Air carrier captain reported an excursion from cleared altitude resulting in a low altitude warning....

Attributes
ACN	1465182
Time
Date	201707
Local Time Of Day	0601-1200
Place
Locale Reference	BOS.Airport
State Reference	MA
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	Light Transport Low Wing 2 Turboprop Eng
Operating Under FAR Part	Part 121
Flight Phase	Final Approach
Route In Use	Vectors
Flight Plan	IFR
Person 1
Function	Captain Pilot Not Flying
Qualification	Flight Crew Air Transport Pilot (ATP) Flight Crew Instrument Flight Crew Multiengine
Experience	Flight Crew Last 90 Days 200 Flight Crew Total 15750 Flight Crew Type 2500
Events
Anomaly	Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Published Material / Policy Deviation - Procedural Clearance Inflight Event / Encounter CFTT / CFIT

Narrative:

We setup and briefed the copilot's approach to RNAV 4L into boston logan airport. Leveled off at final approach intercept altitude using the autopilot. Set next altitude in altitude preselect. On final approach; we entered VMC conditions the copilot looked at the GPS to verify our position related to the FAF. I was looking outside for traffic as we were VMC. He believed to be over the FAF and began to descend. ATC issued a low altitude alert and we immediately leveled off. ATC asked if we had the runway and traffic to follow in sight to which we replied; yes we did. ATC then cleared us for the visual approach which continued without incident.

Original NASA ASRS Text

Title: Air carrier Captain reported an excursion from cleared altitude resulting in a low altitude warning. The crew corrected the deviation and landed without incident.

Narrative: We setup and briefed the copilot's approach to RNAV 4L into Boston Logan airport. Leveled off at final approach intercept altitude using the autopilot. Set next altitude in altitude preselect. On final approach; we entered VMC conditions the copilot looked at the GPS to verify our position related to the FAF. I was looking outside for traffic as we were VMC. He believed to be over the FAF and began to descend. ATC issued a low altitude alert and we immediately leveled off. ATC asked if we had the runway and traffic to follow in sight to which we replied; yes we did. ATC then cleared us for the visual approach which continued without incident.

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.

Air carrier captain reported an excursion from cleared altitude resulting in a low altitude warning. The crew corrected the deviation and landed without incident.