What was found
NR 1 ENGINE VIBRATION ABNORMAL ON START BETWEEN 35-45 PERCENT N2. AT REDUCED TAKEOFF POWER 1.81 EPR, N1 INDICATION 96 PERCENT VS 91 PERCENT ON NR 2 AND NR 3 ENGINES. IN FLIGHT VIBRATION IN TAIL SECTION OF CABIN NOTED. EMERGENCY WAS NOT DECLARED, ENGINE WAS NOT SHUT DOWN, RETURNED FROM FLT TO DCA. REPLACED NR 1 ENGINE PER MM 72-00-00, TS088R9, TS089R6, TS090R3. ENGINE RUNUP CHECKS OK. (M)