Date: 201403
Local time of day: 1201-1800
Location: LGA.Airport, NY
Flight conditions: VMC
Light: Daylight
Aircraft: B737 Next Generation Undifferentiated
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Approach
Airspace: Class B LGA
ATC / advisory: Tower LGA
Aircraft component: Autoflight System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Workload; Situational Awareness; Distraction; Confusion; Human-Machine Interface
Second reporter function: First Officer; Pilot Flying
Anomaly: Aircraft Equipment Problem Less Severe; Deviation - Altitude Crossing Restriction Not Met; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance
Detector: Person Air Traffic Control; Person Flight Crew
When detected: In-flight
Result: Aircraft Automation Overrode Flight Crew; Flight Crew Returned To Clearance; Flight Crew Took Evasive Action; Flight Crew FLC Overrode Automation; Flight Crew Became Reoriented; General Maintenance Action
Contributing factors: Aircraft; Procedure
Primary problem: Ambiguous

Narrative: While on a visual approach to Runway 22 at LGA; Runway 22 ILS was tuned and identified. We were cleared for the visual from 4;000 FT. 1;900 FT was set in the MCP and APP was selected. VOR/LOC and glideslope both turned green in the FMA and the aircraft began to descend. The aircraft began to descend more rapidly than normal and began accelerating. The aircraft descended well below the glide path but the cockpit indications of both myself and the First Officer show on glideslope. Approaching 1;900 FT we began to override the automation; both flight directors OFF; autopilot OFF and altitude hold selected. The aircraft leveled off at 1;500 FT. We continued with the approach without automation and landed on [Runway] 22. I then wrote this up in the maintenance logbook.Aircraft flew below glideslope with autopilot ON and glideslope COUPLED while cockpit indications showed on glideslope.

Callback: The reporter stated that he flies into LGA quite frequently and knows the approaches well. The Runway 22 descent from 4;000 FT to landing would normally not be interrupted and by Company procedure the crew would set the MDA or DA after coupling the localizer and glideslope. In this case he chose to set the GREKO constraint at 1;900 FT. The aircraft's initial descent rate from 4;000 FT was between 900 - 1;000 feet per minute and with an accelerating airspeed. This reporter was the pilot not flying and because of his LGA flying sensed the descent was not correct; but the PFD and ND both indicated ON glideslope but very slightly left on the localizer. Passing 1;900 FT about 2.5 miles prior to GREKO the reporter alerted the flying pilot about the deviation. The flying pilot attempted to shallow or stop the rate of descent but because the aircraft was coupled had no direct control. The reporter directed him to turn both flight directors OFF; the autopilot OFF and level; which was accomplished by 1;500 FT. The entire ILS capture; to descend on glideslope; and then acceleration happened very quickly. ATC called as they leveled and asked if they were 'OK;' but no altitude alert was issued. They did not go-around because once the automation was off the aircraft approached the correct profile. While level at 1;500 FT they flew into the actual Runway 22 glideslope but the reporter did not remember how the transition occurred or how the instrumentation looked leaving the apparent false glideslope.

Second narrative: [Report narrative contained no additional information]


Synopsis: A B737NG flying an autopilot coupled approach to LGA Runway 22 in VMC; apparently captured a false glideslope at 4;000 FT and with the localizer and glideslope engaged began a higher than normal rate of descent. The crew turned both flight directors OFF; autopilot OFF and leveled at 1;500 FT well below the glideslope prior to GREKO which has a 1;900 FT crossing restriction.