Medium Transport; High Wing; 2 Turboprop Eng
Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Less Severe
Aircraft Equipment Problem Dissipated
Aircraft; Procedure
While level at 12;000 MSL we were cleared to descend via the MARNR RNAV STAR to SEA. I deleted the back calculated crossing altitude at MARNR during the briefing of the arrival and its crossing altitudes. When we captured VPATH at RAYNI we reset the altitude alerter to 6;000. With the VNAV page I reviewed the [expected] descent rates.  The initial descent rate from RAYNI to GRYND was 2;200 with all other descent rates approximately 1;800 FPM. After crossing RAYNI the aircraft pitched rapidly to -3;500 FPM; then to -100 FPM; then finally settled out at about 1;700 FPM descent. The remainder of the arrival was steady. I've observed this same series of events during the HAWKZ arrival...although not as dramatic...once as pilot flying and we reached -4;200 FPM initial descent landing north at Seattle. The VPATH undershoots by approximately two times the required descent rate and takes a few minutes to settle on a final descent rate. There is a real good possibility that a Flight Attendant or a passenger may be hurt because of the excessive deviation in descent rates before the VPATH settles on a descent rate.Possible problem with the VPATH/autopilot/FMS interface with the new arrivals. This problem does not occur on RNP approaches. I've only observed this on these two arrivals.
A First Officer reported repeated examples of excessively aggressive initial VNAV/AUTOFLIGHT pitch/rate of descent changes associated with the MARNR and HAWKZ RNAV STARs to SEA.