B737 Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Flight Crew Last 90 Days 163; Flight Crew Total 2903; Flight Crew Type 2903
Distraction; Confusion; Human-Machine Interface; Troubleshooting
Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude
Flight Crew FLC Overrode Automation; Flight Crew Overcame Equipment Problem; Flight Crew Took Evasive Action
Aircraft
For the second time in a month; in two different aircraft; I have had the same issue with VNAV busting an altitude constraint at the same fix. Inbound to SFO on the PIRAT2 arrival. Clearance is to cross ARGGG at 8;000 ft.; then maintain 4;000 ft. In VNAV path with 8;000 ft. programmed at ARGGG and 4;000 ft. set in the MCP window. All is normal until a couple of miles prior to ARGGG when the VNAV PDI pops to full scale high and the aircraft chases the path and ignores the constraint. I disconnected the autopilot at 7;800 ft. and popped back up to 8;000 ft. until crossing the fix. Same thing on both occurrences. This will bite someone as it is the end of a red eye crossing from Hawaii.
B737 Captain reported that in 2 different aircraft at this location; the aircraft did not follow the programmed VNAV flight path.