Narrative:

Approximately 700 ft. High at ltown crossing restriction; on the BLUZZ2 arrival to mem. Earlier; while 20 nm. Prior to top of descent point; we experienced the boeing 'VNAV anomaly' when descending from FL350 to FL340. Crew corrected the issue and resumed a normal VNAV descent via the BLUZZ2; landing north. At approximately FL180 the aircraft again experienced the boeing 'VNAV anomaly;' as it performed an uncommanded level off at FL180 and began an uncommanded deceleration. Attempting to remedy the problem; the first officer (pilot flying) selected flch; 280 kts.; and 14;000 ft. (At or above altitude for ltown; our next fix). Recognizing that the autopilot was not responding in a timely manner; I took control of the aircraft; disengaged the autopilot and attempted to intercept the vertical flight path. I then recognized we would not intercept the path in time to make the 16;000 ft. Restriction at ltown; and notified mem approach that we would 'cross ltown high.' we crossed ltown at approximately 16;700 ft. Shortly after ltown we recaptured the descent path. I returned control of the aircraft to the first officer; who flew the rest of the uneventful arrival in flch. After landing; we wrote up the FL180 VNAV anomaly in the aircraft maintenance logbook.crew failed to recognize the difficulty in making the ltown altitude restriction in a timely manner. Complicating our ability to recognize this was a previous unknown place (well past top of descent) for the boeing VNAV anomaly to occur. We never expected it to happen in the middle of an arrival descent. Mem altimeter setting was 30.24 in hg; making aircraft tend to arrive high and/or fast at ltown. A frequency change to mem approach occurred at FL180; distracting the crew at the worst possible moment. The VNAV anomaly occurred just as the crew was running the in range checklist; again worst possible moment. Include this scenario in simulator training. Flight training provides a demo of the boeing VNAV anomaly; but never mentions that it can occur at any time; not just at top of descent. This crew now recognizes and will be ready to react quickly to the VNAV anomaly at any point in flight. It's probably too much to ask; but please request that air traffic controllers avoid frequency changes during climbs/descents right at FL180.

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Original NASA ASRS Text

Title: B767 Captain reported that an FMS malfunction resulted in an altitude deviation.

Narrative: Approximately 700 ft. high at LTOWN crossing restriction; on the BLUZZ2 arrival to MEM. Earlier; while 20 nm. prior to top of descent point; we experienced the Boeing 'VNAV Anomaly' when descending from FL350 to FL340. Crew corrected the issue and resumed a normal VNAV descent via the BLUZZ2; landing north. At approximately FL180 the aircraft again experienced the Boeing 'VNAV Anomaly;' as it performed an uncommanded level off at FL180 and began an uncommanded deceleration. Attempting to remedy the problem; the First Officer (Pilot Flying) selected FLCH; 280 kts.; and 14;000 ft. (at or above altitude for LTOWN; our next fix). Recognizing that the autopilot was not responding in a timely manner; I took control of the aircraft; disengaged the autopilot and attempted to intercept the vertical flight path. I then recognized we would not intercept the path in time to make the 16;000 ft. restriction at LTOWN; and notified MEM Approach that we would 'cross LTOWN high.' We crossed LTOWN at approximately 16;700 ft. Shortly after LTOWN we recaptured the descent path. I returned control of the aircraft to the First Officer; who flew the rest of the uneventful arrival in FLCH. After landing; we wrote up the FL180 VNAV Anomaly in the Aircraft Maintenance Logbook.Crew failed to recognize the difficulty in making the LTOWN altitude restriction in a timely manner. Complicating our ability to recognize this was a previous unknown place (well past top of descent) for the Boeing VNAV Anomaly to occur. We never expected it to happen in the middle of an arrival descent. MEM altimeter setting was 30.24 in Hg; making aircraft tend to arrive high and/or fast at LTOWN. A frequency change to MEM Approach occurred at FL180; distracting the crew at the worst possible moment. The VNAV Anomaly occurred just as the crew was running the In Range Checklist; again worst possible moment. Include this scenario in simulator training. Flight Training provides a demo of the Boeing VNAV Anomaly; but never mentions that it can occur at any time; not just at top of descent. This crew now recognizes and will be ready to react quickly to the VNAV Anomaly at any point in flight. It's probably too much to ask; but please request that Air Traffic Controllers avoid frequency changes during climbs/descents right at FL180.

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.