A PILOT REPORTS HIS ACR CONVINCED HIM THAT AFTER FLYING A FLAG LEG TO A DOMESTIC DEST; A DOMESTIC LEG WAS LEGAL EVEN IF FLT TIME EXCEEDED 8 HRS IN 24.

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CREW WAS CONVINCED THAT OPERATING OAK-ZZZ1 AFTER A LENGTHY TRANSPACIFIC FLT WITH AN AIR RETURN WAS APPROPRIATE AND LEGAL. BOTH THE CAPT AND I EXCEEDED FLT TIME BY 8 HRS IN A 24 HR PERIOD. AFTER RETURNING TO ZZZ2 FOR A COMPONENT MALFUNCTION WHILE ENRTE TO OAK; WE FLEW 1 HR 53 MINS FOR THAT SEGMENT BACK TO ZZZ2. THE FLT SEGMENT FROM ZZZ2-OAK WAS 5 HRS 16 MINS. THE FLT SEGMENT FROM OAK-ZZZ1 WAS 1 HR 24 MINS. PRIOR TO LEAVING ZZZ2 FOR THE SECOND TIME; WE CONTACTED UNION REPRESENTATIVE MR X FOR GUIDANCE ON OUR SITUATION. HE ADVISED US THAT WE WOULD BECOME ILLEGAL TO FLY THE OAK-ZZZ1 SEGMENT. STILL UNSURE OF THE LEGALITIES OF OUR SITUATION; THE CAPT CONTACTED SCHEDULING TO SEE IF WE WOULD OR WOULD NOT BE LEGAL TO FINISH OUR TRIP. THE HEAD OF SCHEDULING; MR Y; ADVISED THE CAPT THAT EVERYTHING WOULD BE OK FOR US TO TAKE THE FLT AND FINISH OUR DAY SAYING; 'GO LOOK IN YOUR OPS MANUALS AND THE REGS ON BOARD THE ACFT.' THE CAPT AND I DID JUST THAT AFTER LEAVING ZZZ2 FOR OAK. IN THE FOM; WE REVIEWED SECTION X AND SECTION Y. IN THE FAR; WE REVIEWED FAR 121.471 AND FAR 121.481. WE CAME TO THE CONCLUSION THAT WE WERE OPERATING THE ZZZ2-OAK SEGMENT AS A FLAG CARRIER AND THE OAK-ZZZ1 SEGMENT AS A DOMESTIC CARRIER; AS PER FOM SECTION X. AFTER REVIEWING FAR 121.481(B)(C) SEVERAL TIMES; WE ALSO CAME TO THE CONCLUSION THAT AS A CARRIER OPERATING UNDER FLAG RULES FOR THIS SEGMENT; WE WOULD BE HELD TO THE FLT TIME LIMITATIONS OF THIS REG. AFTER TALKING WITH MR X IN THE AFTERNOON; HE CLEARLY EXPLAINED TO US THAT CONTINUING ON TO DEST WAS NOT IN OUR BEST INTEREST BECAUSE OF FAR 121.481(B) WITH THE KEY PHRASE BEING 'IT SHALL GIVE HIM AN INTERVENING REST PERIOD; AT OR BEFORE THE END OF 8 SCHEDULED HRS OF FLT DUTY.' ACCORDING TO THIS FAR; WE SHOULD HAVE GOTTEN OFF THE AIRPLANE IN OAK AND BEEN RELEASED FROM ALL DUTIES FOR AT LEAST TWICE THE FLT TIME. ANOTHER REMEDY TO THIS SITUATION COULD HAVE BEEN TO RELEASE ME AND THE CAPT FROM ALL DUTIES UPON THE RETURN TO ZZZ2 AND CALL IN THE RESERVE PLTS TO TAKE THE AIRPLANE AND THE PAX TO OAK AND THEN ON TO ZZZ1. ON THE HUMAN FACTORS SIDE; I BELIEVE IT WAS FOOLISH OF THE COMPANY TO ALLOW ME AND THE CAPT TO CONTINUE WITH SUCH A LONG DAY OF FLYING AFTER DEALING WITH AN FMC MALFUNCTION. FATIGUE AND STRESS DEFINITELY WERE PLAYERS IN THIS SITUATION OF POSSIBLY NOT HAVING A POSITIVE OUTCOME. FATIGUE AND STRESS HAS SHOWN TO LOWER SITUATIONAL AWARENESS; AS WELL AS; IMPAIRS THE PLT'S ABILITY TO MANAGE HIGHER THAN NORMAL WORKLOADS DURING FLT. FOR MYSELF; I BELIEVE THE COMPANY ACTED IRRESPONSIBLY AND DID NOT ERR ON THE CONSERVATIVE SIDE OF SAFETY LIKE THEY PREACH FOR US TO DO. CREW KNOWLEDGE: CREW FLEW THE TRIP BUT STILL QUESTIONS THEIR LEGALITY AS EXPRESSED BY THE COMPANY.