Q400
Flight Crew Air Transport Pilot (ATP)
Workload; Situational Awareness; Human-Machine Interface; Distraction; Communication Breakdown
Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance
Flight Crew Returned To Clearance
Aircraft; Company Policy; Human Factors; Procedure
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.
We were in cruise flight and had been assigned go as fast as possible and to disregard all speed restrictions on arrival.  I said something similar to 'level off' and 'stop your descent' and 'return to 12;000 FT'.I will list two primary causes: First; attempting to execute a plan 1 or 2 seconds after that plan had become untenable. The plan was untenable because; due to programming deficiencies in the current FMS software; the aircraft was not capable of executing it. Secondly; lack of altitude awareness after turning off the auto pilot and hand flying due to experiencing widely known FMS deficiencies. Contributing to the two primary causes above: 1. The First Officer had set up the FMS according to the company SOP designed to prevent just this type of problem. He tried to [comply with the SOP] after it was too late to follow this course of action although; at the time; it looked reasonable to him. This plan included crossing both MARNR and RAYNI at 12;000. It looked reasonable because VNAV showed a rate of 1;700 FPM required to cross MARNR at 12;000. 2. The clearance from ATC to go as fast as possible and to disregard all speed restrictions on the arrival. 3. The clearance from ATC to 'descend via' came too late; therefore a new plan was required. 4. The altitude alerter was correctly set at 6;000 FT; thus the First Officer lacked the cue of a 12;000 FT altitude preset when turning off the auto pilot to hand fly. 5. My divided attention. I was listening to ATIS; monitoring ATC; and monitoring the position and movement of the aircraft in space and progress along the arrival. I was not monitoring the First Officer's fingers pushing buttons. 6. My failing to note the First Officer was stressed and at about 110% of capacity. A little more awareness on my part could have prevented the button pushing that led to this event. I understand that I used some politically charged words referring to the current programming and functioning of our FMS system as 'deficient'. I refer to it as 'deficient' because it does not work in the manner that it is designed to work and requires 'work arounds' [developed by management] pilots that are disseminated via SOP Bulletins and other means. I believe the combination of these FMS deficiencies; the company 'solutions;' and the [demanding] RNAV arrivals into SEA are going to get some pilots in trouble. We were lucky. I don't believe that our deviation was of sufficient magnitude or duration or that it caused any issues that would bring it to light outside of this report.
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.