B767-300 and 300 ER
Flight Crew Air Transport Pilot (ATP)
Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter CFTT / CFIT
Flight Crew Became Reoriented
Human Factors
At 4;000 feet MSL on downwind; we were cleared by Seattle Approach Control for the Bay Visual Approach to RWY 16C.  First Officer was flying.  When we were handed off to Seattle Tower; the Tower Controller asked us if we wanted to switch to RWY 16L.  I told him we would prefer to stay with 16C; and he cleared us for a visual approach to 16C.  First Officer then began descending on autopilot using vertical speed mode and maneuvered for the runway using heading select mode.  I was performing the Before Landing Checklist and then noticed that we were more than one and a half dots low on the ILS glideslope.  I called out the deviation and the First Officer responded 'correcting.'  We continued to go further below the glideslope and I was about to call it out again when the First Officer disengaged the autopilot and leveled off at approximately 1;600 feet to intercept the glideslope.  I believe that the use of vertical speed mode instead of flight level change mode to descend before intercepting the localizer course eliminates a safeguard against getting below the glideslope. I should have questioned the use of vertical speed mode prior to interception of the localizer course. Also; I had my eyes outside too much and did not sufficiently back up the First Officer. I believe that using vertical speed mode for descent in the terminal area should only be used when already on final course and to intercept the glideslope from above.  Prior to that; I feel that flight level change mode to a set altitude is a better technique.
Upon turning base I determined I was high (above glidepath) and corrected using Vertical Speed (VS) with the approach mode armed. I was primarily using outside reference and when [I] crosschecked instruments [I] was below glidepath and reduced the descent with VS and shortly thereafter disengaged the autopilot and manually leveled at 1;600 MSL to stabilize and intercept the glideslope. We continued the approach to a normal visual landing.
B767-300ER flight crew reported they got low on a night visual approach to SEA in part because the Pilot Flying was using vertical speed mode.