What was found
ON CLIMBOUT, EN ROUTE WHLE SELECTING FLAPS UP THE CAPTAINS PRIMARY FLIGHT DISPLAY INDICATED A BROKEN POLE OF 230 KTS, AND THE OVERSPEED WARNING HORN WOULD GIVE A CONTINUOUS ALARM IF THE AIRSPEED EXCEEDED 230 KTS. TURNED BACK TO RDU AND LANDED WITHOUT FURTHER EVENT. MAINTENANCE REMOVED AND REPLACED THE NR 1 AIR DATA INERTIAL REFERENCE UNIT (ADIRU) IAW AWA T/C R2A34304, OPERATIONAL CHECK WAS GOOD. VENDOR EVALUATION TEST OF ADIRU S/N 2891 WAS NFF, TEST CONDUCTED IAW CMM TP1011, REV 22.