A DC10 ON LOC APCH TO SAN DEVIATED BELOW THE CHARTED ALTITUDE.

Narrative

WE WERE ARRIVING IN SAN DIEGO ON THE BARET4 ARR FOR A LOC APCH TO RWY 27. THE CAPT WAS THE PLT MONITORING; THE FO WAS THE PF. THE ARR AND APCH HAD BEEN INSERTED INTO THE FMS AND BRIEFED. THE APCH WAS IN THE DATABASE AND HAD BEEN BRIEFED TO FLY USING 'LOC ONLY' AND A PROFILE DSCNT. WX WAS VFR BUT AS WE ARRIVED IT WAS VERY HAZY AND WE WERE LOOKING INTO THE SUN. EARLY IN THE ARR; APCH GAVE US A VECTOR TO INTERCEPT THE LOC AND A SERIES OF STEPDOWN DSCNTS. BECAUSE OF THE DISTANCE; WE MANUALLY TUNED THE LOC AND ARMED 'LOC ONLY.' AT ONE POINT; THE ROLL MODE DROPPED OUT OF 'LOC ARMED.' THE FO CAUGHT IT; AND WE RE-ARMED 'LOC ONLY.' AS WE INTERCEPTED THE LOC; APCH INSTRUCTED US TO MAINTAIN 4000 FT UNTIL VYDDA; CLRED US FOR THE APCH; AND SWITCHED US TO TWR. AS WE CROSSED OVER VYDDA; WE REVIEWED THAT ALL OF THE PARAMETERS HAD BEEN MET TO DIAL OUR ALT DOWN TO MINIMUMS. THE FO DID THAT AND I CONFIRMED THE CORRECT ALT. AT THAT TIME; THE VERT DSCNT INDICATOR SHOWED THAT WE WERE SLIGHTLY HIGH ON THE PROFILE. I NOTED THAT; AND THAT AS SOON AS MINIMUMS HAD BEEN DIALED IN; THE ACFT BEGAN TO DSND. I MONITORED THE DSCNT UNTIL THE VERT DSCNT INDICATOR INDICATED THAT WE WERE BACK ON THE PROFILE. AT THAT TIME I CONTACTED TWR; RECEIVED LNDG CLRNC; DIALED IN GND FREQ AND LOOKED OUT VISUALLY FOR TFC. WE ALSO COMPLETED CONFIGURING THE ACFT FOR LNDG AND COMPLETED THE 'BEFORE LNDG CHKLIST.' WHEN I LOOKED BACK OUTSIDE; I STILL COULD NOT SEE THE RWY AND I ALSO SENSED A HIGH RATE OF DSCNT. I LOOKED BACK INSIDE AND NOTED THAT THE DSCNT RATE WAS OVER 1000 FPM; AND THAT WE WERE LOW ON THE PROFILE. AT THE SAME TIME; TWR CALLED AND RPTED US LOW ON THE APCH. I IMMEDIATELY INSTRUCTED THE FO TO DISCONNECT THE AUTOPLT AND CLB BACK UP TO OUR NEXT XING RESTR ALT OF 2000 FT. HE DID THIS EXPEDITIOUSLY; AND THEN CONTINUED HAND FLYING FOR AN UNEVENTFUL LNDG. AFTER SHUTDOWN; WE CONDUCTED AN EXTENSIVE DEBRIEF. ALTHOUGH WE WERE UNABLE TO DETERMINE EXACTLY WHY THE ACFT FAILED TO FOLLOW THE PROFILE DSCNT; BOTH THE PF AND PLT MONITORING FAILED TO CORRECTLY MONITOR THE VERT DSCNT OF THE ACFT. THROUGH A VERY THOROUGH DEBRIEF; BOTH CREW MEMBERS LEARNED VALUABLE LESSONS THAT WE WILL TAKE WITH US FOR THE REST OF OUR AVIATION CAREERS.