Narrative:

I was pilot monitoring/pilot not flying. We arrived into seattle on the Hawkz4 arrival landing south expecting ILS 16R. The weather was marginal VFR at the surface and IMC aloft with some moderate precipitation being reported around seattle. We also experienced occasional light to moderate turbulence on the descent. We made visual contact with the ground around 2;000 feet MSL. We received a descend via Hawkz4 arrival followed by a 'descend and maintain 4;000' after passing rayuu. As we leveled off at 4;000; approach vectored us to a heading of 070 degrees. The 070 degree heading was essentially our base turn prior to final. Note; throughout the arrival and approach into seattle; multiple aircraft kept reporting they were about to 'cross' the localizer to approach control. As we approached the final approach course ATC gave us a clearance of 'turn right heading 130 to intercept the localizer. Cleared ILS 16R approach.' the 130 degree heading put us between celak (12.7 DME from seattle with 4;000 altitude crossing) and bugne (10.1 DME from seattle with 3;200 altitude fix). I read back the clearance and my captain selected 1;900 ft on the altitude window (1;900 ft was the FAF altitude); and either selected flight level change (flch) or vertical speed (vs); I can't remember what he specifically selected. He also armed the approach by selecting app on the guidance panel. Once we were established inbound on the localizer; we broke out of IMC around 2;000 ft between celak and bugne. As we leveled off at 1;900 I realized that we were way too low on the approach. As I was about to say something to my captain; approach control gave us an altitude alert! I immediately told ATC that we're 'climbing' and ATC said 'maintain 2;100.' we leveled off at 2;100 and ATC said 'did you misunderstand maintain?' my captain said 'say yes' and so I said 'misread.' we eventually captured the glide slope and made a successful landing into seattle. During the debrief portion of the parking checklist; my captain and I talked about what happen. We both agreed we heard an intercept heading and cleared for the ILS 16R but weren't sure if we ever heard a 'maintain' altitude until established. I admitted to my captain that because of the massive amount of air traffic congestion on the radio I may have missed the maintain altitude. My captain said he was almost certain that he didn't hear a maintain altitude either. This is a classic mistake of losing situational awareness. While we were both task saturated; we both should have been able to catch our error by descending below the altitudes of celak and bugne.

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Original NASA ASRS Text

Title: EMB175 First Officer reported a descent to 1900 feet between CELAK and BUGNE during the SEA ILS Runway 16R. ATC issued a low altitude alert and a clearance to climb to 2100 feet. Depicted altitude between these fixes is 3200 feet.

Narrative: I was Pilot Monitoring/Pilot Not Flying. We arrived into Seattle on the Hawkz4 Arrival Landing South expecting ILS 16R. The weather was Marginal VFR at the surface and IMC aloft with some moderate precipitation being reported around Seattle. We also experienced occasional light to moderate turbulence on the descent. We made visual contact with the ground around 2;000 feet MSL. We received a Descend via Hawkz4 Arrival followed by a 'descend and maintain 4;000' after passing RAYUU. As we leveled off at 4;000; Approach vectored us to a heading of 070 degrees. The 070 degree heading was essentially our base turn prior to final. Note; throughout the arrival and approach into Seattle; multiple aircraft kept reporting they were about to 'cross' the localizer to Approach Control. As we approached the final approach course ATC gave us a clearance of 'turn right heading 130 to intercept the localizer. Cleared ILS 16R Approach.' The 130 degree heading put us between CELAK (12.7 DME from Seattle with 4;000 altitude crossing) and BUGNE (10.1 DME from Seattle with 3;200 altitude fix). I read back the clearance and my captain selected 1;900 ft on the altitude window (1;900 ft was the FAF altitude); and either selected Flight Level Change (FLCH) or Vertical Speed (VS); I can't remember what he specifically selected. He also armed the approach by selecting APP on the guidance panel. Once we were established inbound on the localizer; we broke out of IMC around 2;000 ft between CELAK and BUGNE. As we leveled off at 1;900 I realized that we were way too low on the approach. As I was about to say something to my captain; Approach Control gave us an altitude alert! I immediately told ATC that we're 'climbing' and ATC said 'maintain 2;100.' We leveled off at 2;100 and ATC said 'did you misunderstand maintain?' My captain said 'say yes' and so I said 'misread.' We eventually captured the glide slope and made a successful landing into Seattle. During the debrief portion of the Parking Checklist; my captain and I talked about what happen. We both agreed we heard an intercept heading and cleared for the ILS 16R but weren't sure if we ever heard a 'Maintain' altitude until established. I admitted to my captain that because of the massive amount of air traffic congestion on the radio I may have missed the maintain altitude. My captain said he was almost certain that he didn't hear a maintain altitude either. This is a classic mistake of losing situational awareness. While we were both task saturated; we both should have been able to catch our error by descending below the altitudes of CELAK and BUGNE.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.