Narrative:

The flight was uneventful until the visual approach for runway 16L in den. The visual backed up by the ILS to 16L approach was briefed. The approach checklist was completed on the downwind leg. We were radar vectored on the base leg for runway 16L. The controller asked if we had the runway/airport. The captain replied yes so I called the field in sight. He then gave us another vector for the visual that took us inside the final approach fix on a left dogleg base visual approach to final and asked us to maintain 170 KTS until final. The captain asked me to extend the final approach course from the runway in the FMC and set 5;400 ft in the altitude window. If I recall correctly we were out of approximately 7;000 ft and configured at 170 KTS at flaps 10. I've seen different captains use different techniques for altitudes they call for while on the visual approach so I didn't think anything of his technique in this case. I did however call his configuration on the MCP. I said verbatim. 'You're in level change; and 5;400' is set in the altitude window; you're slightly below the glideslope.' the captain stated; 'I know; I've got it.' the captain continued his descent on the left dogleg to final. As we got lower I stated; 'you are getting a little too low out here; are you sure you have it?' the captain replied again 'yea; yea; I've got it.' if I recall correctly we were not yet below 1;000 ft AGL because I was still waiting to make the 1;000 ft call; but we were still below the glide slope to the runway. I was anticipating the captain clicking the off autopilot for the visual and leveling us out until we joined the glide path on final. As we continued from this point all of my attention went to visually looking outside the aircraft to see where we were in relation to the runway; because for a few seconds I thought that the captain might have made an error in identifying the runway surface while on the visual approach. As we began to turn final I could see that he was turning for the correct runway but too low for our distance out; so I stated 'we're still way too low out here are you sure you want to be at this altitude.' the captain promptly stated 'no.' I was about to call for a go-around because I though the captain was going to continue the descent but then he immediately disengaged the autopilot and began corrective actions. At the same time he leveled off and added power we got the gear horn. The tower controller issued us a low altitude alert. I responded to the controller with our call sign to let him know that we were aware and correcting. At this point we were in a slow climb to capture the glideslope; the captain then called for gear down; flaps 15; landing checklist; and then flaps 30. We quickly captured the glideslope and continued to a normal landing. I waited until we were parked at the gate to ask the captain what happened on the approach. He stated that he got confused and mistook the 5;400 ft altitude that he saw in the FMC after extending the final approach course from the runway; for a fix and altitude that was further out on the approach.

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Original NASA ASRS Text

Title: A B737 Captain flying a night visual approach to DEN Runway 17L; requested MCP ALT 5;400 FT with LVL CHG engaged and descended below glideslope to about 1;000 FT AGL as the First Officer questioned his situational awareness until ATC called a low level alert.

Narrative: The flight was uneventful until the visual approach for Runway 16L in DEN. The visual backed up by the ILS to 16L approach was briefed. The Approach Checklist was completed on the downwind leg. We were radar vectored on the base leg for Runway 16L. The Controller asked if we had the runway/airport. The Captain replied yes so I called the field in sight. He then gave us another vector for the visual that took us inside the final approach fix on a left dogleg base visual approach to final and asked us to maintain 170 KTS until final. The Captain asked me to extend the final approach course from the runway in the FMC and set 5;400 FT in the altitude window. If I recall correctly we were out of approximately 7;000 FT and configured at 170 KTS at flaps 10. I've seen different captains use different techniques for altitudes they call for while on the visual approach so I didn't think anything of his technique in this case. I did however call his configuration on the MCP. I said verbatim. 'You're in level change; and 5;400' is set in the altitude window; you're slightly below the glideslope.' The Captain stated; 'I know; I've got it.' The Captain continued his descent on the left dogleg to final. As we got lower I stated; 'You are getting a little too low out here; are you sure you have it?' The Captain replied again 'yea; yea; I've got it.' If I recall correctly we were not yet below 1;000 FT AGL because I was still waiting to make the 1;000 FT call; but we were still below the glide slope to the runway. I was anticipating the Captain clicking the off autopilot for the visual and leveling us out until we joined the glide path on final. As we continued from this point all of my attention went to visually looking outside the aircraft to see where we were in relation to the runway; because for a few seconds I thought that the Captain might have made an error in identifying the runway surface while on the visual approach. As we began to turn final I could see that he was turning for the correct runway but too low for our distance out; so I stated 'we're still way too low out here are you sure you want to be at this altitude.' The Captain promptly stated 'no.' I was about to call for a go-around because I though the Captain was going to continue the descent but then he immediately disengaged the autopilot and began corrective actions. At the same time he leveled off and added power we got the gear horn. The Tower Controller issued us a low altitude alert. I responded to the Controller with our call sign to let him know that we were aware and correcting. At this point we were in a slow climb to capture the glideslope; the Captain then called for gear down; flaps 15; Landing Checklist; and then flaps 30. We quickly captured the glideslope and continued to a normal landing. I waited until we were parked at the gate to ask the Captain what happened on the approach. He stated that he got confused and mistook the 5;400 FT altitude that he saw in the FMC after extending the final approach course from the runway; for a fix and altitude that was further out on the approach.

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.