Narrative:

During descent into sea; we descended 300 ft below charted altitude. While in cruise at 13;000; the captain went offline to check ATIS and communicate with sea operations. I was instructed to descend via the marnr 3 arrival; landing south. Before initiating descent; I verified that the FMS altitudes complied with all charted altitudes. I then set altitude alerter to 6;000 and set a VNAV descent to cross marnr at 12;000. The FMS showed a descent rate of approximately 1;800 FPM to cross marnr.at this point; I initiated a vto [FMS commanded] descent to meet the crossing restriction. As we were now less than 3 NM from marnr; the newly adjusted descent jumped to approximately 2;500 FPM. I was uncomfortable with the high rate of descent and quickly changed the VNAV descent into a 1;500 FPM vertical speed descent. [As a result we were] no longer protected by the FMS programmed altitudes. Between marnr and rayni [required crossing 'at or above 12;000 ft] we descended to 11;700 before the captain was back online and noticed the altitude deviation. I then corrected back to the charted 12;000 altitude. With both pilots back to flying and cross-checking altitudes and navigation we flew the remainder of the descent; approach and landing in compliance with all regulations and procedures. ATC did not mention the inadherence to charted altitudes. Causethe deviation occurred because of inadequate understanding of charted altitudes and high workload.

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Original NASA ASRS Text

Title: A Q-400 descended below the crossing restriction at RAYNI when cleared to descend 'via' the MARNR RNAV STAR to SEA. A late descent clearance; a previous clearance to disregard speed restrictions and maintain maximum forward speed and the resultant demands that exceeded the capability of the autoflight system were contributing factors.

Narrative: During descent into SEA; we descended 300 FT below charted altitude. While in cruise at 13;000; The Captain went offline to check ATIS and communicate with SEA Operations. I was instructed to descend via the MARNR 3 arrival; landing south. Before initiating descent; I verified that the FMS altitudes complied with all charted altitudes. I then set altitude alerter to 6;000 and set a VNAV descent to cross MARNR at 12;000. The FMS showed a descent rate of approximately 1;800 FPM to cross MARNR.At this point; I initiated a VTO [FMS commanded] descent to meet the crossing restriction. As we were now less than 3 NM from MARNR; the newly adjusted descent jumped to approximately 2;500 FPM. I was uncomfortable with the high rate of descent and quickly changed the VNAV descent into a 1;500 FPM vertical speed descent. [As a result we were] no longer protected by the FMS programmed altitudes. Between MARNR and RAYNI [required crossing 'at or above 12;000 FT] we descended to 11;700 before the Captain was back online and noticed the altitude deviation. I then corrected back to the charted 12;000 altitude. With both pilots back to flying and cross-checking altitudes and navigation we flew the remainder of the descent; approach and landing in compliance with all regulations and procedures. ATC did not mention the inadherence to charted altitudes. CauseThe deviation occurred because of inadequate understanding of charted altitudes and high workload.

Data retrieved from NASA's ASRS site as of July 2013 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.