Date: 201606
Local time of day: 1201-1800
Location: LGA.Airport, NY
Flight conditions: Mixed
Weather elements / visibility: Cloudy; Rain; 7
Light: Daylight
Aircraft: B737 Next Generation Undifferentiated
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; Confusion
Second reporter function: Pilot Not Flying; First Officer
Anomaly: Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT
Detector: Automation Aircraft Terrain Warning
When detected: In-flight
Result: Flight Crew Became Reoriented; Flight Crew FLC complied w / Automation / Advisory
Contributing factors: Human Factors; Weather
Primary problem: Human Factors

Narrative: Cleared for the LOC 31 LGA; inbound to FABRY; level at 1;700 feet; fully configured; I set 0 in the MCP altitude window. Crossing FABRY; I set 800 fpm down with the Vertical Speed wheel as previously briefed. When we checked in with Tower; we were told to continue as there would be multiple aircraft departing before our arrival. The DDA had been set on our BARO bugs at 650 feet. 'Approaching minimums' call was made; I said 'Going Outside.' As we descended below the ceiling; 'Minimums' was called. Acquiring the runway environment; I said 'Landing'. Trying to reconstruct the events; I believe I left the Vertical Speed in the 800 fpm down; previously set. We briefly went below DDA; and simultaneously we heard a 'Caution; Obstacle.' By this time; we were below the ceiling and had visual contact with the ground. I immediately disconnected the autopilot and autothrottles and adjusted our vertical path. We leveled and continued toward the runway. We were given clearance to land and we landed on Runway 31. My approach briefing should have taken into consideration the significant headwind we experienced on the final portion of the approach. In selecting 800 fpm for my descent; I had failed to account for the effect the wind would have on our groundspeed. In addition; when flying non-precision approaches such as this LOC 31; I need to be ever vigilant of the necessity to 'fly' the MCP. During this approach; while trying to obtain the visual cues necessary while 'going outside' I failed to correctly manipulate the MCP and we ended up descending below DDA.


Second narrative: We were cleared for the Localizer approach to Runway 31 at LGA airport. Aircraft was properly configured; and all checklists were finished. The landing runway and airport environment were difficult to determine clearly; due to mixed visibility. Although I could see the ground below; it was hard to see forward. Callouts on the approach were made; but the Captain inadvertently descended below the DDA minimums of 650 feet; while trying to keep the runway in sight through rain and windshield wipers. At the same time of passing the DDA; a 'caution; obstacle' alert sounded. The Captain disconnected the autopilot and autothrottles; and immediately corrected our flight path; which involved climbing less than 100 feet. The runway was still in sight; since we were below the ceiling; Tower cleared us to land and we continued to an uneventful landing. At no time did Tower issue a low altitude alert and there was no traffic conflict.


Synopsis: B737NG flight crew flying the LGA Runway 31 LOC Approach reported the aircraft descended below the calculated glide path to arrive at the derived decision altitude (DDA) prior to where the correct glide path would have placed the aircraft. The flight crew received an EGPWS 'Caution; Obstacle' alert; leveled the aircraft; and proceeded to land uneventfully.