AN EMB145 ACR PILOT SCHEDULED FOR A DAY OF 7+55 PART 121 FLYING; FLEW A 1+00 PART 91 TEST FLT BELIEVING THAT IT WAS NOT PART OF THE 8 HR IN 24 HR MAX FLT TIME.

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AN ACR FLT CREW STARTED OFF ON A NORMAL 4 DAY TRIP. THE FIRST DAY WAS A 5 LEG DAY THAT HAD A BLOCK TIME OF 7 HRS 55 MINS -- 5 MINS UNDER THE MAX 8 HR DUTY DAY. APCHING ZZZ; THE AIRSPD INDICATION WAS NOT WORKING PROPERLY AND WE REVERTED TO THE BACK-UP SYS AND DECLARED AN EMER AS A PRECAUTION. THE FLT LANDED IN ZZZ UNEVENTFULLY AND AFTER MAINT ACTION FIXED THE PROB; THE ACFT HAD TO HAVE A TEST FLT TO BE PUT BACK INTO SVC AGAIN. WE FLEW THE ACFT ON A 1 HR TEST FLT (PART 91) TO BE ABLE TO PUT THE ACFT BACK INTO SVC AND USE THAT SAME ACFT FOR THE REMAINING LEGS OF SCHEDULED FLYING. THE TEST FLT WAS SUCCESSFUL. WITH THE TEST FLT; HOWEVER; THAT WOULD PUT THE TOTAL BLOCK TIME TO 8 HRS 55 MINS -- 55 MINS OVER THE MAX 8 HR DUTY DAY. THE CREW DID FLY THE REST OF THE DAY OUT AND HAD A TOTAL BLOCK TIME DAY OF 10 HRS 15 MINS. TO MY UNDERSTANDING; THE COMPANY CAN ADD A PART 91 FLT TO THE END OF A DAY OF PART 121 FLYING OR AT THE END OF A MULTI-DAY TRIP TO EXTEND PAST THE 8 HR DAY LEGALLY; BUT THE COMPANY CANNOT INSERT A PART 91 FLT INTO A DAY OF PART 121 FLYING AND GO OVER 8 HRS. THAT IS BREAKING THE 8 HR DUTY DAY REGS. SO BY FLYING BACK FROM ZZZ1; WE AS A CREW BROKE 8 HR DUTY DAY.