Date: 201312
Local time of day: 1801-2400
Location: LGA.Airport, NY
Flight conditions: IMC
Aircraft: MD-80 Series (DC-9-80) Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Final Approach
Airspace: Class B LGA
ATC / advisory: Tower LGA
Aircraft component: MCP
Component problem: Improperly Operated
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness; Training / Qualification
Second reporter function: First Officer; Pilot Not Flying
Anomaly: Deviation - Altitude Crossing Restriction Not Met; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT
Detector: Person Flight Crew; Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Contributing factors: Human Factors
Primary problem: Human Factors

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.


Second narrative: Approaching LGA; we were vectored for the LOC Runway 31 on a heading that would take us just inside the initial fix (IF). We extended the approach course off the FAF; and were cleared for the approach to maintain 2;000 FT until established. The altitude from IF to FAF was 1;700 FT. The pilot flying requested the MDA be set in the altitude select window; as he would follow the vertical descent path indicator to MDA; which would clear the FAF at 1;700 FT. We both switched between map and arc mode on the nav display to identify the IF with DME. After crossing the IF I was configuring; performing mechanical checklist items and switching to Tower frequency. I noticed the pilot flying started a descent; and when I was able to look back at the nav display; it appeared we were still outside the FAF and below 1;700 FT. It took me a second to convince myself of this; but I commanded 'stop descending' and stated we were outside the FAF and should be at 1;700 FT. The pilot flying immediately corrected and was approaching the correct altitude when LGA Tower advised of a low altitude alert. The airport was in sight and we were cleared to land. Weather conditions at the time were not good VFR; visibility about 7 in light rain.I have never been used to selecting the MDA while descending outside the FAF; unless I was absolutely certain we were on a path to cross the FAF above the minimum altitude. The alerter would have made us both aware of the deviation several seconds sooner. We also increased our workload by changing the map display close in; when we could have just used normal nav and the arc mode on the nav display; and DME to positively indentify all of the remaining fixes; instead of switching modes.


Synopsis: 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.