FLC OF AN MLG CARGO ACFT EXCEEDED THE FLT TIME DUTY LIMITATIONS BY 5 MINS.

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I RPTED FOR DUTY AT XT55 EST IN EL PASO. FLEW TRIP INTO ILW. DUE TO SNOW AND ICE IN ILW; DEP WAS DELAYED; HOWEVER MORE THAN 4 HRS OF DUTY TIME AVAILABLE FOR APPROX 2 HR TRIP. UPON REACHING PWM; ARPT WAS BELOW LNDG MINIMUMS; COMPANY REQUESTED WE HOLD WHICH WE DID UNTIL IT WAS NECESSARY TO LEAVE HOLDING IN ORDER TO REMAIN WITHIN 16 HRS OF DUTY. AS WE WERE HDG FOR ALTERNATE THE PORTLAND WX CAME UP SO WE LANDED THERE; AND BLOCKED IN AT XL30 WHICH WAS WITHIN THE 30 MINS OF DUTY TIME AFTER BLOCK-IN PUT THE END OF MY DUTY TIME AT XX00 EST WHICH WAS 5 MINS PAST THE 16 HR DUTY TIME LIMITATION THAT ENDED AT XX55 EST. I BELIEVE THAT I CAN ASSURE THAT THIS DOES NOT REOCCUR BECAUSE OF MY BETTER UNDERSTANDING OF THE REGULATION LIMITING MY DUTY TIME TO 16 HRS. I WILL INSURE THAT I AM ON THE GND AND HAVE COMPLETED ALL REQUIRED WORK PRIOR TO THE 16 HR LIMITATION. REGARDLESS OF COMPANY PRESSURE TO HAVE THE AIRPLANE IN THE ASSIGNED PLACE. I WILL ALSO CONTINUALLY REMIND MY FELLOW PLTS THAT THIS REGULATION IS CONCRETE AND CANNOT BE WAIVED BY THE PLT OR THE COMPANY.