What was found
(CAN) WHEN ENG POWER REDUCED FOR DESCENT, NR 1 ENGINE EXPERIENCED SIGNIFICANT VIBRATION/COMPRESSOR STALLING. CABIN AIR SUPPLY VALVE (WHICH IS AT P2.5 ENG STATION) FAILED IN OPEN POS WHICH ALLOWED P3 COMPRESSOR AIR TO ENTER COMPRESSOR AT LOWER STAGE & CREATE STALL CONDITION (WHEN ENGINE POWR REDUCED). COMPRESSOR STALLING EXPERIENCED ON 3 SEPARATE EVENTS BEFORE BEING CORRECTLY DIAGNOSED. DURING 1ST EVENT, ENG EXHIBITED SIGNIFICANT VIBRATION & TORQUE FLUXS, WHICH REQD ENGINE TO UNDERGO INFLT SHUTDOWN. OTHER 2 EVENTS WERE CONTROLLED IN FLT WITH REDUCTION OF POWER & DID NOT REQUIRE ENG TO BE SHUTDOWN. REPLACED OF CABIN AIR SUPPLY PNEUMATIC VALVE RECTIFIED PROBLEM.