Narrative:

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.

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Original NASA ASRS Text

Title: During an RNAV coupled approach this Bombardier Q400 crew descended slightly below a VNAV waypoint crossing restriction.

Narrative: 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.

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.