Medium Transport; High Wing; 2 Turboprop Eng
Flight Crew Air Transport Pilot (ATP)
Confusion; Situational Awareness
Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT
Air Traffic Control Issued Advisory / Alert; Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Procedure; Human Factors; Weather
We had loaded the RNAV Z for 16R in Seattle anticipating a visual. The LOC was out of service We VNAV'd to VASHN and did not receive the approach clearance and were now anticipating vectors. Approach cleared us to maintain 5;000 Feet and we initiated a V/S descent. After a few miles we received a vector 070 and we both activated the approach and entered 070 in the heading box. Approach gave us 130 to intercept the course for 16R and we initiated the intercept. At this time the path was active and showing the glideslope was below us. We were cleared to cross FINKA at 1;900 Feet; cleared for the RNAV approach to 16R. I initiated a descent to catch the glide path and we transitioned to VPath around 3;800 FEET. As we approached 3;000 Feet on VPATH and about 4 miles from FINCA we broke out and realized we were below the runway glide path. SEA Approach issued an altitude alert at that time and told us to check our altitude. We called the airport in sight for a visual and as we maintained 3;000 Feet or so until the glide path suddenly came up above us and behaved normally for the visual. We also received a 'Caution; Obstacle.' Call at that point; perhaps for the closure rate with downtown. We had loaded the VASHN transition and thus did not have the waypoints on the activated final approach course. This made us more reliant on the glide path. We might have loaded the PAE transition; but were rushed a little with the vectors and some speed reductions. We also anticipated breaking out earlier based on the ATIS; and though we had briefed the full approach; I was thinking it would be a quick transition to a visual. I'm still unclear why we had an erroneous Glidepath. It is possible there was latent VNAV from the HAWKZ or the transition from VASHN affected the activation of the approach somehow. Both of us had the same indications after the same inputs.
Twin turboprop Captain reports arriving over VASHN expecting a clearance for the RNAV Z to 16R at SEA; but instead receives vectors to the final approach course. A 130 heading to intercept is issued and the RNAV glideslope appears below the aircraft so the descent is continued until breaking out at 3;000 Feet about four miles from FINKA. The descent is arrested at the same time ATC issues a low altitude alert and the EGPWS alerts with a 'Caution; Obstacle.' The approach is continued to a normal landing.