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.
Synopsis
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.
Second reporter narrative
The Captain planned on flying in LNAV/VNAV path down to the FAF. The airport was visual from at least 35 miles out. While cleared for the approach; the aircraft pitch mode changed from VNAV PATH to VNAV Speed during the descent. The Captain wanted to correct back to the path with Level Change. The MCP was reset to the correct next minimum altitude on the approach for KARME. The aircraft nosed over and I don't recall the autopilot changing back to VNAV PATH. The Captain was heads-down looking at or making inputs to the FMC and not necessarily verbalizing all his changes with me.While trying to keep up with the Captain; we were both heads-down for a while until Approach told us to check our altitude. At that point; we were about 600 FT to 700 FT below the minimum for KARME. The Captain started an immediate climb back up to the minimum altitude as set in the window. Looking down on the LEGS page it looked like we were already past KARME. The Captain and I were both confused; which led us to be too high for a safe approach. The Captain performed a go-around and flew another visual approach backed up by the ILS. Verbalize everything so both pilots understand what to verify and monitor during each phase of flight.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.