Narrative:

On radar vectors to ILS 35L den. Localizer identification on both sides by pilot not flying. Given heading to intercept localizer; maintain 11;000 ft until established; cleared for ILS 35L. Our intercept point was just outside cruup. As localizer intercepted; pilot flying announced there was a right cross track error reference extended centerline in FMC. Pilot not flying stated established on localizer. We both verified the localizer frequency; course; and identification correct. Lateral raw data correct. Glide slope started to come down off max low position indicating we were approaching cruup. Pilot flying set 10;000 ft in MCP and started V/south descent. Pilot flying announced we would make chola 10;000 ft restriction and set MCP to 9;000 ft. At 9;600 ft ATC asked if we were on glide slope. Pilot not flying advised no we were not on glide slope. Pilot not flying (captain) engaged altitude hold at 9;500 ft. Aircraft leveled off. At this point; both pilot flying/pilot not flying identified we both lost our situational awareness as to distance from runway. ATC advised we need to meet crossing restrictions. Pilot not flying asked if there was a problem. ATC said possibly. We maintained 9;500 ft and recaptured the profile; rest of approach and landing normal.1. Both pilots briefly lost distance situational awareness with the lateral cross track distraction.2. At no time was separation; terrain clearance or any safety compromised. 3. This incident was a pure human error (mistake). No safety compromised. Once raw data verified; stick with it. I should have taken control of plane while first officer got his situational awareness back. Instead I was distracted instead of staying on a sure thing.

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Original NASA ASRS Text

Title: B737 flight crew reports descending early during the ILS 35L at DEN in IMC. While troubleshooting a discrepancy between FMC position and raw data LOC; situational awareness is lost. ATC notices the early descent and questions the crew.

Narrative: On radar vectors to ILS 35L DEN. LOC ID on both sides by pilot not flying. Given heading to intercept LOC; maintain 11;000 FT until established; cleared for ILS 35L. Our intercept point was just outside CRUUP. As LOC intercepted; pilot flying announced there was a right cross track error reference extended centerline in FMC. Pilot not flying stated established on LOC. We both verified the LOC frequency; course; and ID correct. Lateral raw data correct. Glide slope started to come down off max low position indicating we were approaching CRUUP. Pilot flying set 10;000 FT in MCP and started V/S descent. Pilot flying announced we would make CHOLA 10;000 FT restriction and set MCP to 9;000 FT. At 9;600 FT ATC asked if we were on glide slope. Pilot not flying advised no we were not on glide slope. Pilot not flying (Captain) engaged ALT HOLD at 9;500 FT. Aircraft leveled off. At this point; both pilot flying/pilot not flying identified we both lost our situational awareness as to distance from runway. ATC advised we need to meet crossing restrictions. Pilot not flying asked if there was a problem. ATC said possibly. We maintained 9;500 FT and recaptured the profile; rest of approach and landing normal.1. Both pilots briefly lost distance situational awareness with the lateral cross track distraction.2. At no time was separation; terrain clearance or any safety compromised. 3. This incident was a pure human error (mistake). No safety compromised. Once raw data verified; stick with it. I should have taken control of plane while First Officer got his situational awareness back. Instead I was distracted instead of staying on a sure thing.

Data retrieved from NASA's ASRS site as of July 2013 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.