What was found
DESCENDING BELOW 24K FEET, RECEIVED AN IAS COMPARATIVE FLAG ON THE PFD. HAD ABOUT A 5-8 KT DIFFERENCE BETWEEN FIRST OFFICER`S AND CAPTAIN'S AIRSPEED. DIFFERENCE WENT AS HIGH AS 30 KTS, BY THE TIME THE AIRSPEED UNREALABLE QRH CHECKLIST WAS COMPLETED. DETERMINED THROUGH STANBY INSTRUMENTS AND QRH CHECKLIST THAT CADC 1 WAS BAD. COULD NOT CONFIRM ANY FAULTS. CAPTAIN`S PITOT TUBE DRAIN HOLES FOUND TO BE CLEAR IAW MM 34-11-02-2. REMOVED AND REPLACED THE NR 1 ADIRU IAW MM 34-43-03-4. REMOVED AND REPLACED THE CAPTAIN'S PITOT AIR DATA MODULE. IAW MM 34-16-01-5. LEAK CHECKED GOOD IAW 34-16-00-5. OK FOR SERVICE.