What was found
CLIMBING THROUGH 38000' IN MAX CLIMB DETENT 290KTS TWO DISTINCT BANGS, SIMILAR TO COMPRESSOR STALLS, WERE FELT THROUGH FLOOR, RUDDERS & CONTROLS. ALL INSTRUMENT READINGS WERE NORMAL EXCEPT 1/4 SCALE ON RT VIB METER. DISCONNECTED A/P DES TO 24000 SLOWED 250KTS. STILL FELT SLIGHT YAW PRESS UNTIL BELOW 250KTS & 6000'. GND RUNS CHECKED SATISFACTORY. WHILE TROUBLESHOOTING, PERFORMED BOROSCOPE INSPECTION OF RT ENG 1ST & 4TH STAGE COMPRESSOR BLADES COATING INTEGRITY IAW PW306A MM 72-00-00 & FOUND 4TH STAGE COATING TO BE DAMAGED. REPLACED RT ENG ASSEMBLY WITH A SERVICEABLE ENG ASSEMBLY IAW G200 MM 71-00-00. ALL POST INSTALLATION OPERATION CHECKS & LEAK CHECKED GOOD.