Narrative:

Cleared for the visual approach on left downwind for runway 19L at dulles international. Captain said he wanted me to turn base abeam the final approach fix (FAF) because there was a small shower abeam the FAF and dead ahead of us on downwind. As I approached 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 degree angle of bank. I called for the gear to be lowered and flaps to 15 degrees. The captain set my speed back on my command and I continued my turn to intercept final. The captain looked at the GS bug and mentioned not to get high and began resetting my altitude window and programming a vertical speed. The autoplt was disengaged by me on base so his manipulation of the flight guidance panel was to assist me on approach. I told him to arm the approach mode so the flight director would give me guidance once we captured the ILS signal. I had already programmed a slight descent on base 300 FPM and had set 1700 ft (the FAF altitude) and this began a descent out of 2000 ft which was our altitude on downwind. Our downwind was approximately 3 mi distance, but parallel to runway centerline. I had not seen the runway yet, but the captain had kept the field in sight out of his side of the aircraft. When I rolled out to intercept final, it took me 5-10 seconds to pick up the runway. The visibility was 5 mi, but at night it took a moment to pick up the runway in the sea of lights. I had kept above GS bug during the turn to final, but used it as my primary reference on descent. Picking up the runway visually on rollout was at 2 O'clock position, 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 reference to ground. Tower called us about the same time I picked up the runway visually about having a low altitude warning for us by their equipment. I assumed it was because of my early rollout, undershoot, being left off course and on GS altitude. 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 descent by seeing the runway earlier, especially considering the fact it was at night and a captain's side turn. After the fact too, I should not have used the GS bug as a reference unless I was established on the localizer signal.

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Original NASA ASRS Text

Title: 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.

Narrative: 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.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.