Q400
Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Less Severe; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Returned To Clearance
Aircraft; Human Factors
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.
On arrival; we were cleared the MARNR Three. We had prepared for a 34 arrival; but when on approach we found they were landing to the south; and so we re-briefed the arrival; trying to get a comfortable arrival descent profile on the MARNR programmed in the box. The pilot monitoring was off primary radio and talking to Company. We had the initial descent programmed in; but on approaching MARNR at 12;000; the FMS VNAV leveled off 200 feet high resulting in an uncomfortable float. I changed the pitch mode to VS; but I failed to account for the level leg to RAYNI. We continued down to 11;700 before I saw the deviation from the flight path and took the autopilot off and corrected manually to reverse the descent and I corrected to approximately 11;800 at RAYNI (Restriction 12;000 or above) where we continued on the arrival; but subsequently we put all the stepdown altitudes in the altitude alerter to prevent this from happening again.I have three pages of information on how to fly these arrivals. Some techniques range from changing the default angle in the FMS to the official crew read files information on which altitudes to put in for which points. With so much complex and conflicting information; I am very tempted to fly this arrival 'old school' by putting in the stepdown altitude and flying it in the VS mode. I am very uncomfortable putting the final stepdown of 6;000 feet in the arrival once we have been cleared to descend via the arrival. We need to simplify how we fly this arrival and publish it with the chart recommended altitudes or alterations. Currently; there's too much conflicting information. The bottom line is this: Should we fly an arrival that results in an uncomfortable ride for our passengers unless it is altered for every arrival? On short flights this extra complication presents significant risk especially in conjunction with runway changes; and the large number of tasks that must be accomplished on the approach.
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.