Q400
Flight Crew Air Transport Pilot (ATP)
Confusion; Situational Awareness
Deviation - Altitude Crossing Restriction Not Met; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Provided Assistance; Flight Crew Became Reoriented; Flight Crew FLC Overrode Automation; Flight Crew Returned To Clearance
Company Policy; Human Factors
Cleared to descend via [the] arrival. Cruise altitude 12;000 FT. (Suggested by [ATC] as better ride than 14;000 FT due to weather.) Arrival previously modified at gate in accordance with Company recommended VNAV altitudes on descent of 14;000 FT at Waypoint X and 6;000 FT at Waypoint A; with all other altitudes removed to insure smooth descent. Altitude gate at Waypoint X listed as between 15;000 FT to 12;000 FT. Upon receipt of clearance reset FMS Waypoint X altitude from 14;000 FT to 12;000 FT; reset altitude preselector from 12;000 FT to 6;000 FT. Engaged VNAV VTO to Waypoint X with a level leg. Aircraft began descent at Waypoint X. Crossing Waypoint Y; ATC queried our current altitude; (approximately 9;600 FT-9;500 FT); noted gate altitude was 10;000 FT minimum. Aircraft's direct flight path angle between Waypoint X at 12;000 FT and Waypoint A at 6;000 FT had placed it below intermediate minimum crossing altitudes at Waypoint Y and (incipiently) at Waypoint Z. Stopped descent. ATC outer approach then handed us over to final; who almost immediately issued a clearance to descend to 3;000 FT.  Encountered a booby trap by removing FMS altitudes and inserting recommended altitudes; then being given a different cruise altitude to initiate the process; throwing the whole plan slightly off. High workload flight reducing ability of pilot not flying to monitor. Pilot flying had cockpit lighting well dimmed; requiring effort to read charts. Deviation angle was very shallow; not becoming evident until call by ATC. Failed to cross-check current altitudes against next crossing restriction altitudes.
During an RNAV coupled approach this Bombardier Q400 crew descended slightly below a VNAV waypoint crossing restriction.