B757 flight crew reported descending 300 ft below the glideslope on approach to jfk.

Attributes
ACN	1452524
Time
Date	201705
Local Time Of Day	0001-0600
Place
Locale Reference	JFK.Airport
State Reference	NY
Environment
Flight Conditions	IMC
Light	Night
Aircraft 1
Make Model Name	B757 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	Initial Approach
Flight Plan	IFR
Person 1
Function	Pilot Flying Captain
Qualification	Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 99 Flight Crew Total 16000 Flight Crew Type 3000
Person 2
Function	First Officer Pilot Not Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 75 Flight Crew Total 6500 Flight Crew Type 3500
Events
Anomaly	Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Clearance Inflight Event / Encounter CFTT / CFIT

Narrative:

Cleared for approach to [jfk] runway 4L; maintain 2000 until established. ILS signal was verified. Localizer/GS engaged; as localizer began to come in but not yet captured; GS engaged and started down; localizer then captured. At that time we noticed a yellow line thru the GS and PNF queried ATC as to GS function. Aircraft continued to descend on autopilot and ATC informed us GS was functioning normally and minimum altitude was 1500 in this area; we were at 1200. We turned off the autopilot and climbed back to 1500. We were still just outside the final approach fix; reset localizer/GS and reengaged autopilot with good signals. The rest of the approach to landing was uneventful.

Original NASA ASRS Text

Title: B757 flight crew reported descending 300 ft below the glideslope on approach to JFK.

Narrative: Cleared for approach to [JFK] RWY 4L; maintain 2000 until established. ILS signal was verified. LOC/GS engaged; as LOC began to come in but not yet captured; GS engaged and started down; LOC then captured. At that time we noticed a yellow line thru the GS and PNF queried ATC as to GS function. Aircraft continued to descend on autopilot and ATC informed us GS was functioning normally and minimum altitude was 1500 in this area; we were at 1200. We turned off the autopilot and climbed back to 1500. We were still just outside the final approach fix; reset LOC/GS and reengaged autopilot with good signals. The rest of the approach to landing was uneventful.

Data retrieved from NASA's ASRS site 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.