E175 first officer reported the FMC was not performing properly in VNAV and was a factor in their fa...

Narrative:

Descent to sjc via BRIXX2. Entire descent maintained in magenta 'pth' FMA mode. Vertical cross track error observed within 25-40 ft. Conditions VMC. Both first officer and captain baro altitudes set to 29.96. Perf data was .75/290 speed with 2.8 angle. During descent to 12;000 ft. Descending via BRIXX2; oak handed off (flight) to norcal approach. Request was made for RNAV Z 30L. (Flight) was cleared join RNAV Z 30L from BRIXX2 arrival common waypoints except maintain 12;000 ft. Captain selected RNAV Z 30L common fixes (with altitude of 7;000 ft. On jilan and 210/4800A on yadut) with 'verify' statement followed by 'activate' statement from first officer.later; (flight) was then cleared to 7;000 ft. After freq hand off. Through descent; then cleared for the RNAV Z 30L off the BRIXX2 STAR. Altitude selector then set to 1;600 ft. For woxar while in pth mode with app armed (LNAV engaged /gp armed). While in pth mode received check altitude constraint message ~200-300ft below jilna of 7000ft with jilna as active flight plan fix. Altitude hold selected on guidance panel to prevent further baro altitude deviation by captain. Airplane was at baro altitude ~350-440 ft. Below 7;000 ft. In altitude mode as the vertical deviations showed airplane high of VNAV path and growing at waypoint sequence constraint. First officer selected 210/4800 on yadut for vertical direct-to on flight plan page. Flight path angle too steep message occurred. VNAV was then selected and remaining of RNAV Z 30L constraints followed per procedure. VNAV path computed a fpa not coincidental to 7;000 ft. Altitude constraint in active flight plan at jilan 7;000 ft. Constraint on an rnp ar procedure outside the FAF. No discrepancies reported by ATC. Airport was visual so no missed approach conducted. Issues observed may prevent maintaining ops spec approval of baro VNAV system for rnp ar C400.

Original NASA ASRS Text

Title: E175 F/O reported the FMC was not performing properly in VNAV and was a factor in their failure to meet charted crossing restrictions on arrival into destination .

Narrative: Descent to SJC via BRIXX2. Entire descent maintained in magenta 'PTH' FMA mode. Vertical cross track error observed within 25-40 ft. Conditions VMC. Both First Officer and Captain baro altitudes set to 29.96. Perf data was .75/290 speed with 2.8 angle. During descent to 12;000 ft. descending via BRIXX2; OAK handed off (flight) to NORCAL approach. Request was made for RNAV Z 30L. (flight) was cleared join RNAV Z 30L from BRIXX2 arrival common waypoints except maintain 12;000 ft. Captain selected RNAV Z 30L common fixes (with altitude of 7;000 ft. on JILAN and 210/4800A on YADUT) with 'VERIFY' statement followed by 'ACTIVATE' statement from First Officer.Later; (flight) was then cleared to 7;000 ft. after freq hand off. Through descent; then cleared for the RNAV Z 30L off the BRIXX2 STAR. Altitude selector then set to 1;600 ft. for WOXAR while in PTH mode with APP armed (LNAV engaged /GP armed). While in PTH mode received CHECK ALT CONSTRAINT message ~200-300ft below JILNA of 7000ft with JILNA as active flight plan fix. ALT hold selected on guidance panel to prevent further baro altitude deviation by Captain. Airplane was at baro altitude ~350-440 ft. below 7;000 ft. in ALT mode as the vertical deviations showed airplane high of VNAV path and growing at waypoint sequence constraint. First Officer selected 210/4800 on YADUT for vertical direct-to on Flight Plan page. FLIGHT PATH ANGLE TOO STEEP message occurred. VNAV was then selected and remaining of RNAV Z 30L constraints followed per procedure. VNAV path computed a FPA not coincidental to 7;000 ft. altitude constraint in active flight plan at JILAN 7;000 ft. constraint on an RNP AR procedure outside the FAF. No discrepancies reported by ATC. Airport was visual so no missed approach conducted. Issues observed may prevent maintaining Ops spec approval of Baro VNAV system for RNP AR C400.

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.

Attributes
ACN	1739766
Time
Date	202004
Local Time Of Day	0601-1200
Place
Locale Reference	SJC.Airport
State Reference	US
Environment
Flight Conditions	VMC
Aircraft 1
Make Model Name	EMB ERJ 170/175 ER/LR
Operating Under FAR Part	Part 121
Flight Phase	Descent
Flight Plan	IFR
Component
Aircraft Component	FMS/FMC
Person 1
Function	First Officer Pilot Flying
Qualification	Flight Crew Instrument Flight Crew Air Transport Pilot (ATP) Flight Crew Multiengine
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Altitude Crossing Restriction Not Met Deviation - Procedural Clearance Deviation - Procedural Published Material / Policy

E175 first officer reported the FMC was not performing properly in VNAV and was a factor in their failure to meet charted crossing restrictions on arrival into destination .