Narrative:

Flying as captain; my leg to ZZZ; night VMC; we were vectored to intercept final for runway. When we had intercepted and confirming runway in sight; we were cleared the visual approach; approximately 20 mi out. I directed the first officer to set 2200 ft in the altitude window. That far on final; the ILS signal was 'wobbly;' causing the aircraft to oscillate back and forth; so I decided to hand fly to smooth out the flight path. In so doing; I inadvertently descended below the FAF altitude; probably down to 2000 ft. We received a single 'caution; obstacle' aural warning. Realizing I had descended; I added power and climbed slightly. No further warning was received; and I continued the approach to landing. I didn't even realize my error until 2 days later; when I was studying for my pt; and read that receiving an altitude warning at night requires the crew to go around; regardless of perceived clearance or corrective action. I now know that I should have gone around. Further; I should have let the autoplt muddle through; or perhaps used control wheel steering for roll; to ensure that the altitude hold function kept us from descending below the appropriate altitude. During my pt just 3 days after this event; my instructor did a good job of emphasizing crew requirements upon receiving GPWS alerts and warnings. I think the training department is trying to make sure crews know the rules.

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Original NASA ASRS Text

Title: B737 FLT CREW REPORTS DESCENDING BELOW GS DURING NIGHT VISUAL APPROACH AND RECEIVING GPWS OBSTACLE WARNING.

Narrative: FLYING AS CAPT; MY LEG TO ZZZ; NIGHT VMC; WE WERE VECTORED TO INTERCEPT FINAL FOR RWY. WHEN WE HAD INTERCEPTED AND CONFIRMING RWY IN SIGHT; WE WERE CLRED THE VISUAL APCH; APPROX 20 MI OUT. I DIRECTED THE FO TO SET 2200 FT IN THE ALT WINDOW. THAT FAR ON FINAL; THE ILS SIGNAL WAS 'WOBBLY;' CAUSING THE ACFT TO OSCILLATE BACK AND FORTH; SO I DECIDED TO HAND FLY TO SMOOTH OUT THE FLT PATH. IN SO DOING; I INADVERTENTLY DSNDED BELOW THE FAF ALT; PROBABLY DOWN TO 2000 FT. WE RECEIVED A SINGLE 'CAUTION; OBSTACLE' AURAL WARNING. REALIZING I HAD DSNDED; I ADDED PWR AND CLBED SLIGHTLY. NO FURTHER WARNING WAS RECEIVED; AND I CONTINUED THE APCH TO LNDG. I DIDN'T EVEN REALIZE MY ERROR UNTIL 2 DAYS LATER; WHEN I WAS STUDYING FOR MY PT; AND READ THAT RECEIVING AN ALT WARNING AT NIGHT REQUIRES THE CREW TO GO AROUND; REGARDLESS OF PERCEIVED CLRNC OR CORRECTIVE ACTION. I NOW KNOW THAT I SHOULD HAVE GONE AROUND. FURTHER; I SHOULD HAVE LET THE AUTOPLT MUDDLE THROUGH; OR PERHAPS USED CTL WHEEL STEERING FOR ROLL; TO ENSURE THAT THE ALT HOLD FUNCTION KEPT US FROM DSNDING BELOW THE APPROPRIATE ALT. DURING MY PT JUST 3 DAYS AFTER THIS EVENT; MY INSTRUCTOR DID A GOOD JOB OF EMPHASIZING CREW REQUIREMENTS UPON RECEIVING GPWS ALERTS AND WARNINGS. I THINK THE TRAINING DEPT IS TRYING TO MAKE SURE CREWS KNOW THE RULES.

Data retrieved from NASA's ASRS site as of January 2009 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.