B737-800 FLT CREW ON A NIGHT VISUAL RECEIVES A LOW ALT WARNING ALERT FROM TWR WHEN TURNING SHORT OF LOC COURSE TO RWY 19L AT IAD; VA.

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CLRED FOR THE VISUAL APCH ON L DOWNWIND FOR RWY 19L AT DULLES INTL. CAPT SAID HE WANTED ME TO TURN BASE ABEAM THE FINAL APCH FIX (FAF) BECAUSE THERE WAS A SMALL SHOWER ABEAM THE FAF AND DEAD AHEAD OF US ON DOWNWIND. AS I APCHED THE SHOWER; I HAD TO TURN A LITTLE EARLY CAUSING ME TO TURN BASE 1 MI INSIDE FAF. I COMMENCED THE TURN ON AUTOPLT WHICH WAS SET AT 30 DEG ANGLE OF BANK. I CALLED FOR THE GEAR TO BE LOWERED AND FLAPS TO 15 DEGS. THE CAPT SET MY SPD BACK ON MY COMMAND AND I CONTINUED MY TURN TO INTERCEPT FINAL. THE CAPT LOOKED AT THE GS BUG AND MENTIONED NOT TO GET HIGH AND BEGAN RESETTING MY ALT WINDOW AND PROGRAMMING A VERT SPD. THE AUTOPLT WAS DISENGAGED BY ME ON BASE SO HIS MANIPULATION OF THE FLT GUIDANCE PANEL WAS TO ASSIST ME ON APCH. I TOLD HIM TO ARM THE APCH MODE SO THE FLT DIRECTOR WOULD GIVE ME GUIDANCE ONCE WE CAPTURED THE ILS SIGNAL. I HAD ALREADY PROGRAMMED A SLIGHT DSCNT ON BASE 300 FPM AND HAD SET 1700 FT (THE FAF ALT) AND THIS BEGAN A DSCNT OUT OF 2000 FT WHICH WAS OUR ALT ON DOWNWIND. OUR DOWNWIND WAS APPROX 3 MI DISTANCE; BUT PARALLEL TO RWY CTRLINE. I HAD NOT SEEN THE RWY YET; BUT THE CAPT HAD KEPT THE FIELD IN SIGHT OUT OF HIS SIDE OF THE ACFT. WHEN I ROLLED OUT TO INTERCEPT FINAL; IT TOOK ME 5-10 SECONDS TO PICK UP THE RWY. THE VISIBILITY WAS 5 MI; BUT AT NIGHT IT TOOK A MOMENT TO PICK UP THE RWY IN THE SEA OF LIGHTS. I HAD KEPT ABOVE GS BUG DURING THE TURN TO FINAL; BUT USED IT AS MY PRIMARY REF ON DSCNT. PICKING UP THE RWY VISUALLY ON ROLLOUT WAS AT 2 O'CLOCK POS; SO I HAD UNDERSHOT THE TURN. IT DID NOT TAKE RADICAL MANEUVERING TO READJUST AND INTERCEPT FINAL EITHER IN PITCH OR ROLL. I NEVER 'FELT' LOW EITHER BY VISUAL REF TO GND. TWR CALLED US ABOUT THE SAME TIME I PICKED UP THE RWY VISUALLY ABOUT HAVING A LOW ALT WARNING FOR US BY THEIR EQUIP. I ASSUMED IT WAS BECAUSE OF MY EARLY ROLLOUT; UNDERSHOOT; BEING L OFF COURSE AND ON GS ALT. IN RESOLUTION TO ALL THIS; IT WOULD HAVE BEEN BETTER NOT TO HAVE MADE THE EARLY TURN; BUT TO EXTEND DOWNWIND GIVING ME MORE FINAL. IT WOULD HAVE GIVEN ME MORE TIME TO SET UP MY DSCNT BY SEEING THE RWY EARLIER; ESPECIALLY CONSIDERING THE FACT IT WAS AT NIGHT AND A CAPT'S SIDE TURN. AFTER THE FACT TOO; I SHOULD NOT HAVE USED THE GS BUG AS A REF UNLESS I WAS ESTABLISHED ON THE LOC SIGNAL.