What was found
AFTER CONFIRMING EXISITNG H.S.I. IN AIRCRAFT HAD A MALFUNCTION, A LOANER H.S.I. WAS INSTALLED AND PROPERLY TESTED. THE LOANER UNIT WAS PROVIDED BY HONEYWELL. APPROXIMATELY 20 MINUTES AFTER TAKEOFF, AIRCRAFT RETURNED TO HOME BASE TO REPORT 'PRI DIR' CIRCUIT BREAKER HAD POPPED. EXTENSIVE TROUBLESHOOTING OF SYSTEM CONCLUDED IN A DEFECTIVE LOANER H.S.I. SUBMITTER RECOMMENDATIONS - INCREASE QUALITY CONTROL ON LOANER COMPONENTS FROM HONEYWELL.