A B767 FO; ON 2 OCCASIONS; TOLD HIS CAPT TO GO AROUND AFTER THE TWR RPTED WINDSHEAR AND MICROBURST ALERTS. APPARENTLY; THIS COMPANY'S OPS MANUAL CLRLY STATES THAT LNDG IS PROHIBITED WHEN A TSTM IS WITHIN 3 NM OF THE INTENDED FLT PATH. AT THE GATE; THE FO CONFRONTED THE CAPT AGAIN; AND HE RESPONDED; 'I WAS COMFORTABLE AND I'M THE CAPT.'

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I WAS THE COPLT ON SEP/XA/99; DFW-ATL. DURING VECTORS FOR THE ILS RWY 9R AT ATL; IT BECAME OBVIOUS THAT THERE WAS A TSTM AT THE FIELD. ONBOARD RADAR/EFIS DISPLAYED RED RADAR RETURNS AT THE ARPT. THE CAPT WAS FLYING THE ACFT (AUTOPLT ON). I ADVISED HIM I WAS AGAINST FLYING THE APCH BY STATING 'I THINK WE SHOULD GO AROUND.' HE ELECTED TO CONTINUE. WE SWITCHED TO TWR AT ABOUT BURNY FAF. I QUERIED TWR ABOUT FIELD CONDITIONS. THEIR RESPONSE WAS HVY RAIN THEN 'WINDSHEAR ALERT; LOSS OF 20 KTS DEP END.' TWR THEN ISSUED A 'MICROBURST ALERT' AT WHICH POINT I AGAIN TOLD THE CAPT TO GO AROUND. HE AGAIN OVERRULED ME AND SAID HE WAS GOING TO LAND. WE TOUCHED DOWN IN CLR AIR AND ROLLED INTO HVY RAIN AT MID FIELD. LNDG WAS UNEVENTFUL. AT THE GATE; I TOLD THE CAPT; 'I DON'T THINK WE SHOULD HAVE FLOWN THE APCH.' HIS RESPONSE WAS; 'I WAS COMFORTABLE AND I'M THE CAPT.' COMPANY MANUAL PROHIBITS LNDG WHEN A TSTM IS WITHIN 3 NM OF THE INTENDED FLT PATH. IT ALSO PROHIBITS LNDG AFTER TWR ISSUES A 'MICROBURST ALERT.' WHY DO PLTS CONTINUE TO LAND IN TSTMS?