Narrative:

I was pilot flying. We were being vectored in to lga from the east for localizer 31. Cleared to descend to 2;000 ft and turned towards final by approach control. [We were] then given another turn to intercept and cleared for localizer 31 approach; maintain 2;000 until quene. The final intercept vector was going to bring us well inside quene; so I extended off fabry (FAF); armed the localizer; and switched to arc mode on my navigation display to monitor DME to identify abeam quene; and to monitor localizer capture. After passing abeam quene at 10.3 DME; with localizer capturing and fabry (FAF) next; I called for first officer to set and arm 600 ft which was our MDA; and I initiated descent out of 2;000 ft. Busy time now completing final configuration for landing; slowing down; and completing landing checklist. I had left arc mode on my navigation display. I was not aware that first officer was also in arc mode; thus neither of us were watching the 'football' on the navigation display which was now our only protection for crossing fabry at 1;700 ft; the published altitude at the FAF. Shortly after we were on the localizer and descending; we were told to contact tower. Before tower contact was made; the first officer figured it out and said 'hey; we're really low; we need to climb.' the altitude was approximately 1;100 ft; or 600 ft low; a couple of miles outside fabry. I immediately realized what I had done and climbed quickly back to 1;700 ft just as we reached fabry. We checked in with tower as I was correcting back to 1;700 ft and they issued a low altitude alert. After fabry a normal descent to a normal landing was made. We were IMC through all of this until about 1;100 ft. Ground was occasionally visible; but we were in clouds until that point. We did not get a GPWS warning.this one scared me. I fully grasp that being that far (600 ft) below a hard altitude on an approach will get you (and everyone's lives you have in your hands) killed in many; many places. I honestly don't think complacency was our issue. I was not cavalier about this approach and neither was the first officer. The approach was thoroughly briefed. One thing I did not specify though was who would have what (arc or map) displayed on the navigation display. We needed both for at least a short while; in order to see DME. As pilot flying; it was my responsibility to see that I had displayed what I needed to shoot this approach in accordance with our procedures. I made a mistake when I did not go back to map mode after passing abeam quene. If map mode had not been available; then I would have had to set 1700 feet for fabry; followed by 600 ft for the MDA after passing fabry; and the whole thing would not have happened. If MDA is set outside the FAF; then I must be in map prior to the FAF. I am fortunate to have had a good first officer; who regained situational awareness before I did; and that embarrassment likely the worst I will suffer. I know how I led myself into this; and will not permit that again.

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Original NASA ASRS Text

Title: MD80 flight crew reports inadvertently descending below the FAF crossing altitude during the LOC Runway 31 approach to LGA. The error is detected by the First Officer first and the Captain climbs back to 1;700 FT as the Tower is announcing a low altitude alert.

Narrative: I was pilot flying. We were being vectored in to LGA from the east for LOC 31. Cleared to descend to 2;000 FT and turned towards final by Approach Control. [We were] then given another turn to intercept and cleared for LOC 31 approach; maintain 2;000 until QUENE. The final intercept vector was going to bring us well inside QUENE; so I extended off FABRY (FAF); armed the LOC; and switched to arc mode on my NAV display to monitor DME to identify abeam QUENE; and to monitor LOC capture. After passing abeam QUENE at 10.3 DME; with LOC capturing and FABRY (FAF) next; I called for First Officer to set and arm 600 FT which was our MDA; and I initiated descent out of 2;000 FT. Busy time now completing final configuration for landing; slowing down; and completing Landing Checklist. I had left arc mode on my NAV display. I was not aware that First Officer was also in arc mode; thus neither of us were watching the 'football' on the NAV display which was now our only protection for crossing FABRY at 1;700 FT; the published altitude at the FAF. Shortly after we were on the LOC and descending; we were told to contact Tower. Before Tower contact was made; the First Officer figured it out and said 'hey; we're really low; we need to climb.' The altitude was approximately 1;100 FT; or 600 FT low; a couple of miles outside FABRY. I immediately realized what I had done and climbed quickly back to 1;700 FT just as we reached FABRY. We checked in with Tower as I was correcting back to 1;700 FT and they issued a low altitude alert. After FABRY a normal descent to a normal landing was made. We were IMC through all of this until about 1;100 FT. Ground was occasionally visible; but we were in clouds until that point. We did not get a GPWS warning.This one scared me. I fully grasp that being that far (600 FT) below a hard altitude on an approach will get you (and everyone's lives you have in your hands) killed in many; many places. I honestly don't think complacency was our issue. I was not cavalier about this approach and neither was the First Officer. The approach was thoroughly briefed. One thing I did not specify though was who would have what (arc or map) displayed on the NAV display. We needed both for at least a short while; in order to see DME. As pilot flying; it was my responsibility to see that I had displayed what I needed to shoot this approach in accordance with our procedures. I made a mistake when I did not go back to map mode after passing abeam QUENE. If map mode had not been available; then I would have had to set 1700 feet for FABRY; followed by 600 FT for the MDA after passing FABRY; and the whole thing would not have happened. If MDA is set outside the FAF; then I MUST be in map prior to the FAF. I am fortunate to have had a good First Officer; who regained situational awareness before I did; and that embarrassment likely the worst I will suffer. I know how I led myself into this; and will not permit that again.

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.