Narrative:

In night VMC; we were using LNAV/VNAV to the FAF of the RNAV (GPS) X runway 34L; and planned to fly the final approach segment visually. I was the pilot flying and the first officer was the pilot not flying. With approach's clearance; in LNAV and VNAV path; and past spoon (the IAF); we set the altitude window to zero. Approach subsequently gave us an intermediate altitude restriction (10;500 ft) and I re-programmed the FMC. After the restriction; I used level change at 230 KIAS to continue the descent; the first officer coordinated with me and re-set the altitude window to 8;100 ft (karme restriction). When we were within 200 ft of our descent path; I re-selected VNAV and I recall that the FMA showed VNAV path; so I reset the altitude window. I now believe we were actually in (or reverted to) VNAV speed; because we did not level off at 8;100 ft. Approach called with a low altitude alert (we did not get any alert in the aircraft); and instructed us to climb. We climbed from about 7;500 ft to about 8;000 ft. At this point (past karme); we were too high to continue the approach. We executed a missed approach and were vectored to the ILS 34L.I will ensure that we have captured and are maintaining VNAV path during the approach descent and also crosscheck the legs page. In this case; ATC gave us a change (altitude restriction) after we had set the altitude window to zero. Therefore; it is important to coordinate and ensure all parameters are again met before setting the altitude window to zero the second time.

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Original NASA ASRS Text

Title: B737 flight crew reports descending below the FAF altitude during a night visual approach to Runway 34L at RNO; generating a low altitude warning from ATC. Confusion results in the aircraft getting too high to complete the approach and a go around ensues. The second approach is successful.

Narrative: In night VMC; we were using LNAV/VNAV to the FAF of the RNAV (GPS) X Runway 34L; and planned to fly the final approach segment visually. I was the pilot flying and the First Officer was the pilot not flying. With Approach's clearance; in LNAV and VNAV PATH; and past SPOON (the IAF); we set the altitude window to zero. Approach subsequently gave us an intermediate altitude restriction (10;500 FT) and I re-programmed the FMC. After the restriction; I used Level Change at 230 KIAS to continue the descent; the First Officer coordinated with me and re-set the altitude window to 8;100 FT (KARME restriction). When we were within 200 FT of our descent path; I re-selected VNAV and I recall that the FMA showed VNAV PATH; so I reset the altitude window. I now believe we were actually in (or reverted to) VNAV Speed; because we did not level off at 8;100 FT. Approach called with a low altitude alert (we did not get any alert in the aircraft); and instructed us to climb. We climbed from about 7;500 FT to about 8;000 FT. At this point (past KARME); we were too high to continue the approach. We executed a missed approach and were vectored to the ILS 34L.I will ensure that we have captured and are maintaining VNAV PATH during the approach descent and also crosscheck the LEGS page. In this case; ATC gave us a change (altitude restriction) after we had set the altitude window to zero. Therefore; it is important to coordinate and ensure all parameters are again met before setting the altitude window to zero the second time.

Data retrieved from NASA's ASRS site as of April 2012 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.