Narrative:

Captain was now on the 757 less than 100 hours. We were flying marginal VFR (1200 ft broken and 5 mi) localizer 27 into san, ca. Captain was in LNAV. During descent autoplt disconnected by itself so captain hand flew approach. I (PNF) was in expanded ILS mode for DME on approach. IRS relative position was offset a little bit to the left, so captain elected to go to expanded ILS mode for accurate localizer data, but didn't tell me. He stated a little confusion over where he would be receiving proper localizer DME data. I stated that it would be on the right DME with an 'left' in front. He started to descend from 2000 ft at what he believed to be reebo, but he was looking at the incorrect DME (the left one initially) from map mode which caused him to descend below 2000 ft (reebo) early. Once I realized that he was on ILS expand also, I returned to map mode for situational awareness. At this point, tower stated that she was showing us low at reebo. I confirmed that we were at reebo but our altitude was 1400- 1500 ft MSL. We leveled off with runway in sight until VASI lights gave us correct GS information. Landing uneventful. After landing, we discussed how we had left reebo early. The captain stated that he really should have briefed approach better to clarify which DME would be the correct one for localizer data. I was unaware that he wasn't sure. I was also somewhat confused because I believed he had left reebo accurately. Looking for 6.4 DME to descend, I believe we had, from the expanded ILS localizer DME? It is easy to get high on this approach, and I'm sure that was on our minds as we were descending to 700 ft MSL. Being near dusk with all the cloud cover also made the approach more difficult than usual. Supplemental information from acn 316110: we passed swatt and began descent to 2000 ft MSL. Because the LNAV was displaced, I switched my HSI to expanded ILS to check the course. I switched between map and expanded ILS several times. Because of this switching of HSI's, I believe I started a situation. Looking back on it the confusion began on my part in regards to exactly where the localizer DME was to be presented on my panel. In the future I must be more specific in briefing radios, map, expanded ILS. Then if changes take place both pilots must know and keep up.

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Original NASA ASRS Text

Title: ACR FLC DSNDED BELOW IAP ALT. CAPT (PF) MISINTERPRETED DME SOURCE FROM NAV DISPLAY.

Narrative: CAPT WAS NOW ON THE 757 LESS THAN 100 HRS. WE WERE FLYING MARGINAL VFR (1200 FT BROKEN AND 5 MI) LOC 27 INTO SAN, CA. CAPT WAS IN LNAV. DURING DSCNT AUTOPLT DISCONNECTED BY ITSELF SO CAPT HAND FLEW APCH. I (PNF) WAS IN EXPANDED ILS MODE FOR DME ON APCH. IRS RELATIVE POS WAS OFFSET A LITTLE BIT TO THE L, SO CAPT ELECTED TO GO TO EXPANDED ILS MODE FOR ACCURATE LOC DATA, BUT DIDN'T TELL ME. HE STATED A LITTLE CONFUSION OVER WHERE HE WOULD BE RECEIVING PROPER LOC DME DATA. I STATED THAT IT WOULD BE ON THE R DME WITH AN 'L' IN FRONT. HE STARTED TO DSND FROM 2000 FT AT WHAT HE BELIEVED TO BE REEBO, BUT HE WAS LOOKING AT THE INCORRECT DME (THE L ONE INITIALLY) FROM MAP MODE WHICH CAUSED HIM TO DSND BELOW 2000 FT (REEBO) EARLY. ONCE I REALIZED THAT HE WAS ON ILS EXPAND ALSO, I RETURNED TO MAP MODE FOR SITUATIONAL AWARENESS. AT THIS POINT, TWR STATED THAT SHE WAS SHOWING US LOW AT REEBO. I CONFIRMED THAT WE WERE AT REEBO BUT OUR ALT WAS 1400- 1500 FT MSL. WE LEVELED OFF WITH RWY IN SIGHT UNTIL VASI LIGHTS GAVE US CORRECT GS INFO. LNDG UNEVENTFUL. AFTER LNDG, WE DISCUSSED HOW WE HAD LEFT REEBO EARLY. THE CAPT STATED THAT HE REALLY SHOULD HAVE BRIEFED APCH BETTER TO CLARIFY WHICH DME WOULD BE THE CORRECT ONE FOR LOC DATA. I WAS UNAWARE THAT HE WASN'T SURE. I WAS ALSO SOMEWHAT CONFUSED BECAUSE I BELIEVED HE HAD LEFT REEBO ACCURATELY. LOOKING FOR 6.4 DME TO DSND, I BELIEVE WE HAD, FROM THE EXPANDED ILS LOC DME? IT IS EASY TO GET HIGH ON THIS APCH, AND I'M SURE THAT WAS ON OUR MINDS AS WE WERE DSNDING TO 700 FT MSL. BEING NEAR DUSK WITH ALL THE CLOUD COVER ALSO MADE THE APCH MORE DIFFICULT THAN USUAL. SUPPLEMENTAL INFO FROM ACN 316110: WE PASSED SWATT AND BEGAN DSCNT TO 2000 FT MSL. BECAUSE THE LNAV WAS DISPLACED, I SWITCHED MY HSI TO EXPANDED ILS TO CHK THE COURSE. I SWITCHED BTWN MAP AND EXPANDED ILS SEVERAL TIMES. BECAUSE OF THIS SWITCHING OF HSI'S, I BELIEVE I STARTED A SIT. LOOKING BACK ON IT THE CONFUSION BEGAN ON MY PART IN REGARDS TO EXACTLY WHERE THE LOC DME WAS TO BE PRESENTED ON MY PANEL. IN THE FUTURE I MUST BE MORE SPECIFIC IN BRIEFING RADIOS, MAP, EXPANDED ILS. THEN IF CHANGES TAKE PLACE BOTH PLTS MUST KNOW AND KEEP UP.

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.