ATB - FLT 3120 - CREW HEARD A LOUD BANG FROM RIGHT SIDE OF AIRCRAFT, THEN NR 4 ENGINE SHUTOFF. UPON RETURN TO LAX, IT WAS DISCOVERED THAT NR 4 ENGINE TURBINE SECTION HAS FAILED AND THE ENGINE HAS INTERNAL DAMAGE. SEVERAL BLADES FOUND DAMAGED DURING BORESCOPE INSPECTION. IN ADDITION, THE ENGINE CASE WAS DAMAGED. REPLACED NR 4 ENGINE AND PERFORMED ALL APPLICABLE INSPECTINS AND MAINTENANCE CHECKS. NO ABNORMALITIES OBSERVED. ENGINE RUN UP SATISFACTORY. (M)