B737 Undifferentiated or Other Model
Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 169; Flight Crew Type 402
Confusion; Situational Awareness
Deviation - Altitude Crossing Restriction Not Met; Deviation - Speed All Types; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance
Flight Crew Returned To Clearance; Flight Crew Became Reoriented
Human Factors
At 4;000 feet; cleared to 3;000 feet and cleared for the ILS Runway 16R between AGANE and CELAK at SEA. Aircraft was flying below the glideslope and continued to gradually descend at a normal rate when the PF (Pilot Flying) asked for the touchdown zone to be set in the altitude window. I paused with my fingers on the altitude window selector knob and asked the PF he wanted to set 1;900 feet in the altitude window instead; since we had not captured the glideslope nor met all the remaining altitude constraints prior to the glide path intercept point. We continued a gradual descent below 3;000 feet and below glideslope with automation off. In the confusion he continued to descend and seemed distracted by my question. I noticed he had now descended well below glideslope; was slowing; and continuing to descend. BFI airfield was directly off the nose and visible. I think this may have added confusion to the Captain's situational awareness. I immediately pointed out that we needed to be higher and began steady; slow paced altitude and airspeed callouts to help the Pilot Flying return to the ILS profile. The Pilot Flying continued to hesitate due to lost situational awareness. We crossed BUGNE at approximately 1;500 feet MSL. I made at least two more inputs in a calm and well-paced manner that we needed to climb and cross FINKA at 1;900 feet. The situational awareness and recovery actions of the Pilot Flying improved and we crossed FINKA at 1;900 feet. The aircraft was properly configured with all SOP items and checklists completed; and the approach proceeded to an uneventful landing.
B737NG First Officer reported the Captain descended below the glideslope on approach and appeared to be unaware of the deviation even after being advised.