Narrative:

Approaching the arrival; we were directed by ATC to cross zzzzz intersection at or above FL300. Normal crossing depicted for the zzzzz intersection is FL240-FL320. We discussed and loaded the FMC by first erasing the preloaded speed/altitudes and loading only 280 for speed. The FMC update 13 (U13) software calculated a crossing of approximately 28750 confirming we would be able to meet the follow on restrictions; if cleared; with a little work. Next we loaded 280/30;000 ft; verified VNAV path; top of descent; etc. We were satisfied with the setup. We anticipated a clearance of 'cross zzzzz intersection at or above FL300 and then descend via the arrival'; which were given halfway through our descent to zzzzz intersection and placed the bottom altitude of the arrival in the MCP with verification of VNAV path. As we monitored the automation on descent we started to notice an erratic action of the autopilot attempting to stay on path with pitch and the airspeed was 20-30 knots fast. I deployed speedbrakes smoothly for a few thousand feet to aid in the controllability and attempt to return the speed to 280. As we approached FL300 in an idle descent; we both felt we were going to cross at FL300 and; with past experience; that the aircraft would go to VNAV speed if unable and slow its descent. Instead; the aircraft stayed in VNAV path idle descent and before I could slow the descent; I noted a deviation 29;600 ft approximately 0.3 from zzzzz intersection. The VNAV continued the high speed issue causing deployment of speedbrakes at different times through the rest of the descent and pitch and path seemed erratic; even though it stayed in VNAV path. Overall; it was having a difficult time. We had a shared mental model and were truly attempting to be in 'its head'; but it was giving mixed signals. We had no other altitude crossing issues until flying final on the RNAV Y runway xxl and it attempted to stop at 1800 ft in path and cause us to go high. We recognized it; I disengaged the automation; re-acquired path; and hand flew the rest of the approach without incident. The indications were odd; showing us low per VNAV; by approximately 500 ft; yet visually on glideslope. At about 150 ft AGL; the indications returned to a VNAV 'on path' norm. U13 software: it seemed inaccurate and misleading during this VNAV descent. If this were IMC; the indications and actions of the software would have intensifying confusion and result in task saturation. On final; if IMC; it would have caused a go around without the VMC references. These issues in the software are a cause for concern and need further attention. In reference to the altitude deviation: in hindsight; I could have put some back pressure on the yoke and been more aggressive to attempt to prevent the deviation as I did later with the anomaly on final.

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Original NASA ASRS Text

Title: B737 flight crew reported the aircraft automation had an erratic action with the autopilot trying to stay on path.

Narrative: Approaching the Arrival; we were directed by ATC to cross ZZZZZ intersection at or above FL300. Normal crossing depicted for the ZZZZZ intersection is FL240-FL320. We discussed and loaded the FMC by first erasing the preloaded speed/altitudes and loading only 280 for speed. The FMC Update 13 (U13) software calculated a crossing of approximately 28750 confirming we would be able to meet the follow on restrictions; if cleared; with a little work. Next we loaded 280/30;000 ft; verified VNAV path; Top of Descent; etc. We were satisfied with the setup. We anticipated a clearance of 'cross ZZZZZ intersection at or above FL300 and then descend via the Arrival'; which were given halfway through our descent to ZZZZZ intersection and placed the bottom altitude of the arrival in the MCP with verification of VNAV PATH. As we monitored the automation on descent we started to notice an erratic action of the autopilot attempting to stay on path with pitch and the airspeed was 20-30 knots fast. I deployed speedbrakes smoothly for a few thousand feet to aid in the controllability and attempt to return the speed to 280. As we approached FL300 in an idle descent; we both felt we were going to cross at FL300 and; with past experience; that the aircraft would go to VNAV Speed if unable and slow its descent. Instead; the aircraft stayed in VNAV PATH idle descent and before I could slow the descent; I noted a deviation 29;600 ft approximately 0.3 from ZZZZZ intersection. The VNAV continued the high speed issue causing deployment of speedbrakes at different times through the rest of the descent and pitch and path seemed erratic; even though it stayed in VNAV PATH. Overall; it was having a difficult time. We had a shared mental model and were truly attempting to be in 'its head'; but it was giving mixed signals. We had no other altitude crossing issues until flying final on the RNAV Y Runway XXL and it attempted to stop at 1800 ft in path and cause us to go high. We recognized it; I disengaged the automation; re-acquired path; and hand flew the rest of the approach without incident. The indications were odd; showing us low per VNAV; by approximately 500 ft; yet visually on glideslope. At about 150 ft AGL; the indications returned to a VNAV 'on path' norm. U13 software: It seemed inaccurate and misleading during this VNAV descent. If this were IMC; the indications and actions of the software would have intensifying confusion and result in task saturation. On final; if IMC; it would have caused a go around without the VMC references. These issues in the software are a cause for concern and need further attention. In reference to the altitude deviation: In hindsight; I could have put some back pressure on the yoke and been more aggressive to attempt to prevent the deviation as I did later with the anomaly on final.

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.