Date: 202003
Local time of day: 0601-1200
Location: MEM.TRACON, TN
Flight conditions: VMC
Aircraft: B767-300 and 300 ER
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Cargo / Freight / Delivery
Flight phase: Descent
Airspace: Class B MEM
ATC / advisory: TRACON MEM
Aircraft component: Navigational Equipment and Processing
Component problem: Design
Reporter organization: Air Carrier
Reporter function: Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Human factors: Troubleshooting; Distraction; Human-Machine Interface; Situational Awareness; Time Pressure
Anomaly: Aircraft Equipment Problem Less Severe; Deviation - Altitude Crossing Restriction Not Met; Deviation - Altitude Undershoot; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Became Reoriented; Flight Crew FLC Overrode Automation
Contributing factors: Aircraft; Human Factors
Primary problem: Ambiguous

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.




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