B737-700
Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument
Flight Crew Last 90 Days 465
Human-Machine Interface; Distraction; Situational Awareness; Confusion
Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance; Flight Deck / Cabin / Aircraft Event Other / Unknown; Inflight Event / Encounter CFTT / CFIT
Air Traffic Control Issued Advisory / Alert; Flight Crew Became Reoriented
Human Factors
On downwind to XXR; cleared for visual; I set 2800 feet. Captain verified it. On base leg; we were told of a King Air on the opposite side. We saw it on TCAS. I'd planned and briefed joining final outside FAF; as we were still descending to GSIA (Glide Slope Intercept Altitude). As I turned to base; about 3 miles outside FAF; I looked left to see the runway and adjusted the heading bug slightly to left for better intercept angle; or so I thought. All of a sudden; the sight picture looked lower; I looked up and the aircraft was at 2100-2200 feet MSL. I looked at the alerter and it was set to 1500 feet. Clicked autopilot off; climbed back up to 2800 feet MSL. At this time; ATC gave us a low altitude alert. We acknowledged; and at 2800 MSL; intercepted glideslope and continued on approach to landing. I think I may have inadvertently twisted the altitude alerter instead of heading bug to the left as I was setting a better intercept angle. But because the visual picture looked low; I was able to recognize and correct the situation.
B-737 First Officer reported descending below glideslope intercept altitude during vector to final.