Large Transport; Low Wing; 2 Turbojet Eng
Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 145; Flight Crew Type 6700
Situational Awareness; Distraction
Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT
Air Traffic Control Issued Advisory / Alert; Flight Crew Returned To Clearance
Human Factors
Cleared a visual approach to Runway 10 at BWI by Approach Control and told to switch to Tower. I was heads down changing radio frequency and check-in with Tower and was issued a Low Level Altitude Alert. I did not monitor Pilot Flying who had started descent to glideslope intercept altitude; 1500 ft. AGL. No GPWS alert only Tower Advisory. Pilot Flying climbed up 500 ft. to 2000 ft.; joined LOC and glideslope outside of marker and  landed uneventfully.[Recommend] use of LNAV and VNAV to join visual approach. Delay calling tower until established on final. Do not start descending without being on loc.
Cleared on the visual descending to intercept altitude and onto final ATC advised us of low altitude alert. I felt clear of all terrain and we got no terrain warnings inside our aircraft. Leveled off anyway and intercepted at 2;000 ft. instead of 1;500 ft.
Air carrier flight crew reported distraction caused failure to monitor pilot flying starting early descent which resulted in ATC issuing low altitude alert.