What was found
ON DEPARTURE, LOST A-SYSTEM HYDRAULIC PRESSURE, TURNED BACK AND LANDED WITHOUT FURTHER EVENT. AIRCRAFT TOWED BACK TO GATE. MAINTENANCE FOUND THE NR 1 ENGINE DRIVEN HYDRAULIC PUMP HAD FAILED, REMOVED AND REPLACED PUMP. A-SYSTEM HYDRAULICS FLUSHED, OPERATIONAL CHECK WAS GOOD. VENDOR TEARDOWN OF ENGINE DRIVEN HYDRAULIC PUMP S/N 183862 CONFIRMED FAILURE OF UNIT. THRUST BEARING FAILED ON INTERNAL DRIVE SHAFT CAUSING DAMAGE TO THE CYLINDER BLOCK BEARING. PISTON SHOES WERE ALSO FOUND WORN BEYOND LIMITS. REPLACED O-RINGS, BACK-UP SEALS, BEARINGS, PISTON SHOE ASSEMBLIES, STROKING PISTON, INTERNAL & EXTERNAL DRIVE SHAFT, THRUST BEARING RETAINER, ROTATING GROUP ASSEMBLY, AND CYLINDER BLOCK. UNIT WAS BENCH TESTED IAW CMM