What was found
NR 1 ADC FAILED IN FLIGHT AT 30,000 FT, SPEED TAPE RED X OUT. NR 1 ADC REPLACED. ACFT DEPARTED AND AFTER 1 HOUR OF FLIGHT FAULT RED CROSS NR 1 ASI RETURNED AND ACFT RETURNED TO DEPARTURE. ADC WAS A TESTED WITH NO FAULTS FOUND. DURING FIRST FLIGHT IT PRODUCED PREVIOUS FAULT THAT WAS REPORTED ON ITS SERVICE REPORT. FAULT WAS CONFIRMED TO BE THE NR 1 ADC BY SWAPPING WITH THE NR 2 AND THE FAULT FOLLOWED. REPLACED FAULTY UNIT WITH ANOTHER TESTED UNIT. OPS CHECK AND LEAK CHECKS ILLUSTRATED, NO DEFECTS NOTED.