Narrative:

On descent into san (runway 27) we were held to 6000 ft and 180 KTS assigned until just outside vydda. We were then cleared the RNAV runway 27 approach. I selected open descent (idle) to get down to 4000 ft, then 3600 ft (swatt). Twice during this descent, the descent mode switched to a vertical speed descent (1000 FPM). After the second mode switch or change, and being quite high on glide path, I turned the autoplt and both flight directors off in order to capture the glide path. I had ground contact at all times. The airport was IMC, but on final I had VMC conditions. Once the speed was back and I was fully configured I descended through the glide path. The glide path went from full scale high to full scale low almost immediately. The first officer called the glide path out and I leveled off at approximately 1600 ft. This was at reebo. Tower called with a low altitude alert. I re-engaged the flight directors and captured the glide path and continued the approach. Contributing factors were being held high on altitude, automation preventing from open descent (idle) to a vertical speed mode twice, and my failure to realize the difference between a GS and a glide path when I was trying to capture the glide path from above.

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Original NASA ASRS Text

Title: AUTOFLT DISTRACTIONS RESULT IN A DEV BELOW PUBLISHED APCH ALT ON APCH TO SAN, CA.

Narrative: ON DSCNT INTO SAN (RWY 27) WE WERE HELD TO 6000 FT AND 180 KTS ASSIGNED UNTIL JUST OUTSIDE VYDDA. WE WERE THEN CLRED THE RNAV RWY 27 APCH. I SELECTED OPEN DSCNT (IDLE) TO GET DOWN TO 4000 FT, THEN 3600 FT (SWATT). TWICE DURING THIS DSCNT, THE DSCNT MODE SWITCHED TO A VERT SPD DSCNT (1000 FPM). AFTER THE SECOND MODE SWITCH OR CHANGE, AND BEING QUITE HIGH ON GLIDE PATH, I TURNED THE AUTOPLT AND BOTH FLT DIRECTORS OFF IN ORDER TO CAPTURE THE GLIDE PATH. I HAD GND CONTACT AT ALL TIMES. THE ARPT WAS IMC, BUT ON FINAL I HAD VMC CONDITIONS. ONCE THE SPD WAS BACK AND I WAS FULLY CONFIGURED I DSNDED THROUGH THE GLIDE PATH. THE GLIDE PATH WENT FROM FULL SCALE HIGH TO FULL SCALE LOW ALMOST IMMEDIATELY. THE FO CALLED THE GLIDE PATH OUT AND I LEVELED OFF AT APPROX 1600 FT. THIS WAS AT REEBO. TWR CALLED WITH A LOW ALT ALERT. I RE-ENGAGED THE FLT DIRECTORS AND CAPTURED THE GLIDE PATH AND CONTINUED THE APCH. CONTRIBUTING FACTORS WERE BEING HELD HIGH ON ALT, AUTOMATION PREVENTING FROM OPEN DSCNT (IDLE) TO A VERT SPD MODE TWICE, AND MY FAILURE TO REALIZE THE DIFFERENCE BTWN A GS AND A GLIDE PATH WHEN I WAS TRYING TO CAPTURE THE GLIDE PATH FROM ABOVE.

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.