Narrative:

Enroute level at 13;000 MSL. After being cleared to descend via the marnr 3 landing south the pilot flying read the waypoint altitudes from his FMS while I verified with the chart. The first two waypoints (marnr & rayni) were set at 12;000 feet. All others were the back calculated altitudes. All waypoint altitudes met with each crossing restriction. The pilot flying set the initial vpath for marnr and the altitude alerter was dialed down to 6;000 feet at grify. I notified the pilot flying that I was leaving ATC to talk to company on comm #2; pilot flying had ATC on comm #1. There was a lot of company talk on the radio; I had to wait for my call into operations. Once talking with operations I gave our position report and ETA; they let us know our parking spot. While talking with operations the plane started to level off abruptly at 12;000 feet; VNAV leveling at marnr. I (pilot monitoring) completed my operations call and notified the pilot flying that I was back and had ATC. I noticed that we were at about 11;750 feet MSL and looked at our next waypoint. We were in between marnr and rayni and supposed to be at or above 12;000 feet MSL. As I was about to call altitude I noticed the pilot flying recognizing the situation and correcting; climbing back to 12;000. Once level at 12;000 we proceeded to dial each step down to protect the crossing restrictions. When the VNAV was abruptly leveling at 12;000 the vertical mode was switched to vs with the leg limitation not protected. In vs the plane continued to descend below 12;000.

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Original NASA ASRS Text

Title: Q400 flight crew reports descending below the RAYNI crossing restriction on the MARNR 3 arrival to SEA due to anomalous autopilot operation in VNAV and the flying pilot switching to Vertical Speed and descending below 12;000 feet. The First Officer laments the many special techniques required for the Q400 to smoothly and successfully fly RNAV arrival procedures.

Narrative: Enroute level at 13;000 MSL. After being cleared to descend via the MARNR 3 landing south the pilot flying read the waypoint altitudes from his FMS while I verified with the chart. The first two waypoints (MARNR & RAYNI) were set at 12;000 feet. All others were the back calculated altitudes. All waypoint altitudes met with each crossing restriction. The pilot flying set the initial Vpath for MARNR and the altitude alerter was dialed down to 6;000 feet at GRIFY. I notified the pilot flying that I was leaving ATC to talk to Company on Comm #2; pilot flying had ATC on Comm #1. There was a lot of company talk on the radio; I had to wait for my call into Operations. Once talking with operations I gave our position report and ETA; they let us know our parking spot. While talking with operations the plane started to level off abruptly at 12;000 feet; VNAV leveling at MARNR. I (Pilot Monitoring) completed my operations call and notified the pilot flying that I was back and had ATC. I noticed that we were at about 11;750 feet MSL and looked at our next waypoint. We were in between MARNR and RAYNI and supposed to be at or above 12;000 feet MSL. As I was about to call altitude I noticed the pilot flying recognizing the situation and correcting; climbing back to 12;000. Once level at 12;000 we proceeded to dial each step down to protect the crossing restrictions. When the VNAV was abruptly leveling at 12;000 the vertical mode was switched to VS with the leg limitation not protected. In VS the plane continued to descend below 12;000.

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.