FLC OF A WDB EXCEEDED THE FLT TIME DUTY LIMITATIONS.

Narrative

BASICALLY; I BELIEVE THE CREW AND PLT SCHEDULING MISINTERPRETED AND MISUSED FAR 121.503F WHICH ALLOWS A TRANSCONTINENTAL FLT SCHEDULE OF UP TO 10 HRS. THE CAPT CONVINCED A NEWLY HIRED FLT SCHEDULER THAT WE COULD USE THIS FAR TO FLY LAX-JFK-LAX IN OUR DC10; PART 121. THE CAPT ALSO CONVINCED ME AND THE FO; HOWEVER; HE NEVER SHOWED US THE FAR. TO MY KNOWLEDGE; THE DUTY DAY WAS NOT EXCEEDED; ONLY THE FLT TIME LIMITATION. I REALIZED THE ERROR TODAY WHEN I DISCUSSED THE FLT WITH A CAPT; CHK AIRMAN WHO ENCOURAGED ME TO SUBMIT THIS RPT. HE SAID THE RULE IS ONLY APPLICABLE TO 1 LEG (EG; LAX-JFK); NOT A ROUND-TRIP. FURTHERMORE; SINCE THE CREW DID NOT HAVE A 'PAIRING' WHICH SHOWS US THE SCHEDULED FLT TIME; I DID NOT KNOW HOW LONG THE LEGS WERE SCHEDULED TO BE. A FEW DAYS LATER; I WAS GIVEN A PAIRING AND I SAW THAT THE LEGS WERE SCHEDULED OVER 10 HRS! LAX-JFK: 4 HRS 45 MINS; JFK- LAX: 5 HRS 45 MINS; TOTAL: 10 HRS 30 MINS. I HAD TO ASSUME THAT THE CAPT AND SCHEDULER WOULD HAVE WORKED THIS OUT BEFORE GIVING US THE TRIP.