A B757 CAPT DESCRIBED A WINDSHEAR ENCOUNTER AT DEN WHICH CAUSED HIM TO EXECUTE A MISSED APCH.

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WE WERE ON DAY 4 OF A 4 DAY TRIP. THE COPLT FLEW THE LEG. THE ATIS INDICATED THAT DENVER WAS LNDG TO THE S. WE WERE GIVEN VECTORS AND TOLD TO EXPECT RWY 17R. AS WE DSNDED THROUGH 10000 FT MSL; WE ENCOUNTERED MODERATE TURB. AT APPROX 8000 FT MSL THE TURB SUBSIDED AND THE COPLT DISENGAGED THE AUTOPLT TO PERFORM A HAND FLOWN APCH. SEVERAL SECONDS LATER HE DISCONNECTED THE AUTOTHROTTLE AND TOOK OVER THROTTLE OP MANUALLY. WE WERE CLRED FOR A VISUAL APCH TO RWY 17R. THE ACFT DSNDED TO APPROX 100 FT BELOW THE GS XING ALT WHILE STILL 1/2 MI OUTSIDE THE FAF AND I TOLD THE COPLT TO MAINTAIN HIS ALT UNTIL REACHING THE GS. AT GS INTERCEPT; WE BEGAN OUR DSCNT. AT APPROX 1400 FT AGL; WE BEGAN TO SINK BELOW THE GS AND; IN SPITE OF NORMAL MANEUVERS TO ARREST THE DSCNT RATE; AT 1000 FT AGL WE WERE OVER 1 DOT LOW. I INSTRUCTED THE COPLT TO ADD MORE PWR AND REJOIN THE GS AND; IF NOT ABLE TO DO SO PRIOR TO 500 FT AGL; TO EXPECT TO MISS THE APCH. HE DID SO; HOWEVER; THE PWR WAS STILL NOT ADEQUATE AND WE GOT 5 KTS SLOW IN THE MANEUVER. FURTHER ATTEMPTS TO RESUME A NORMAL APCH WERE NOT ADEQUATE AND AT 500 FT I INSTRUCTED THE COPLT TO GO AROUND BECAUSE WE WERE BELOW BUG SPD. THE GAR WAS PERFORMED WITHOUT PROB. DURING THE SUBSEQUENT APCH; I BELIEVE THE COPLT OVERREACTED SO AS NOT TO REPEAT THE PREVIOUS EVENT AND; IN DOING SO; ACTUALLY BECAME HIGH AND FAST. BECAUSE WE WERE MARGINALLY WITHIN THE LIMITS OF A STABILIZED APCH; I ALLOWED HIM TO CONTINUE. OUR TOUCHDOWN WAS AT THE END OF THE TOUCHDOWN ZONE MARKINGS; BUT; BECAUSE THE RWY IS VERY LONG; WERE EASILY ABLE TO STOP WITH NORMAL BRAKING AND REVERSE. SAFETY WAS NOT COMPROMISED. THE COPLT WAS VISIBLY SHAKEN AND I BELIEVE OVERLY HARD IN JUDGING HIMSELF. I WISHED TO GIVE HIM TIME TO COOL OFF AND TO DEBRIEF THE APCH AT THE GATE. THE TWR; HOWEVER; WISHED TO HAVE AN EXPLANATION FOR THE MISSED APCH AND ASKED THAT WE GIVE ONE AS WE TAXIED IN; OSTENSIBLY SO THAT THEY COULD COMPLETE THEIR RPTS. BECAUSE OF NUMEROUS RPTS OF NEWS MEDIA MISUSE OF INTERCEPTED RADIO XMISSIONS; AND NOT WANTING TO EMBARRASS THE COPLT FURTHER; I TOLD THE TWR ONLY THAT THE MISS WAS PRECAUTIONARY AND GAVE A FICTITIOUS REASON. LATER; PRIOR TO TAXI OUT FOR THE RETURN LEG; WE LEARNED THAT WINDSHEAR HAD BEEN EARLIER RPTED FROM FL180 TO TOUCHDOWN BY ANOTHER ACFT. IT WAS NOT UNTIL RETURNING TO ATL THAT I HAD TIME TO RECONSIDER THE EVENTS OF THE APCH; AND I RECALLED THAT I HAD FORGOTTEN TO CALL THE TWR ON THE LANDLINE. UPON REFLECTION I BELIEVE THAT WE HAD ENCOUNTERED A MINOR DECREASING WINDSHEAR; INSIDIOUS IN THAT IT WAS MASKED BY ITS SLOW ONSET. THE DEG OR PERFORMANCE DEGRADATION WAS MINOR; MAKING THE NEED FOR A GAR NOT IMMEDIATELY APPARENT. NO WINDSHEAR ALERT WAS GIVEN BY THE GPWS. THE DECISION TO GAR WAS TIMELY AND APPROPRIATE; HOWEVER; I REGRET THAT I FAILED TO IMMEDIATELY RECOGNIZE THE CONDITION FOR WHAT IT WAS AND FOR NOT GIVING THE TWR A MORE ACCURATE OBSERVATION. I FEEL LIKE THE LONG PREVIOUS DUTY DAY; SHORT LAYOVER; AND LONG SUBSEQUENT DUTY DAY CONTRIBUTED TO DELAYED JUDGEMENT WITH REGARD TO THESE CIRCUMSTANCES.