AN ACR MD88 FLC EXPERIENCES A VIBRATION IN THE R JT8D-219 ENG WHILE INFLT. THE VIBRATION WAS VERIFIED BY MAINT AND BY OTHER FLCS.

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ENRTE; CRUISE/CLB FL310 CLBING TO FL350. FELT VIBRATION IN FLOOR BOARDS AND HEARD RUMBLE AS ENGS ACCELERATED THROUGH 80 PERCENT N1. TURNED OFF AUTOTHROTTLES AND MOVED EACH THROTTLE SEPARATELY TO TROUBLESHOOT WHICH ENG VIBRATION WAS ASSOCIATED WITH. AT 80 PERCENT N1; R-HAND ENG; WE DUPLICATED THE VIBRATION/RUMBLE; WHICH WAS NOT FELT BY EITHER CREW MEMBER BEFORE. ARRIVED MKE. DEBRIEFED WITH MAINT. ACFT WAS DISPATCHED MKE-LAX-SAN-MKE. CREW RPTED SAME VIBRATION AS WE HAD THE NIGHT BEFORE. WROTE UP PROB; MAINT INSPECTED ENG TO INCLUDE ENG RUNS; VIBRATION METER CHK; BOROSCOPE; OIL SAMPLE; CHIP DETECTOR CHK. FOUND NO PROBS. ACFT WAS AND IS FLYING WITH SAID VIBRATION TODAY; CONFIRMED AFTER FLYING ACFT 3 DAYS LATER. P&W SVC NOTICE STATES THAT THEY ARE AWARE OF A PROB OF 'RUMBLING' AND HAVE NOT ADDRESSED IT ANY FURTHER. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THIS MD88 CAPT SAID THAT 5 DIFFERENT FLCS HAVE FLOWN THIS ACFT SINCE HE RPTED THE ENG VIBRATIONS. HE SAID THAT HE HAS TALKED TO SEVERAL MGRS ABOUT THE ACFT/ENG AND THEY APPARENTLY FEEL THAT IF MAINT SAYS THAT THE ACFT IS SAFE TO FLY IT WILL STAY ON THE SCHEDULE. THE MAINT DEPT HAS DONE A NUMBER OF TESTS OF THIS ENG AND EVERYONE SEEMS TO AGREE THAT IT IS THE SOURCE OF THE VIBRATIONS. THE RPTR SAID THAT MAINT DID A SOAP TEST; A CHIP DETECTOR CHK; A BORE SCOPE INSPECTION OF THE SEVENTH STAGE COMPRESSOR AND SEVERAL ENG RUNS THAT SUPPOSEDLY VERIFIED THE VIBRATION MAGNITUDE AND THE RPM RANGE IN WHICH IT OCCURS. HE ALLEGES THAT HE WAS SHOWN A MESSAGE FROM PRATT & WHITTNEY ABOUT THE VIBRATION PROB ON SOME JT8D-219 ENGS; BUT TO THE RPTR; THE MESSAGE DOES NOT PRECISELY ADDRESS THE SYMPTOMS ON THIS PARTICULAR ENG. THE MESSAGE IS APPARENTLY ABOUT A RANGE OF N2 RPM THAT DOES NOT EQUAL THE 79-80 PERCENT N1 RPM THAT IS THIS ENG'S MAX VIBRATION AREA. THE RPTR FURTHER SAYS THAT A MAINT SUPVR TOLD HIM THAT THIS ENG HAD BEEN DAMAGED SEVERAL YRS AGO IN A FLT THROUGH A HAIL STORM. AFTER SUFFERING THE HAIL DAMAGE; THE ENG HAD TO BE REBALANCED BECAUSE OF SEVERE VIBRATION AT RPM'S BTWN 75-85 PERCENT N1. THE SUPVR DID NOT KNOW WHAT WAS DONE TO REBALANCE THE ENG. THIS CAPT SAID THAT HE WILL BE FLYING THIS ACFT AGAIN AND HE WILL BE SENDING US AN UPDATE ON THE VIBRATION.