Narrative:

Enroute [to] denver centennial. Cleared to descend via the PUFFR4 RNAV arrival. As pilot monitoring (pm); I selected the 'bottom' altitude in the alt selector (9;000 ft). Approximately 30 miles from the FMS-generated top of descent; and in LNAV/VNAV; the aircraft began descending to cross slmon above FL210. Neither the pilot flying (PF) or I thought much of it as we were at FL320. I remarked that the descent seemed early and we should monitor. The aircraft continued to descend and approximately 10 miles east of slmon we were descending through FL215. I remarked to the PF; 'we need to make sure this captures the altitude.' the aircraft continued to descend below FL210 while were still several miles from the fix. I told the PF; 'altitude; take the aircraft we are not leveling!' the PF disconnected the autopilot and we were approximately 200 feet low passing slmon. The PF chose vs mode for the next several crossing restrictions. As we continued the PF again engaged VNAV. The next restriction was huukk at 15;000 ft. The aircraft remained level until 5 miles before reaching huukk. Again the aircraft descended early in an attempt to cross the next fix above 14;000 ft. Again I voiced to the PF; 'altitude; it is descending early!' the PF was momentarily confused thinking we were to cross huuck at 14;000 ft. I stated; 'hand-fly and climb to 15;000 ft. We are 200 feet low!' the PF disconnected the ap and manually controlled the aircraft for the remainder of the arrival and approach.we debriefed after landing and could not determine any reason why the aircraft should have descended prematurely in either instance. However; we did determine that the PF needed to be more proactive in disengaging the automation when things started to go poorly. He was attempting to use vertical speed mode to climb back to the proper altitude when we were low. When doing that he failed to disengage the VNAV mode so the flight director was giving him conflicting information. As pm I attempted to notify him of the mode selections but it's possible I was giving too much information during a stressful moment. The PF also stated that he was trying to be 'gentle' so the passengers would not feel the changes in pitch. To this I say who cares about the coffee let's not bust the altitude!live and learn. Lesson learned is even though RNAV arrivals are to be flown in LNAV/VNAV be sure to cross check everything. Trust but verify!

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Original NASA ASRS Text

Title: Corporate Aircraft First Officer reported having issues with the FMC VNAV descent mode in complying with altitude restrictions on an arrival.

Narrative: Enroute [to] Denver Centennial. Cleared to descend via the PUFFR4 RNAV arrival. As Pilot Monitoring (PM); I selected the 'bottom' altitude in the Alt Selector (9;000 FT). Approximately 30 miles from the FMS-generated top of descent; and in LNAV/VNAV; the aircraft began descending to cross SLMON above FL210. Neither the Pilot flying (PF) or I thought much of it as we were at FL320. I remarked that the descent seemed early and we should monitor. The aircraft continued to descend and approximately 10 miles east of SLMON we were descending through FL215. I remarked to the PF; 'We need to make sure this captures the altitude.' The aircraft continued to descend below FL210 while were still several miles from the fix. I told the PF; 'Altitude; take the aircraft we are not leveling!' The PF disconnected the autopilot and we were approximately 200 feet low passing SLMON. The PF chose VS mode for the next several crossing restrictions. As we continued the PF again engaged VNAV. The next restriction was HUUKK at 15;000 FT. The aircraft remained level until 5 miles before reaching HUUKK. Again the aircraft descended early in an attempt to cross the next fix above 14;000 FT. Again I voiced to the PF; 'Altitude; it is descending early!' The PF was momentarily confused thinking we were to cross HUUCK at 14;000 FT. I stated; 'Hand-fly and climb to 15;000 FT. We are 200 feet low!' The PF disconnected the AP and manually controlled the aircraft for the remainder of the arrival and approach.We debriefed after landing and could not determine any reason why the aircraft should have descended prematurely in either instance. However; we did determine that the PF needed to be more proactive in disengaging the automation when things started to go poorly. He was attempting to use vertical speed mode to climb back to the proper altitude when we were low. When doing that he failed to disengage the VNAV mode so the flight director was giving him conflicting information. As PM I attempted to notify him of the mode selections but it's possible I was giving too much information during a stressful moment. The PF also stated that he was trying to be 'gentle' so the passengers would not feel the changes in pitch. To this I say who cares about the coffee let's not bust the altitude!Live and learn. Lesson learned is even though RNAV arrivals are to be flown in LNAV/VNAV be sure to cross check everything. Trust but verify!

Data retrieved from NASA's ASRS site 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.